To grasp immediately the nature of the field of solar and heliospheric physics, one need only look at the striking image of the 2010 solar eclipse shown in Figure 10.1. The solar atmosphere extends outward from the Sun’s surface, apparently without end. In fact, this extended atmosphere—the heliosphere—does end at roughly 140 AU, where the solar wind runs into the galaxy at the heliopause. Our Earth, its upper atmosphere, and its magnetosphere—and the other planets of the solar system—are all embedded deep inside this extended stellar atmosphere. Hence, the vast physical domain stretching from the center of the Sun out to the heliopause defines our home in space. The heliopause is where our home ends and the rest of the universe begins.
To achieve the key science goals set forth by this decadal survey in Chapter 1, it is necessary to understand the physical processes that create and sustain connections within the heliosphere. Figure 10.1 demonstrates, however, that gaining such an understanding requires overcoming major fundamental physics challenges. The most striking feature of the image is its beautifully intricate structure—obviously due to the Sun’s magnetic field—that has its origins deep below the solar surface. Understanding the processes that produce the solar field is a prerequisite for achieving the goals of the survey, but the recent, unexpected deep solar minimum demonstrates that understanding of the Sun’s dynamo is still rudimentary. Consequently, the Panel on Solar and Heliospheric Physics (SHP) identified as the first of four SHP science goals for the upcoming decade, SHP1, to determine how the Sun generates the quasi-cyclical variable magnetic field that extends throughout the heliosphere.
The extended structure of Figure 10.1 shows that the magnetic field provides a coupling across the solar and heliospheric domain, from the solar interior to the galaxy. In fact, the field not only couples the solar and heliospheric domain but also is essential for its creation. All current theories for heating the corona and accelerating the wind postulate the magnetic field as the central agent. Although this is not immediately obvious from Figure 10.1, because it is only a snapshot, the magnetic coupling of the Sun and heliosphere is always dynamic. The field at the solar surface and low corona is observed to vary dra-
FIGURE 10.1 White-light image of the solar corona out to 4 solar radii during the solar eclipse of November 7, 2010. SOURCE: Courtesy of M. Druckmüller, Brno University of Technology, M. Dietzel, Graz University of Technology, S. Habbal, Institute for Astronomy, and V. Rušin, Slovak Academy of Sciences.
matically on temporal scales ranging from seconds to decades. In situ measurements of the heliosphere indicate that that variability creates a broad range of temporal scales, from subseconds for discontinuities produced by explosive events to decades for the solar cycle evolution. Furthermore, the heliosphere is inherently turbulent, and so it produces internal dynamics down to kinetic dissipation scales. All those heliospheric dynamics couple directly to Earth’s magnetosphere and upper atmosphere, again by the magnetic field, producing the myriad forms of space weather that we must understand and mitigate. The SHP panel therefore identified as its second major science goal for the coming decade, SHP2, to determine how the Sun’s magnetism creates its dynamic atmosphere.
In addition to its ceaseless dynamics, a defining feature of our home in space is that it is filled with high-energy radiation, both electromagnetic—ultraviolet (UV) rays, X rays, and gamma rays—and particulate. High-energy particle radiation, especially protons, is the greatest threat to human exploration of the solar system and to deep-space missions. The most dangerous bursts of energetic particles are due to the giant
explosions of the solar magnetic field and matter known as coronal mass ejections/eruptive flares. Those events also produce the most destructive space weather at Earth, with damaging consequences ranging from communication and GPS blackouts to the loss of high-orbiting satellites and power transformers on the ground.
In recent years, scientists have made substantial progress in understanding the Sun’s magnetic explosions. Researchers know where on the Sun they are likely to occur but are far from understanding when and how large the explosions will be and, in particular, how so much of their energy produces particle radiation. It should be emphasized that the processes of magnetic energy storage and release are commonly observed in all laboratory and cosmic plasmas; therefore, understanding them in the Sun and heliosphere will be a fundamental advance for all physics. Thus, the SHP panel’s third major science goal, SHP3, is to determine how magnetic energy is stored and explosively released.
As stated above, the heliopause is where the Sun’s extended atmosphere ends and the galactic medium begins. That interface where two regions collide is generally rich in unexplored and unique physics. For example, the dominant energy form in the region is due not to the solar wind plasma or magnetic field but rather to interstellar neutrals that stream freely into the heliosphere and charge exchange with the solar wind ions. The magnetic structure of the region is also unique in that the field is wound into such a tight spiral due to solar rotation that it is almost cylindrical rather than radial. It should also be noted that the structure of the outer heliosphere has important effects close to home: it determines the penetration of high-energy galactic cosmic rays into near-Earth space. In situ measurements by the Voyager spacecraft and new methods to globally image the outer boundary of the heliosphere are spurring a revolution in understanding of this region. During the next decade, the Voyager spacecraft should pass the heliopause and enter interstellar space. Extending their presence robotically, humans will have left their home in space for the first time and entered the universe—a truly historic event. The coming decade will be critical for gaining an understanding not only of how our space environment is created and driven, but also of how it ends. That is the SHP panel’s final major science goal for the decade, SHP4, to discover how the Sun interacts with the local galactic medium and protects Earth.
Associated with each of the SHP panel’s four science goals are several SHP actions that, if carried out, promise substantial progress in achieving the goals. Box 10.1 summarizes the SHP panel’s four major science goals and 14 associated actions.
To achieve the four major science goals listed in Box 10.1, the SHP panel developed a strategy that consists of a set of imperatives for the federal agencies involved in solar and heliospheric research. The imperatives—actions that are essential for future progress—are listed briefly below according to the relevant agency (or agencies) and discussed in detail below in this chapter.
1. Complete the development and launch of the Interface Region Imaging Spectrograph (IRIS) and Solar Probe Plus (SPP) missions, and deliver U.S. contributions to the European Space Agency–National Aeronautics and Space Administration (ESA–NASA) Solar Orbiter mission. The measurements from those missions are central to the SHP panel’s strategy for addressing SHP science goals 1, 2, and 3 in the next decade (§10.5.3.1).
2. Augment the heliophysics Explorer budget to expand launch opportunities and add new cost-effective mid-size launch vehicles (§10.5.3.2).
BOX 10.1 SOLAR AND HELIOSPHERIC PHYSICS PANEL’S MAJOR SCIENCE GOALS AND ASSOCIATED ACTIONS
SHP1. Determine how the Sun generates the quasi-cyclical variable magnetic field that extends throughout the heliosphere.
a. Measure and model the near-surface polar mass flows and magnetic fields that seed variations in the solar cycle.
b. Measure and model the deep mass flows in the convection zone and tachocline that are believed to drive the solar dynamo.
c. Determine the role of small-scale magnetic fields in driving global-scale irradiance variability and activity in the solar atmosphere.
SHP2. Determine how the Sun’s magnetism creates its dynamic atmosphere.
a. Determine whether chromospheric dynamics is the origin of heat and mass fluxes into the corona and solar wind.
b. Determine how magnetic free energy is transmitted from the photosphere to the corona.
c. Discover how the thermal structure of the closed-field corona is determined.
d. Discover the origin of the solar wind’s dynamics and structure.
SHP3. Determine how magnetic energy is stored and explosively released.
a. Determine how the sudden release of magnetic energy enables both flares and coronal mass ejections to accelerate particles to high energies efficiently.
b. Identify the locations and mechanisms that operate in impulsive solar energetic-particle sites, and determine whether particle acceleration plays a role in coronal heating.
c. Determine the origin and variability of suprathermal electrons, protons, and heavy ions on timescales of minutes to hours.
d. Develop advanced methods for forecasting and nowcasting of solar eruptive events and space weather.
SHP4. Discover how the Sun interacts with the local galactic medium and protects Earth.
a. Determine the spatial-temporal evolution of heliospheric boundaries and their interactions.
b. Discover where and how anomalous cosmic rays are accelerated.
c. Explore the properties of the heliopause and surrounding interstellar medium.
3. Substantially enhance the following key elements of the NASA heliophysics research and analysis programs so that the science community can use the new measurements to attain the SHP science goals outlined above (§10.5.3.4):
• Create new heliophysics science centers composed of teams of theorists, numerical modelers, and data experts to tackle major science problems.
• Implement a graduated increase in individual principal investigator grant programs to 20 percent of Heliophysics Division funding, primarily by increasing the size of individual grants.
• Augment the mission operations and data analysis program to facilitate systems science using the Heliophysics Systems Observatory.
• Ensure that all newly confirmed missions have Phase E budgets adequate to ensure mission success and that they sponsor a guest-investigator program as part of their success criteria.
• Form a consolidated heliophysics instrument and technology development program for innovative instrument concepts.
• Provide flight and training opportunities through increased funding of the Low Cost Access to Space program for science-payload development and data analysis.
4. To attack SHP science goals 3 and 4, develop the Interstellar Mapping and Acceleration Probe (IMAP), a mission to observe the interaction of the heliosphere with the interstellar medium, and measure suprathermal ion populations with unprecedented resolution (§10.5.2.2). IMAP is the SHP panel’s highest-priority mid-size mission.
5. To attack SHP science goals 2 and 3, support U.S. participation in the Japanese-led Solar-C mission that will measure the magnetic coupling of the lower atmosphere into the corona (§10.5.2.3).
6. To maximize the science returns on limited resources, extend the Explorer mission development model to mid-size strategic missions (§10.5.3.3).
7. To attack SHP science goal 3, develop the Solar Eruptive Events (SEE) mission to image electron and ion acceleration in SEEs with unprecedented resolution. This is the SHP panel’s highest-priority mission concept for an LWS-class strategic mission (§10.5.2.4).
9. To develop solar-sail propulsion for future Heliophysics Division missions, invest about $50 million as “seed money” in a full-scale solar sail demonstration mission by partnering with the Office of the Chief Technologist’s Technology Demonstration Missions program (§10.5.2.8).
1. Provide base funding sufficient for efficient and scientifically productive operation of the Advanced Solar Technology Telescope, which is also central to achieving the SHP panel’s science goals (§10.5.4.1).
2. Establish a midscale projects funding program similar to NASA’s Explorer model (§10.5.4.2).
3. Fund the development and operation of the Frequency-Agile Solar Radiotelescope (FASR) to produce three-dimensional images of the solar atmosphere with high temporal and spatial resolution (§10.5.4.3).
4. Fund the development and operation of the Coronal Solar Magnetism Observatory (COSMO), a large-aperture coronagraph, chromosphere, and prominence magnetometer (§10.5.4.4).
5. Double the size of the National Science Foundation (NSF) small-grants programs to support the effort required for analyzing and modeling the new data sets (§10.5.4.5).
6. Broaden the definition of NSF’s Solar-Terrestrial Research Program to include outer-heliosphere research (§10.5.4.6).
1. Ensure continued support of current ground-based observations of the Sun, especially the line-of-sight and vector measurements of the solar magnetic field that are being used routinely for space weather operations (§10.5.5.1).
2. Ensure continued support of laboratory facilities for accurate measurement of atomic properties and for instrument calibrations (§10.5.5.2).
3. Develop real-time and near-real-time instrumentation and data streams from future missions and ground-based facilities. In collaboration with other agencies, fly missions devoted to space weather objectives. Establish an interagency clearinghouse and archive for space weather data (§10.5.5.3).
4. Ensure continuity of real-time solar wind measurements from L1 (§10.5.5.4).
5. Identify key long-term solar and heliospheric data sets and recommend approaches to ensure that they are continued and archived (§10.5.5.5).
6. Continue support of laboratory plasma physics at current or higher levels to complement spacecraft measurements in understanding basic heliophysical processes (§10.5.5.6).
7. NASA, the National Oceanic and Atmospheric Administration (NOAA), and the Department of Defense are encouraged to develop a plan for a mission at the L5 Lagrangian point to conduct high-priority helioseismology studies and develop advanced capabilities to forecast space weather (§10.5.5.7).
There has been considerable progress along with many surprising discoveries in the disciplines that constitute solar and heliospheric physics since publication in 2003 of the National Research Council (NRC) decadal survey, The Sun to the Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics.1 In the following four subsections, organized by the four SHP science goals outlined in Section 10.1, the panel describes a small sampling of recent developments. Accomplishments or goals that address the three decadal survey guiding motivations (M1-M3) are also noted.2
As emphasized in the 2003 decadal survey,3 an enduring major science goal is to determine how the Sun generates its quasi-cyclical variable magnetic field. The practical goal of such research is to learn enough to be able to help to predict the changing space environment and its societal impact (motivation M2). Here the SHP panel sketches four of the many notable accomplishments of the past decade toward meeting these goals.
With a wide array of ground- and space-based sensors, solar activity in its many forms was observed to fall in 2008-2009 to low levels not seen for nearly a century. Solar activity is driven by the solar magnetic field, and the recent decline in the polar magnetic field flux is shown in Figure 10.2. The sharp decline was not generally expected. However, researchers noted that before the activity minimum measurements by ground-based instruments and space-based instruments on the Solar and Heliospheric Observatory (SOHO) showed unusually small amounts of magnetic flux near the poles of the Sun. Considered by some to be a useful precursor of the strength of an activity cycle, the low flux levels led to predictions that the current solar cycle maximum would be the lowest since polar flux measurements became available—as appears to be the case. Other venerable solar activity precursors that suggested a high level of activity proved to be spectacularly unreliable. Vital observational work in this research is continuing with ground-based and space-based assets, particularly because there is a chance that the Sun is entering a sustained period of low activity.
The effects of record low solar activity were observed throughout the heliosphere. Among the prominent effects with societal consequences were these: cosmic ray fluxes near Earth reached the highest levels on record, and reduced heating of Earth’s upper atmosphere by solar UV radiation led to less drag on satellites.
1 National Research Council (NRC), The Sun to the Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics, The National Academies Press, Washington, D.C., 2003; and NRC, The Sun to the Earth—and Beyond: Panel Reports, The National Academies Press, Washington, D.C., 2003.
2 The motivations referred to in this section are those outlined in the introduction to Part II of this report: M1, Understand our home in the solar system; M2, Predict the changing space environment and its societal impact; M3, Explore space to reveal universal physical processes.
3 NRC, The Sun to the Earth—and Beyond: Panel Reports, 2003, p. 12.
FIGURE 10.2 Average value of the magnetic flux density at the Sun’s polar regions during the last four cycles. The polar flux density peaks at the times of maximum. The recent cycle minimum had the weakest polar flux ever recorded. SOURCE: Courtesy of Leif Svalgaard, Stanford University. Red, data from Wilcox Solar Observatory; blue, data from Mt. Wilson Observatory scaled to match.
Those surprising events and the rapid development of flux-transport dynamo models have led to intense recent work in solar dynamo modeling. Two key issues that involve the transport of magnetic flux have emerged: the speed with which meridional flows move poloidal magnetic flux to the solar poles and the extent to which this moving flux stays near the surface or diffuses inward on its journey to the poles. Because of the importance of those issues, there is a critical need for measurements of meridional flow and its variability. The measurements are difficult, and a consensus view on how to proceed has yet to emerge.
Observational spatial resolution has greatly improved through image-processing techniques applied to ground-based observations made with the 1.6-m New Solar Telescope (NST) at the Big Bear Solar Observatory and other 1-m-class apertures and direct observations made with the 0.5-m Hinode satellite. But it is the combination of these state-of-the-art observations with advanced numerical modeling that has, after 400 years of speculation, revealed the main physical processes at work in sunspot penumbral filaments, bright umbral dots, bright faculae, and small-scale magnetic elements. Figure 10.3 shows a numerical simulation of a sunspot and an actual photograph. Good observational and modeling progress has also been made with respect to the more complicated upper layers of the solar atmosphere.
Continuing and improved helioseismic measurements of the solar interior from instruments including those on the SOHO and SDO spacecraft, launched in 1995 and 2010, respectively, and from the GONG ground-based network, have shown cycle-related changes in large-scale internal zonal and meridional flows. The varying flows may play a driving role rather than just being consequences of the solar activity cycle, and they narrow the wide range of realistic solar dynamo models. Helioseismic observations of active regions have shown subsurface helical flows whose strength is closely related to flare activity. In addition, a new analytic method shows surprisingly deep-seated effects of large active regions well before they emerge at the surface (Figure 10.4). Either or both of those findings may develop into useful forecasts of strong solar activity with societal significance.
FIGURE 10.3 Numerical simulation of a sunspot (left) and a very-high-resolution image (right) from the New Solar Telescope at Big Bear Observatory, which is operated by the New Jersey Institute of Technology. Detailed comparisons have elucidated the physics of such solar features. SOURCE: Left, courtesy of M. Rempel, High Altitude Observatory. Right, courtesy of Big Bear Solar Observatory. For further information on the simulation, see M. Rempel, Numerical sunspot models: Robustness of photospheric velocity and magnetic field structure, Astrophysical Journal 750(1):62, 2012.
The Sun sustains life on Earth by its nearly steady output of light. Measurements of total solar irradiance (TSI) with different space-based instruments have consistently shown a cycle variation on the order of 0.1 percent but give conflicting results about the absolute value of the TSI. The conflicts have recently been resolved in favor of a lower value that improves agreement with climate research results. The original conflicting results and new results are shown in the two panels of Figure 10.5. The new results indicate that the TSI at the recent cycle minimum was less than at previous minima. It has also been suggested that this level is about as low as can be expected even if a long cessation of solar activity occurs (as during the Maunder minimum).
Major advances have been made during the past decade in understanding the dynamic and coupled nature of the Sun’s extended atmosphere, which reaches from the chromosphere and corona to the end of the heliosphere (motivations M1-M3). The advances highlight the need for direct sampling of the solar wind in the deep inner heliosphere as close to the Sun as possible, with high-resolution measurements of plasma and electromagnetic fluctuations; high-spatial- and high-temporal-resolution observations of the solar atmosphere from the photosphere into the corona; accurate magnetic field measurements in the low-beta chromosphere and corona; and advanced theoretical models that can couple closely to the upcoming observations.
FIGURE 10.4 Upper panel shows the surface magnetic field (grey) with a travel-time perturbation map constructed at a depth of 42-75 million meters (blue) about a day before an active region appeared at the surface as seen in the lower panel. SOURCE: Adapted from S. Ilonidis, J. Zhao, and A. Kosovichev, Detection of emerging sunspot regions in the solar interior, Science 333:993-996, 2011. Reprinted with permission from AAAS.
FIGURE 10.5 Recent improvements in the accuracy and precision of measurements of total solar irradiance have resolved several basic issues. The upper panel shows the original discordant measurements before recalibration. SOURCE: G. Kopp and J. Lean, A new, lower value of total solar irradiance: Evidence and climate significance, Geophysical Research Letters 38(1):L01706, doi:10.1029/2010GL04577, 2011. Copyright 2011 American Geophysical Union. Reproduced by permission of American Geophysical Union.
In situ measurements of the solar wind plasma velocity distribution functions and electromagnetic fluctuations permit us to identify the dominant kinetic physics responsible for the generation and dissipation of Alfvénic fluctuations and other types of solar wind turbulence (motivations M1 and M3). Progress has been made in measuring and understanding the types of turbulent fluctuations in the solar wind and the likely dissipation mechanisms, but important questions remain. Observations from spacecraft—such as Cluster, Wind, and ACE—and archival data from Helios have been used to probe the power in fluctuations perpendicular and parallel to the local magnetic field as a function of scale and have demonstrated that these fluctuations are highly anisotropic. Higher time-resolution measurements have allowed researchers to trace the evolution of the turbulent cascade and dissipation past ion kinetic scales toward electron scales. More work is needed to determine the relative fraction of fluctuations dissipated by ions and electrons and to distinguish between different mechanisms for dissipation, such as ion-cyclotron resonance, kinetic Alfvén waves, or the formation of small-scale current sheets. It is also unclear where broadband Alfvénic fluctuations and other types of solar wind turbulence, such as 1/f noise, originate, either in the corona or during propagation in the inner heliosphere. Further theoretical work and joint analysis of solar wind plasma and electromagnetic fields will allow a determination of whether nonlinear plasma physics can explain the highly nonadiabatic expansion of the solar wind.
In the past decade, substantial progress has also been made in understanding the development of kinetic instabilities in the solar wind due to nonthermal ion and electron distribution functions; in particular, how temperature anisotropies are limited by the mirror, firehose, and cyclotron instabilities (motivations M1 and M3). A surprising result is that the mirror instability appears to play a stronger role than the cyclotron instability in limiting temperature anisotropy. Instabilities have also been proposed to explain the observed restriction of differential ion flow to the local Alfvén speed, but observational confirmation of these theories has remained elusive.
In situ measurements of the solar wind are also a powerful diagnostic of the evolving connection between the corona and interplanetary space, and measurements in the past decade have revealed new features in the solar wind and related them to the structure and dynamics of the inner heliosphere and evolving Sun. The anomalously low levels of solar activity in the recent solar minimum were associated with substantial decreases in the density and pressure of the solar wind. Charge state and composition allow changes in the coronal sources of the solar wind to be tracked independently of changes in the solar wind speed due to the evolution of the plasma as it expands into interplanetary space. It is now understood that the three distinct forms of wind—fast, slow, and transient (associated with ICMEs)—can be identified clearly by their ionic charge-state signatures (using O7+/O6+) without assumptions about the dynamic evolution of the wind.
Figure 10.6 shows the fractions of the three solar wind components for the decade 1998-2008. The compositions of the three winds provide vital clues to their sources in the Sun and the mechanisms of their formation. The panel notes the particular importance of furthering understanding of the slow wind, as its source and origin have constituted one of the outstanding problems in solar and heliospheric physics. With new insights into the slow wind’s origin and with the upcoming Solar Probe Plus and Solar Orbiter missions, researchers are poised to solve this problem definitively in the coming decade.
FIGURE 10.6 Sunspot number (top) and three solar wind components (bottom) during 1998-2008: interplanetary coronal mass ejections (yellow), coronal hole wind (green), and noncoronal hole wind (orange). SOURCE: L. Zhao, T.H. Zurbuchen, and L.A. Fisk, Global distribution of the solar wind during solar cycle 23: ACE observations, Geophysical Research Letters 36:L14104, doi:10.1029/2009GL039181, 2009. Copyright 2009 American Geophysical Union. Reproduced by permission of American Geophysical Union.
An unexpected discovery has been the observation of ubiquitous reconnection events in the solar wind, often on a very large scale of about 1 million kilometers. The reconnection events constitute a major puzzle in that they do not appear to accelerate particles to speeds greater than the local Alfvén speed. Again, observations nearer to the Sun will be critical for understanding the physical properties of this type of reconnection and its role in driving solar wind dynamics. Figure 10.7 details the structure of a giant reconnection event in the solar wind. Is there evidence of remnants of plumes in the inner heliosphere? Are there remnants or observable consequences of type II spicules? Do nanoflares indicate a heating process that continues throughout the solar atmosphere? Are small-scale nanoflares and turbulent current sheets (reconnection sites) physically related to solar wind discontinuities, and do they represent a form of turbulence heating mechanism that may heat the corona from the solar wind acceleration region to the outer heliosphere? The need for high-resolution observations can be seen in all levels of the solar atmosphere. The Solar Optical Telescope of the Hinode satellite has provided high-resolution images of the solar chromosphere in the Ca II and Hα spectral lines. At the solar limb, these images reveal a very rich, relentlessly dynamic, and highly structured environment. A new type of spicule has been discovered that may play a crucial role in providing mass and energy transfer to the corona. Figure 10.8 shows images of these solar spicules.
The solar atmosphere and wind are both created and structured through the medium of the Sun’s powerful magnetic field; consequently, accurate measurements of the field in the corona and wind are
FIGURE 10.7 Structure of a giant reconnection event in the solar wind as inferred from the combination of Cluster, Wind, and ACE data. SOURCE: Reprinted by permission from Macmillan Publishers Ltd.: Nature, T.D. Phan, J.T. Gosling, M.S. Davis, R.M. Skoug, M. Øieroset, R.P. Lin, R.P. Lepping, D.J. McComas, C.W. Smith, H. Reme, and A. Balogh, A magnetic reconnection X-line extending more than 390 Earth radii in the solar wind, Nature 439:175-178, 2006.
FIGURE 10.8 Images of solar spicules on the disk (upper) made with the Swedish Solar Telescope, La Palma, Spain, and on the limb (lower) made with the Solar Optical Telescope on Hinode. SOURCE: Upper, Courtesy of Bart De Pontieu, Lockheed Martin Solar and Astrophysics Laboratory. Lower, Hinode is a Japanese mission developed and launched by ISAS/JAXA, with NAOJ as domestic partner and NASA and STFC (United Kingdom) as international partners. It is operated by these agencies in cooperation with the European Space Agency and NSC (Norway).
essential for understanding our home in space. The lack of such measurements of the corona is one of the greatest obstacles to advancing solar and heliospheric science. Hinode and SDO can determine the full vector field accurately in the photosphere, but the plasma beta is high there, and so it is not possible to extrapolate the magnetic field into the low-beta corona reliably. Two major advances during the past decade promise to overcome the magnetic-field measurement obstacle: the first observations of the full chromospheric vector field on the disk, and the first maps of the coronal field above the solar limb.
The prevalence of high-resolution extreme UV (EUV) narrowband images from instruments on the TRACE (Transition Region and Coronal Explorer) and SDO (Solar Dynamics Observatory) spacecraft have revealed that warm (1 million Kelvin) coronal loops are up to 3 orders of magnitude overdense and hence cannot be in steady state as previously believed (Figure 10.9). Observations and theoretical models of the structures imply that they are still unresolved. How coronal structures are heated is still unknown, but it has been suggested that including the coupling between the chromosphere and corona will prove essential in determining the heating mechanism.
Great progress has also been made in the past decade in achieving closure between observations and predictions from theory or models. The first three-dimensional magnetohydrodynamic (MHD) ab initio numerical simulation of the corona was successfully performed. The chromosphere now stands as the modeling frontier with qualitatively more challenging simulations requiring radiation coupled to MHD (R-MHD) and no assumption of local thermodynamic equilibrium. Fully three-dimensional R-MHD numerical simulations spanning the upper convection zone through the corona—treated as a coherent system—are starting to appear but cannot yet address all the salient physical ingredients on scales larger than a few granules or one supergranule. This state of affairs should improve in the coming decade given the expected rapid progress in numerical hardware and software.
Major solar flares and associated fast coronal mass ejections (CMEs) are the most powerful explosions and particle accelerators in the solar system, and they produce the most extreme space weather. In the past decade, substantial progress was made in understanding how magnetic energy is stored and explosively released on both large and small scales (motivation M1).
RHESSI hard X-ray (HXR) imaging-spectroscopy measurements have shown that accelerated electrons often contain about 50 percent of the solar-fare energy release and provided strong evidence that energy release-electron acceleration is associated with magnetic reconnection. In one occulted flare, HXR and microwave measurements from the acceleration region, high above the thermal soft-X-ray loop tops (Figure 10.10), showed that essentially all electrons in this region were accelerated to over about 15 keV with no detectable thermal plasma. The accelerated-electron and magnetic-field energy densities were comparable. In large flares, the energy in ions over 1 MeV and in electrons over 20 keV appears comparable. RHESSI gamma-ray imaging of flare-accelerated ions of about 30 MeV shows emission from two small footpoints rather than an extended region. In the largest flare, the footpoints straddled the flare-loop arcade (Figure 10.11), indicating that ion acceleration is also related to magnetic reconnection.
For the first time, flares were detected in total solar irradiance (TSI) with the SORCE/TIM instrument. For the X17 October 28, 2003, flare, the TSI showed both impulsive (HXR-like) and gradual (soft-X-ray-like) components with a peak increase of about 100 ppm. The total radiated energy (over about 1032 ergs) and associated-CME kinetic energy (about 1033 ergs) were comparable. In addition, the SDO/EVE instrument has discovered an EUV late phase in flares, a second enhancement delayed many minutes after the X-ray peak. In addition, global EUV observations with SDO/AIA and STEREO/EUVI have revealed long-distance “sympathetic” interactions between magnetic fields in flares, eruptions, and CMEs during August 1-2, 2010,
FIGURE 10.9 Top: Solar Dynamics Observatory (SDO) observation of 1 million-kelvin active region loops on February 23, 2011. Bottom: First three-dimensional magnetohydrodynamic ab initio model for coronal loops. SOURCE: Top, Courtesy of NASA/SDO and the AIA science team. Bottom, B.V. Gudiksen and A. Nordlund, An ab initio approach to solar coronal loops, Astrophysical Journal 618(2):1031-1038, 2005, reproduced by permission of the AAS.
FIGURE 10.10 Hard X-ray (HXR; blue) and microwave (magenta) images of the electron-acceleration/energy-release region in the December 31, 2007, solar flare, in which the bright HXR footpoints were occulted (with HXR and microwave limbs indicated). SOURCE: Courtesy of S. Krucker, University of California, Berkeley, “Electron Acceleration in Solar Flares: RHESSI Hard X-ray Observations,” presentation at the Interdisciplinary Workshop on Magnetic Reconnection, February 11, 2010, Southwest Research Institute, San Antonio, Tex.; modified after S. Krucker, H.S. Hudson, L. Glesener, S.M. White, S. Masuda, J.-P. Wuelser, and R.P. Lin, Measurements of the coronal acceleration region of a solar flare, Astronomical Journal 714(2):1108-1119, 2010. Reproduced by permission of the AAS.
probably due to perturbations of the magnetic-field topology, indicating that ion acceleration is also related to the magnetic reconnection responsible for the flare.
The acceleration profiles of CMEs below about 4 solar radii (RS) are closely synchronized with flare-HXR energy releases, implying that the fast CME expansion creates a flare current sheet below the CME, where reconnection results in the particle acceleration responsible for the hard X rays (Figure 10.12). Theory and remote-sensing observations agree that many ejected CMEs contain a magnetic flux-rope structure. The shocks produced by fast CMEs (presumably the main solar energetic-particle [SEP] accelerators) can now often be identified in SOHO/STEREO coronagraph images. Indeed, STEREO/HI images CMEs from the deep corona to 1 AU, revealing their solar origins, propagation to Earth, and space weather impacts.
In a surprising discovery, STEREO A&B detected a burst of 1.6- to 15-MeV energetic neutral hydrogen atoms (ENAs) arriving from the Sun hours before SEPs arrived from the E79 December 5, 2006, solar event. The ENAs were produced by either flare or CME-shock-accelerated protons charge-exchanging with coronal ions. Thus, ENAs provide a new probe of ion acceleration near the Sun.
The largest SEP events are generally thought to be due to CME-driven shock acceleration that can quickly reach energies of 0.1-1 GeV near the Sun. In the past decade, accelerated ions and enhanced resonant waves predicted by diffusive shock-acceleration theory were detected in a few events. Although large
FIGURE 10.11 RHESSI imaging of 2.223-MeV neutron-capture γ-ray line emission (produced by ions over about 30 MeV) and 200- to 300-keV HXR emission (produced by electrons over about 200 keV) for the October 28, 2003, flare, superimposed on a TRACE image of the flare loop arcade. The ions over 30 MeV are confined to two footpoints straddling the loop arcade, similar to the electrons over 200 keV except for a significant (roughly 104 km) displacement between the ion and electron footpoints. SOURCE: G.J. Hurford, S. Krucker, R.P. Lin, R.A. Schwartz, G.H. Share, and D.M. Smith, Gamma-ray imaging of the 2003 October/November solar flares, Astrophysical Journal 644:L93-L96, doi:10.1086/505329, 2006. Reproduced by permission of the AAS.
FIGURE 10.12 Ultra-high-resolution numerical simulation of a reconnection-initiated coronal mass ejection and eruptive flare. White contours indicate high current densities. Note the vertical flare current sheet below the erupting plasmoid. The plasmoid undergoes sudden acceleration coincident with the onset of the flare (reconnection in this sheet). A similar result is obtained in ideally driven models. SOURCE: J.T. Karpen, S.K. Antiochos, and C.R. DeVore, The mechanisms for the onset and explosive eruption of coronal mass ejections and eruptive flares, Astrophysical Journal 760(1):81, 2012. Reproduced by permission of the AAS.
SEP events are usually associated with strong shocks at 1 AU, there is seldom a one-to-one correspondence between observations and theory, and this shows the need for near-Sun measurements.
Solar-cycle 23 produced about 100 large SEP events, including the largest ground-level event (observed with neutron monitors) since 1956. New instrumentation on ACE, SOHO, and Wind permitted important advances in measuring SEP elemental, isotopic, and ionic-charge-state composition from H to Ni (1 ≤ Z ≤ 28). SEP composition and spectral variations depend on ionic charge-to-mass (Q/M) ratios, probably because of interactions with excited waves and variable shock geometry, but testing acceleration and transport models will require near-Sun measurements where most of the acceleration occurs. SEPs remain a major radiation hazard for spacecraft and astronauts, but the discovery that relativistic electrons provide about a 1-hour warning of arriving SEP ions has provided a new forecast tool (motivation M2).
FIGURE 10.13 The effect of “radio noise” from the powerful October 28, 2003, flare on GPS carrier signal-to-noise strength (C/N0). The red and blue traces show GPS C/N0 for signals at two receiving stations. The purple trace shows the radio flux density at 1,415 MHz measured by the U.S. Air Force Radio Solar Telescope Network station at San Vito, Italy. The green curve shows the radio flux density inferred from the GPS L1 carrier fade (1,575 MHz). Note the inverse correlation between the C/N0 fade and the flare’s radio flux density. SOURCE: Adapted from P.M. Kintner, Jr., B. O’Hanlon, D.E. Gary, and P.M. Kintner, Global Positioning System and solar radio burst forensics, Radio Science 44:RS0A08, doi: 10.1029/2008rs004039, 2009. Copyright 2009 American Geophysical Union. Reproduced by permission of American Geophysical Union.
Radio bursts from major flares interfere with wireless communications and GPS (Figure 10.13), and the “Halloween” solar storm events of 2003 disrupted aircraft navigation systems. Strong magnetic fields in CME-driven disturbances can generate powerful geomagnetic storms that accelerate radiation-belt “killer” electrons and induce ground-level currents that disrupt electric-power grids. The past decade has seen substantial progress in modeling CMEs, shocks, and SEPs from solar eruptions.4 One system uses CME and real-time solar wind data to drive models that forecast effects on power grids (motivation M2). Furthermore, uncertainties in forecasting CME Earth-arrival times have been reduced from ±12 hours to ±3 hours by using STEREO observations more than 1 day in advance.
The most common SEP events, with 104 events per year near solar maximum, are small “impulsive” events associated with coronal jets that are enriched in 3He and heavy ions up to Z ≈ 80 by amounts that depend on mass-to-charge ratios. ACE and SOHO observations indicate Fe charge states substantially higher than ambient values, most likely because of electron stripping during acceleration in the low corona. 3He and Fe are also enriched in many large SEP events; this indicates that remnant suprathermal particles from previous impulsive flares are an important source of seed particles for CME-shock acceleration.
The past decade has seen a new appreciation of the frequency of occurrence of the halo solar wind (HSW) and its importance in local dynamics. The HSW, a nonthermal tail extending far beyond the thermal ion and electron distributions, makes an important contribution to the local pressure even if the relative density of the halo is small compared with the rest of the solar wind. The HSW and suprathermal tails provide information on nascent particle acceleration in local sites and transport mechanisms for remotely accelerated particles. Distribution functions of locally accelerated suprathermal tails in the heliosphere
and heliosheath commonly have a power-law index and gradual rollover at higher speeds. Observations reveal that local acceleration occurs in solar wind compression regions and that interplanetary transport modifies spectra of remotely accelerated particles. The variability of the power law and its implications for acceleration models have been topics of focused study.
The past decade produced one of the most dramatic advances in space physics. As the Voyager spacecraft approached the termination shock (TS) and entered the heliosheath, a series of groundbreaking discoveries were made. These in situ measurements, combined with all-sky heliospheric images by the Interstellar Boundary Explorer (IBEX) and Cassini mission, led to major advances in our understanding of how the solar system interacts with the interstellar medium (SHP science goal 4; decadal survey key science goal 3).
Voyager 1 (V1) crossed the TS in December 2004 at 94 AU in the Northern Hemisphere, and Voyager 2 (V2) crossed in the Southern Hemisphere in August 2007 at 84 AU. On their locations on the shock, neither Voyager found any evidence of the acceleration of the higher-energy anomalous cosmic rays (ACRs) observed in interplanetary space. That puzzle inspired several new ideas of where and how particles may be accelerated (motivation M3).
In addition, the solar wind did not get heated at the TS nearly as much as expected (Figure 10.14); apparently, 80 percent of the supersonic-flow energy went into suprathermal particles (Voyager measures only thermal plasma). Earlier calculations suggested that acceleration of pickup ions may be the primary dissipation mechanism at the TS. However, that had not been explicitly incorporated into heliospheric models, and so the V2 observation that heating of solar wind protons accounted for only about 20 percent of the dissipation was generally surprising. Remarkably, suprathermal-tail energy spectra have remained power laws with index around -1.5, which is consistent with suprathermal neutral-atom distributions observed in Cassini/INCA images.
Those discoveries affect other disciplines, including astrophysics and plasma physics; the heliosphere has become a test bed for other astrospheres and plasma sheaths. Moreover, the heliosheath plasma environment is unlike any other plasma in the heliosphere.
Beginning about April 2010, V1 observed the heliosheath plasma to become nearly stagnant (zero speed). The V1 plasma instrument is not functional, but the Low-Energy Charged-Particle (LECP) instrument can determine two components of the local plasma velocity by observing its effect on energetic ions. Voyager scientists reported that the flow velocity near the ecliptic plane was near zero, and the north-south component was also near zero on the basis of higher-energy observations. There is no consensus interpretation of those unexpected observations, but one idea is that V1 entered a “transition region,” possibly a precursor of the heliopause, starting in April 2010.
To determine the plasma-flow direction at the Voyager locations more precisely, both spacecraft were commanded to execute a series of “roll” maneuvers, allowing the LECP instruments to determine the plasma velocity normal to the ecliptic plane. The rolls have occurred, but results are not yet available. Such maneuvers may also be used after the Voyagers cross the heliopause.
Another major surprise came from global energetic neutral-atom (ENA) maps by IBEX and Cassini. IBEX discovered a completely unpredicted, narrow (about 20°) ribbon of ENA emissions from the outer heliosphere, apparently ordered by the local interstellar magnetic field (ISMF), as indicated by comparison with global MHD models. More than a half-dozen theories have tried to explain the ribbon origin. The hypothesized physical mechanisms operate in disparate regions from the TS to beyond the heliopause and out to the local bubble. CASSINI found a similar, but much broader, feature at higher energies (Figure 10.15).
FIGURE 10.14 The radial speed (a), density (b), and temperature (c) in the heliosheath (red) are compared with that expected (black) on the basis of the crossing of Neptune (x-axis scale in hours). Note that the velocity drop and density increase were much lower than expected, and the temperature increase was less than 10 percent of that expected. SOURCE: Reprinted by permission from Macmillan Publisher Ltd.: Nature: J.D. Richardson, J.C. Kasper, C. Wang, J.W. Belcher, and A.J. Lazarus, Cool heliosheath plasma and deceleration of the upstream solar wind at the termination shock, Nature 454:63-66, 2008, doi:10.1038/nature07024.
The IBEX ribbon appears to evolve on timescales as short as 6 months, demonstrating that the heliosphere-interstellar-medium interaction is more dynamic than expected.
It was not known to what extent the ISMF would play a role in shaping the outer heliosphere. The two Voyagers crossed the TS at different distances, indicating a distinct north-south (and east-west) asymmetry of the global heliosphere. That effect was attributed to a tilt of the ISMF related to the velocity vector of the solar system relative to the interstellar cloud (Figure 10.16). Earlier measurements by SOHO/SWAN also indicated an ISMF influence. The IBEX and Cassini images are apparently organized by the ISMF as well (see Figure 10.16). The ISMF orientation and magnitude are poorly constrained. Models indicate that the ISMF may be strong and provide most of the pressure in the local cloud. The orientation of the local ISMF differs from that of the large-scale interstellar field, but the exact orientation is still uncertain.
In 2009, the galactic cosmic-ray (GCR) intensity at Earth reached the highest level of the space age (Figure 10.17), owing mainly to the reduced interplanetary magnetic field and an extended period of low solar activity. The extended solar minimum, reduced sunspot number, and record cosmic-ray intensity have led to suggestions that we may be entering an extended period of minimum activity such as was
FIGURE 10.15 The unexpected ribbon seen in 0.9- to 1.5-keV energetic neutral atoms (ENAs) with IBEX and the 5- to 13-keV INCA belt. These maps depict integrated line-of-sight global maps of ENAs. Previous models, based on ENA production in the heliosheath, predicted concentrated, uniform emission near the nose. None of the earlier models predicted the ribbon or belt. SOURCE: Interstellar Boundary Explorer Mission Team.
FIGURE 10.16 Crossing of the termination shock by Voyager 1 (V1) and Voyager 2 (V2) at different locations (left) indicated a strong influence of the interstellar magnetic field in distorting the heliosphere (Opher et al. 2006). Left panel shows a side view of the heliosphere where color contours indicate the magnetic-field intensity. Related results were obtained by IBEX (right panel), where the ribbon location appears to be influenced by the interstellar magnetic field that shapes the heliosphere. SOURCE: Left, M. Opher, E.C. Stone, and P.C. Liewer, The effects of a local intersteller magnetic field on Voyager 1 and 2 observations, Astrophysical Journal 640:L71, 2006. Reproduced by permission of the AAS. Right, D.J. McComas, F. Allegrini, P. Bochsler, M. Bzowski, E.R. Christian, G.B. Crew, R. DeMajistre, H. Fahr, H. Fichtner, P.C. Frisch, H.O. Funsten, et al., Global observations of the interstellar interaction from the Interstellar Boundary Explorer (IBEX), Science 326:959-962, doi:10.1126/science.1180906, 2009.
observed (in sunspot, 14C, and 10Be data) during the Dalton minimum (1800-1820) or the Maunder minimum (1645-1715).
In the following sections, the panel outlines steps that will continue the recent pace of progress while offering opportunities for significant breakthroughs. The first four subsections of this section summarize opportunities related to each of the SHP panel’s four major science goals (§10.2). The panel then outlines new opportunities provided by the continuation of existing space-based and ground-based programs (§10.4.6.4 and §10.4.7.2) and by the completion of programs that are now in development (§10.4.6.1-§10.4.6.3 and §10.4.7.1). Goals addressed by proposed new programs are described in Section 10.5.
The variable magnetic field that creates the heliosphere and produces space weather events results from processes in the solar interior and at the surface. Consequently, probing the solar interior and surface to determine the origins of the Sun’s magnetic activity is a major goal for the next decade (SHP actions
FIGURE 10.17 The galactic cosmic-ray intensity in 2009 was a record for the space age (Mewaldt et al., 2010). At the same time, the interplanetary magnetic field strength (E.J. Smith and A. Balogh, Geophysical Research Letters 35:L22103, 2008) and solar wind dynamic pressure (D.J. McComas et al., Geophysical Research Letters 35:L18103, 2008) were about 40 percent lower than during the previous solar minimum. Note that the cosmic-ray maximum was about 2 years later than projected. SOURCE: R.A. Mewaldt, A.J. Davis, K.A. Lave, R.A. Leske, E.C. Stone, M.E. Wiedenbeck, W.R. Binns, E.R. Christian, A.C. Cummings, G.A. de Nolfo, M.H. Israel, et al., Record-setting cosmic-ray intensities in 2009 and 2010, Astrophysical Journal Letters 723:L1-L6, 2010. Reproduced by permission of the AAS.
1a and 1b; motivation M1). The processes are not yet well understood, mainly because of the challenges of measuring the solar interior and surface in crucial locations and on timescales of multiple solar cycles.
Helioseismology is revealing surprising properties of much of the solar interior. Currently, models of the Sun that agree with helioseismology measurements are based on element abundances that disagree with recent values obtained by spectral line analysis. The discrepancy challenges the foundations of astrophysics and must be solved in the next decade. It is widely thought that the polar regions and tachocline play large roles in the solar cycle, and these locations are difficult to observe with helioseismology from a single near-Earth location. A goal for the next decade is to start to probe these regions in sufficient detail to define whether and how they affect the course of solar activity. Attaining that goal involves simultaneous surface velocity measurements from near Earth and a vantage point separated by a sizable fraction of an astronomical unit (stereohelioseismology).
The ESA-led Solar Orbiter will furnish brief, pioneering observations at moderate inclinations relative to the ecliptic plane during the 2020s. The science return from this mission will be greatly enhanced if NASA supports both additional telemetry coverage and U.S. investigations with non-U.S. instruments. White papers offered other paths to obtain in-ecliptic stereohelioseismic observations, for example, L5 (§10.5.2.5) and Safari. Sustained observation of the polar regions requires a high-inclination solar orbit. Such orbits
are not easily reached, but a successful investment in this decade in solar-sail technology (§10.5.2.8) will enable effective long-term use of high-inclination orbits in the 2020s to probe portions of the solar interior that are important for the solar cycle.
Magnetic fields at the visible surface of the Sun—the photosphere—have been studied for decades. The well-observed photosphere links the obscured internal sources of activity with the faint corona and sparsely sampled heliosphere. Conditions measured in this narrow layer provide scientists’ main insights into the origin of the solar cycle and sources of space weather. One long-standing goal is to understand how magnetically active regions erupt and disperse at the surface. There are major uncertainties about the influence of small spatial-scale magnetic fields on the solar cycle and on TSI. Open issues include how much small-scale fields contribute to the magnetic and TSI budgets of the Sun and how their properties change over the course of a solar cycle (SHP action 1c). Attacking that frontier is a major goal of the 4-meter-aperture Advanced Technology Solar Telescope (ATST).
An important societal goal is to improve the quality of real-time models of the solar magnetic field to assist in forecasting space weather events and heliospheric conditions (motivation M2). Those models use observed magnetic boundary conditions as the basis of outward extrapolation of the magnetic field. Improving the quality and extending the measurements above the surface with optical and radio methods are goals that will lead to better understanding and improved space weather forecasts. At present, limited measurements of the solar magnetic field are made by a few ground-based observatories and with the Hinode and SDO space missions. Augmenting those measurements with ground-based data from the proposed FASR and COSMO facilities, along with the space-based data from the proposed JAXA-led Solar-C mission, would greatly contribute to meeting the goal.
Great strides in understanding stellar interiors and activity cycles have been made with new high-precision measurements of oscillations and variability. A goal for the next decade is to use such observations to help solve fundamental questions about the dynamo process, internal structure and dynamics, rotation, and activity cycles of stars similar to the Sun. Information from a broad array of stars will sharpen understanding of the Sun’s physics (motivation M3).
Section 10.3.2 describes a few of the impressive advances of the past decade, but researchers are still far from understanding how the Sun’s variable magnetic field structures and powers an atmosphere that extends from the bottom of the chromosphere out to the distant boundaries of the heliosphere. For example, how the corona couples dynamically to the solar wind is uncertain. The chromosphere clearly plays an important role in the injection of energy into the corona, but it is observed only in a narrow way and is poorly understood. The mechanisms that heat the corona and accelerate the wind constitute one of the central problems in all space science. It is expected that in the coming decade the revolutionary new observations from the missions and projects discussed in this survey, combined with next-generation theory and models, will resolve many of these outstanding problems.
To make progress in understanding the solar wind’s dynamics and structure, it is necessary to go as close to the Sun as possible to measure the properties of the wind at its origin (motivations M1-M3). That is the goal of Solar Probe Plus and Solar Orbiter, new missions that will each make unique observations of the structure and evolution of the connection between the corona and interplanetary space. SPP will measure solar wind characteristics well within the Alfvén radius where the solar magnetic field still controls the dynamics of the wind. The probe will truly be a discovery mission in that it will explore a region of the heliosphere that has never been visited before. Solar Orbiter will bring a suite of instruments designed for coordinated in situ and remote imaging both close to the Sun and out of the ecliptic plane.
The chromosphere is another key region of our home in space that has yet to be understood. Major goals for the upcoming decade are to measure the structure and dynamics of the chromosphere accurately and to understand their role in the origin of the heat and mass fluxes into the corona and wind (motivation M1). Attacking those problems requires simultaneous observation of emission from the photosphere to the corona at high spatial, temporal, and spectral resolution. That is the motivating strategy for Solar-C and for the IRIS Explorer mission. IRIS will deliver pioneering observations of chromospheric dynamics in preparation for Solar-C, which will observe the full, coupled solar atmosphere, including all detailed plasma and magnetic measures, with spatial resolution never before achieved—about 0.1 arc-seconds. With its vast improvement in resolution and coverage, Solar-C, like SPP, will be a discovery mission.
The defining feature of the photosphere-corona system is its magnetic coupling; understanding this coupling requires accurate measurement of the magnetic field in the corona, especially the full vector field so that the free energy can be measured. Measurement of the coronal field has challenged solar and heliospheric physics for decades, but with the recent advances in both ground-based and space-based instrumentation, researchers are finally poised to meet this challenge. Solar-C will determine the vector field in the chromosphere with high resolution and over extended duration, thereby permitting reliable extrapolation of the field into the corona. At the same time, ATST, FASR, and COSMO would measure the coronal magnetic field directly from the ground. With those revolutionary capabilities, it will be possible to follow the buildup and release of magnetic energy in the corona and address many of the most fundamental questions in solar and heliospheric science and space weather, such as the physical processes that produce flares and CMEs.
One of the fundamental questions is the origin of the thermal structure of the closed-field corona. Researchers have long observed coronal loops but have never definitively seen the heating process, which is expected to occur at scales well below that provided in present images. The heating process is expected to have clear signatures in the internal structure of coronal loops. Solar-C is designed specifically to have the spatial, temporal, and spectral resolution required to reveal this internal structure and dynamics. Those observations will be pioneering. Simultaneously with observing the plasma structure, Solar-C, ATST, and FASR would constrain the properties of electric currents in the corona and thereby probe the heating mechanism itself. With those missions and projects in the coming decade, enormous progress will be made toward achieving one of the central goals in solar and heliospheric science: understanding how the Sun produces the hot closed corona.
Solar cycle 23 was the best-observed cycle of the space era,5 and scientists now have greatly improved understanding of basic processes in large solar eruptions as well as more sophisticated models of these events. However, key questions remain. Expected progress toward SHP science goal 3 is outlined below for associated SHP actions 3a-d:
• Determine how the sudden release of magnetic energy enables both flares and CMEs to accelerate particles to high energies efficiently. There are rather complete models of particle acceleration and transport
5 Since the last solar maximum, Hinode, STEREO, Fermi, and SDO have joined SOHO and RHESSI to provide solar imaging over 360° with much greater spatial, temporal, and spectral resolution. In addition, in situ instruments now encircle the Sun. These unprecedented observatories promise exciting new observations of CME and flare eruptions as solar activity increases. Expected to come on line in the coming decade or shortly are ATST, which will measure coronal magnetic fields; SPP and Solar Orbiter, which explore SEP, CME, and interplanetary properties near the Sun; and IMAP, a spacecraft to be placed at L1 to observe ENAs from the heliospheric boundary region that also requires background measurements of the solar wind.
at CME-driven shocks, but key questions remain about conditions near the Sun: Why does SEP acceleration efficiency vary so greatly from event to event, and how do preceding CMEs apparently improve acceleration efficiency? SPP and Solar Orbiter will directly measure the seed populations and physical conditions necessary for particle acceleration, investigating the roles of shocks, reconnection, waves, and turbulence. The near-Sun measurements, backed by 1-AU spacecraft, will relate acceleration-region conditions with 1-AU intensities, spectra, and composition to discover why and how SEP acceleration varies and how particles are transported in radius and longitude (motivation M1). High-resolution CME and shock images from FASR will aid these studies. Figure 10.18 shows a stage of the June 13, 2010, coronal wave with the approximate position of the wavefront that forms a weak shock and the outline of a solar eruption.
• Identify the locations and mechanisms that operate in impulsive SEP sites, and determine whether particle acceleration plays a role in coronal heating. During solar-active periods, low-coronal reconnection activity causes thousands of impulsive SEP events each year. While close to the Sun, SPP and Solar Orbiter will improve the statistical accuracy and temporal resolution of measured intensity and composition variations by 1-2 orders of magnitude over 1-AU data, enabling improved correlations with images of coronal jets and other reconnection sites, tests of models for acceleration and ion fractionation, and searches for quiet-time coronal emission. In addition, the NuSTAR Explorer X-ray mission will search,
FIGURE 10.18 The formation of shocks low in the corona is a critical missing piece in understanding sudden solar energetic particle onsets. This Solar Dynamics Observatory/Atmospheric Imaging Assembly image shows a stage of the June 13, 2010, coronal wave with the approximate position of the wavefront (dashed black curve) that forms a weak shock and the outline of a solar eruption (dotted curve). The shock was formed at about 1.2 RS and observed here at 1.4 RS. SOURCE: K.A. Kozarev, K.E.Korreck, V.V. Lobzin, M.A. Weber, and N.A. Schwadron, Off-limb solar coronal wavefronts from SDO/AIA extreme-ultraviolet observations—Implications for particle production, Astrophysical Journal Letters 733: L257,2011. Reproduced by permission of the AAS.
with more than 100 times the sensitivity of earlier studies, for microflares and nanoflares that may heat the corona. IMAP high-resolution composition and charge-state measurements will trace 1-AU impulsive events while remote-sensing and near-Sun observations provide spatial and temporal structures of solar and interplanetary acceleration regions.
• Determine the origin and variability of suprathermal electrons, protons, and heavy ions on timescales of minutes to hours. Discovering suprathermal-ion production mechanisms is one key to understanding particle acceleration. Pioneering observations by Ulysses, ACE, Wind, and STEREO revealed the importance of suprathermal ions and raised questions about their origin, but these studies had limited time resolution and statistical accuracy. SPP and Solar Orbiter will measure the evolution of ion and electron halo solar wind and suprathermal tails close to the Sun, providing improved opportunities to isolate solar and interplanetary contributions and to test theoretical models. Comprehensive measurements of suprathermalion composition, spectra, and intensity fluctuations by IMAP will relate energetic-particle populations to interplanetary structures and physical processes.
• Develop advanced methods for forecasting and nowcasting of solar eruptive events and space weather. Combining SPP and Solar Orbiter in situ observations with 1-AU imaging and in situ data will provide ground truth for SEP-acceleration models and thereby improve SEP forecasts (motivation M2). The data may also reveal how monitoring critical near-Sun conditions (for example, active-region, shock, and suprathermal-seed properties) can aid forecasting. Multipoint measurements of SEP radial and longitudinal distributions will clarify current environmental-model uncertainties.
It is critical that forecasters develop predictive capabilities for space weather events while maintaining comprehensive measurements for nowcasting solar wind and energetic-particle inputs into geospace (motivation M2). IMAP, like ACE before it, will be a keystone of the Heliophysics Systems Observatory (HSO) by providing comprehensive solar wind data; diagnostics of suprathermal-ion and electron sources; solar wind and energetic-particle inputs into geospace; and evolving interplanetary magnetic-field properties. IMAP will also provide unprecedented measurements of suprathermal-tail variability and determine how seed populations are related to higher-energy particles accelerated by shocks, waves, and disturbances.
FASR would provide many new observations important with respect to space weather, including observations of coronal magnetic fields in active regions and their evolution before, during, and after flares and CMEs; “real-time” observations of coronal-shock locations and properties; measurements of the spectral evolution of electron energy-distribution functions; and radio flux-density spectra in communication bands.
The coming decade offers unique opportunities for additional breakthroughs in understanding how the heliosphere and local galactic medium interact. Those opportunities address associated SHP actions 4a-c:
• Determine the spatial-temporal evolution of heliospheric boundaries and their interactions. Solar-cycle changes in the solar wind dynamic pressure affect the structure of heliospheric boundaries. In the next few years, Voyager and IBEX observations will determine how the global heliosphere responds to increased solar activity. Complementary solar wind, pickup-ion, and anomalous and galactic cosmic-ray observations by ACE, Wind, and STEREO will provide context for evolving solar conditions and measure the cosmic-ray response to global-heliosphere changes. IMAP, with its unprecedented roughly 80 times greater sensitivity and duty cycle in ENA maps (and 10 times higher angular resolution), will deliver definitive measurements of the fine structure and detailed evolution of the global heliosphere. By combining those breakthrough
observations with expected advances in theory and modeling,6 researchers will understand the structure of the heliosheath in detail, including how time-dependent effects propagate in the heliosheath and affect heliospheric structures. IMAP will make it possible to solve the mystery of the IBEX ribbon and INCA belt and to discover the implications of these vast structures for the heliosphere and the local galactic medium (motivation M1).
• Discover whether particles are accelerated in the heliosheath. When Voyager 1 crossed the TS, particles up to about 1 MeV/nucleon peaked at the shock, but, surprisingly, the intensity of higher-energy ACRs continued to rise well after the shock. There are several theories about the location and mechanism of acceleration of these high-energy ACRs. One theory suggests that the highest-energy ACRs originate at the flanks of the heliosphere rather than at the nose because of the blunt TS shape. Another idea invokes random “hot spots” along the TS due to large-scale turbulence. It is also proposed that acceleration occurs in the heliosheath as particles move within random plasma compressions. Still another theory suggests that particle acceleration arises from contracting magnetic islands that result from magnetic reconnection in the heliosheath. The last two ideas predict that the main acceleration occurs near the heliopause.
Distinguishing among those theories requires Voyager energetic-particle, solar wind, and magnetic-field measurements through the heliosheath to the heliopause. In addition, current global-MHD models need to be expanded to include turbulence, magnetic reconnection, and feedback from suprathermal particles. Hybrid and kinetic codes are needed to complement global-MHD studies. Voyager data will test quantitative predictions from the models.
The Voyagers are exploring a new region, the heliosheath, which, unlike the region inside the TS, is not dominated by supersonic solar wind. Compressive magnetic structures and possibly turbulence or magnetic reconnection dominate this region. Heliosheath physics is not yet well understood, but new measurements and models will spark new advances. Understanding heliosheath physics is also important for interpreting IBEX, Cassini, STEREO, and IMAP ENA observations. Finally, understanding the heliosheath is important because of the role it plays in modulating the intensity of galactic cosmic rays that penetrate into the inner solar system and reach Earth. Figure 10.19 shows a schematic of the heliosphere, the heliosheath, and the interstellar medium.
• Explore the properties of the heliopause and surrounding interstellar medium. The Voyagers are expected to cross the heliopause into the local interstellar medium (LISM) within the next decade and will provide the first measurements of interstellar magnetic-field strength and orientation and, it is hoped, measurements of interstellar plasma properties (motivation M3). They will also measure interstellar cosmic-ray spectra, which may distinguish among acceleration-transport models or reveal contributions from nearby sources. Interstellar cosmic-ray measurements have other consequences: they represent the maximum cosmic-ray intensity that Earth has experienced (important for interpreting 10Be archives) and they establish the maximum intensity of the radiation to which future space travelers can be exposed (constraining interplanetary environmental models). Simultaneous near-Earth measurements will establish the absolute intensity drop from interstellar space to Earth.
The Voyagers will provide the first measurements of the structure of the heliopause. What is the role of instabilities and reconnection near the heliopause? How are cosmic rays modulated? How thick is the heliopause? Heliopause models exist, but surprises are inevitable, such as when the TS was crossed. In addition, if the Voyagers enter interstellar space roughly coincidentally with increased solar activity, plasma-wave data may constrain LISM kinetic properties (such as the turbulence level). IBEX and IMAP
6 J.D. Richardson et al., The Heliospheric Interaction with the LISM: Observations and Models, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 227; V. Florinski et al., The Outer Heliosphere-Solar System’s Final Frontier, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 76.
FIGURE 10-19 A schematic image of the regions of the heliosphere, including the approximate locations of Voyagers 1 and 2. Voyager 1 crossed the termination shock at 94 AU in December 2004, and Voyager 2 crossed at 84 AU in August 2007. For details, see http://science1.nasa.gov/missions/voyager/. SOURCE: Courtesy of NASA/JPL/Walt Feimer. NOTE: This figure was prepared several years ago when scientists expected Voyager to discover a bow shock. New results from the Interstellar Boundary Explorer (IBEX) have shown that instead of a shock, there is a softer interaction—a bow wave, similar to how water piles up ahead of a moving boat. For IBEX results, see D.J. McComas, D. Alexashov, M. Bzowski, H. Fahr, J. Heerikhuisen, V. Izmodenov, M.A. Lee, E. Moebius, N. Pogorelov, N.A. Schwadron, and G.P. Zank, The heliosphere’s interstellar interaction: No bow shock, Science 336:1291, doi: 10.1126/science.1221054, May 2012.
will complement those observations and provide context with ENA maps of the global response of the heliosheath to changing solar activity.
IBEX made the first in situ detections of interstellar H and O, including secondary O (neutral oxygen produced when interstellar O+ charge-exchanges in the outer heliosheath), which tracks deflected interstellar flow around the heliosphere. IBEX studies of interstellar H, O, He, and Ne are forcing re-examination of LISM characteristics. Resolving LISM properties (for example, whether the solar system is still in the local interstellar cloud or in transition between clouds) requires interstellar-flow observations with high sensitivity and angular resolution. The detailed neutral and high-precision pickup-ion observations with IMAP will provide abundances of He, N, O, and Ne and the flow vector and temperature of He, O, and Ne, which will strongly constrain models of the ionization state and radiation environment of the interstellar medium.
Key isotope ratios (D/H, 3He/4He, 22Ne/20Ne) will place strong constraints on big bang cosmology and the evolution of matter in the galaxy. From high-precision observations of secondary O and He, the strength and orientation of the local interstellar magnetic field and the structure of the outer heliosheath can be independently deduced. Extended IBEX observations will provide reconnaissance, and IMAP will have all the necessary capabilities for decisive measurements. The Voyagers will provide ground truth through and beyond the heliopause, and theory and modeling will have the challenge of reconciling these observations.
This section summarizes the impact that the SHP panel’s proposed program can have on realizing the key science goals of this decadal survey (see Chapter 1).
The top section of Table 10.1 illustrates how the space missions and ground-based facilities described in this chapter will address the SHP panel’s science goals and associated actions outlined in Box 10.1. Note that implementation of the entire SHP program would make major contributions to all four SHP goals and all 14 SHP actions (for details, see §10.4.1-10.4.4 and descriptions of the individual actions in §10.4.6-10.5).
The bottom section of Table 10.1 illustrates how the SHP panel’s program can help realize decadal survey key science goals 1-4 during the coming decade and beyond. As has already been noted, several space missions and ground-based facilities promise to make transformative contributions to decadal survey key science goal 1 with unique remote-sensing measurements that will discover how the Sun’s magnetic activity transfers matter and energy from the photosphere through the atmosphere and out into the heliosphere. The exploratory Solar Orbiter and SPP missions, including multiple passes through the solar corona by SPP, will address decadal survey key science goal 1 by providing the first in situ observations of the inner heliosphere close to the Sun. These truly transformative observations will reveal new details of solar and interplanetary processes and greatly improve researchers’ ability to understand, model, and predict variability in the space environment. The activities also support decadal survey key science goal 2 in that they will lead to a better understanding of the interplanetary plasma and energetic-particle environments that impinge on the magnetosphere.
Similarly, IMAP will image temporal and spatial variations of the outer heliosphere with about 80 times better sensitivity while providing comprehensive measurements of solar wind and energetic-particle inputs to geospace. Many of the measurements will also reveal details of basic solar and heliospheric processes that are at work in astrophysical systems throughout the galaxy, fulfilling decadal survey key science goal 3. Finally, the planned observations, spanning a broad array of plasma environments, will permit the study and understanding of the fundamental physical processes that act within the heliosphere and in astrophysical plasmas throughout the universe (decadal survey key science goal 4).
10.4.6.1 Science Goals for Solar Probe Plus
The space age has revolutionized understanding of the Sun and heliosphere, but after more than a half-century two of the most fundamental questions in heliophysics remain unanswered: Why is the solar corona so much hotter than the photosphere? How is the solar wind accelerated? Those questions are critical for understanding our solar system, determining the effects of solar inputs on the dynamic heliosphere, and developing predictive space weather capabilities. Achieving closure requires direct samples of electromagnetic fields, plasma, and energetic particles in the solar corona and across the Alfvénic transition between the corona and solar wind. Recognizing the urgency of such observations, the 2003
TABLE 10.1 Illustration of Contributions of Planned and Proposed Missions and Ground-Based Facilities to SHP Panel Goals and Actions and How the Proposed SHP Panel Program Addresses the Four Decadal Survey Science Goals Discussed in Chapter 1
|SHP Panel Science Goals and Actionsa||Missions and Their Contributions|
|Strategic Missions||Opportunity||In Development||Ground-Based|
|Dynamo and Activity Cycle||1a. Polar mass flows, solar cycle||o||o||+||o||++||o||o||+|
|1b. Deep mass flows, dynamo||++||o||++||o||o||o|
|1c. Small-scale fields, Large-scale, variability||+||+||+||+||o||o||++||++||++|
|Sun’s Magnetic Field to Dynamic Atmosphere||2a. Chromosphere dynamics||+||o||++||++||o||o||++||++||++|
|2b. Magnetic free energy||o||o||+||++||+||+||o||++||++||++|
|2c. Corona thermal structure||+||+||++||+||++||++||+||++||+|
|2d. Solar wind origin dynamics and structure||+||+||+||+||o||++||+||+||++||+|
|Magnetic Energy Storage and Release||3a. Eruptive events energy/acceleration||++||++||++||++||o||++||++||+||++||+|
|3b. Impulsive SEP mechanisms, role||++||+||++||+||++||++||o||++||o|
|3c. Suprathermal particle origin, variability||+||++||+||+||++||++||+|
|3d. Solar events forecasting||++||o||++||++||+||+||o||o||+||+|
|Interaction with Galactic Neighborhood||4a. Heliospheric boundary evolution||o||++||o||o|
|4b. Anomalous cosmic-ray origin||o||++||o|
|4c. Heliopause and interstellar medium||++|
|Decadal Survey Key Science Goals|
|1. Determine the origins of the Sun’s magnetic activity and predict the variations in the space environment||++||o||++||++||++||++||++||++||++||++|
|2. Understand the dynamics and coupling of Earth’s magnetosphere, ionosphere, and atmosphere and their response to solar and terrestrial inputs||o||+||o||o||o||o||o||o||o||o|
|3. Determine the interaction of the Sun with the solar system and the interstellar medium||o||++||o||o||o||o||o||o||o||o|
|4. Discover and characterize fundamental processes that occur both within the heliosphere and throughout the universe||++||++||++||++||++||++||++||++||++||++|
KEY: o, supporting role; +, significant contribution; ++, unique to transformative.
a Note that the SHP panel goals and actions are represented in shorthand form with key words. For the complete missions, see Box 10.1.
decadal survey recommended implementation of a solar probe as a large-class NASA mission.7 SPP, started in 2009, will begin its voyage of discovery in 2018 and serve as a keystone of the strategy for solar and heliospheric science in the coming decade.
The goals of SPP are to determine the structure and dynamics of the Sun’s coronal magnetic field, to understand how the solar corona and wind are heated and accelerated, and to determine what mechanisms accelerate and transport energetic particles. To accomplish those goals, SPP is equipped with a tailored payload for the first near-Sun in situ measurements of solar wind ion and electron thermal plasma, supra-thermal and energetic particles, and DC to high-frequency electromagnetic fields. Remote observations include a large-field-of-view white-light imager to provide global context and a directional radio receiver to locate and track flares and shocks. Multiple Venus encounters will gradually lower perihelion from 35 RS to 9.5 RS, producing more than 1,000 hours inside 20 RS, including substantial time within the Alfvén critical point and providing samples of all solar wind types.
SPP will trace the flow of energy that heats and accelerates the solar corona and solar wind (SHP action 2d). Observations of magnetic-reconnection exhausts, jets, shocks, and plasma properties—including wave-particle coupling, heat flux, and mass flux—will directly indicate how the Sun’s convective motion and magnetic field create its dynamic atmosphere and how magnetic free energy is transmitted from the photosphere to the corona. Measurements will determine the energy budget of the solar wind as it evolves from the corona and thereby detect signatures of heating and dissipation responsible for the high temperature of the outer corona and extended heating of the solar wind. As described in Section 10.3.2, composition measurements that determine ionic charge states would facilitate this study by identifying the various types of wind and constraining their solar origin.
SPP will determine the structure and dynamics of the plasma and magnetic fields at the sources of solar wind. Measurements will reveal the steady-state mapping between photospheric sources and coronal structures and emerging solar wind. Full-sky maps of the suprathermal-electron strahl and pitch-angle distribution will unambiguously identify when the spacecraft is on closed magnetic-field lines rooted at both ends in the corona.
The Sun accelerates high-energy particles in solar flares and at CME-driven shocks, where suprathermal particles from multiple sources are the seed particles. Testing particle-acceleration models at 1 AU is hampered by lack of knowledge of source conditions and by acceleration and transport ambiguities. By closing within the Alfvén point, SPP will survey plasma-field and seed-particle properties in the prime-acceleration region of CME-driven shocks, providing ground truth for acceleration models and revealing the causes of energetic-particle variability needed to improve SEP forecasts (motivation M2 and SHP actions 3a and 3d). SPP is expected to observe directly about 10 strong CME-driven shocks within 20 RS, providing comprehensive shock, turbulence, and seed-particle properties for comparison with accelerated-particle spectra, composition, and pitch-angle distributions.
Near the Sun, impulsive SEP events associated with flares appear as sharp spikes, enabling subminute timing comparisons with flares, jets, coronal waves, and radio bursts. Flare-particle studies will test acceleration and charge- and mass-dependent fractionation models, survey neutron-decay protons and electrons, and discover how flare-accelerated particles escape and are transported in longitude (SHP action 3b). Measuring near-Sun suprathermal-particle properties will provide breakthroughs in understanding of the relative importance of local acceleration and solar sources (SHP action 3c).
Sending a spacecraft into the last unexplored region of the heliosphere will produce transformative results throughout the field of solar and space physics and will illuminate fundamental physical processes that occur in stellar atmospheres and energetic astrophysical objects across the universe (motivation M3).
7 NRC, The Sun to the Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics, 2003.
The SHP panel gives its unqualified endorsement to the SPP mission as long as it satisfies cost and schedule guidelines.8
10.4.6.2 Science Goals for Solar Orbiter
In October 2011, ESA’s Science Programme Committee unanimously selected Solar Orbiter as the Cosmic Vision M1 mission, and it was scheduled for launch in 2017. Solar Orbiter will investigate links between the solar surface, corona, and inner heliosphere from as close as 62 RS by using a comprehensive payload that combines remote-sensing and in situ measurements. NASA’s LWS program will provide the launch vehicle and critical science instruments and investigations. Two instruments that had been descoped by NASA were restored with ESA funding, and so the complete payload will fly as originally planned. When close to the Sun, Solar Orbiter will observe emissions, solar wind, and energetic particles from a single area for much longer than is possible from 1 AU and will provide improved insight into the evolution of sunspots, active regions, coronal holes, and other solar features and phenomena. Solar Orbiter’s high spatial and time-resolution observations close to the Sun and its long observations during near corotation with the Sun will probe key questions for understanding the formation of the heliosphere and the generation of space weather events:
• How and where do the solar wind plasma and magnetic field originate in the corona (SHP action 2d)?
• How do solar transients drive heliospheric variability (SHP actions 2d and 3a)?
• How do solar eruptions produce energetic-particle radiation that fills the heliosphere (SHP actions 3a-c)?
• How does the solar dynamo work and drive connections between the Sun and heliosphere (SHP action 1b)?
A unique aspect of the mission occurs when the spacecraft’s orbital plane is increased to about 35° solar latitude, permitting definitive measurements of polar magnetic fields and high-quality observations of solar oscillations in the polar region and thereby supplying a missing link in observations of solar global-circulation patterns (SHP action 1a). Solar Orbiter and SPP observations will overlap in time, permitting many opportunities for coordinated inner-heliosphere measurements that will greatly increase the science return from both missions. Solar Orbiter out-of-ecliptic measurements contemporaneous with near-ecliptic measurements will provide unprecedented insights into the evolving three-dimensional inner heliosphere and outer corona. For example, the remotely observed polar magnetic field combined with in situ observations by SPP will provide tests of the magnetic flux transport model (SHP action 1a).
Data return using U.S. tracking assets to provide enhanced temporal coverage for helioseismology and other uses would provide improved measurements of the solar interior, including such aspects as variations near the bottom of the convection zone and meridional flows, which are important for understanding the generation of solar activity. The scientific return of Solar Orbiter would also be greatly enhanced by providing postlaunch funding opportunities for investigations that would not directly support U.S. instruments but would have important involvement of U.S. investigators.
The SHP panel strongly endorses NASA’s highly leveraged participation in this mission.
8 Solar Probe Plus successfully completed its mission design review in November 2011 and proceeded into preliminary design. As this report went to press, it was scheduled for launch in July 2018.
10.4.6.3 Science Goals for the Interface Region Imaging Spectrograph
IRIS is a small Explorer scheduled for launch in June 2013 for a 2-year prime mission. The IRIS science goals focus on three themes of broad importance in solar and plasma physics, space weather, and astrophysics, aiming to understand how internal convective flows power atmospheric activity:
• Which types of nonthermal energy dominate in the chromosphere and beyond (SHP action 2a)?
• How does the chromosphere regulate mass and energy supply to the corona and heliosphere (SHP actions 2a and 2b)?
• How do magnetic flux and matter rise through the lower atmosphere, and what role does flux emergence play in flares and mass ejections (SHP actions 2a, 2b, and 3a)?
10.4.6.4 Goals for the Heliophysics Systems Observatory
The Heliophysics Systems Observatory (Table 10.2) will provide unique opportunities to observe solar and interplanetary activity and heliosphere-interstellar interactions evolve over a new solar cycle.9 Since February 2011 and continuing through 2019, STEREO and near-Earth observatories (SDO, Hinode, SOHO, and ground-based facilities) have combined to provide a 360° view of the Sun. This total-Sun view can observe far-side active regions emerge and develop and can observe far-side flares and CMEs that often cause near-Earth solar-particle events. The solar origins of Earth-directed CMEs and flares will be observed with ultra-high precision by SDO, which delivers continuous measurements of the photospheric vector magnetic field and of coronal XUV structure. In addition, STEREO and near-Earth spacecraft (ACE, Wind, SOHO, and GOES) now provide broad longitudinal coverage of solar wind, pickup ions, CMEs, shocks, SEPs, and radio bursts, enabling greatly improved nowcasts and forecasts of the interplanetary environment. Simultaneously, the Voyagers traverse the heliosheath, and IBEX maps the outer heliosphere and samples interstellar flows.
An important goal enabled by the HSO is the comprehensive study of solar activity and explosive events. SOHO, ACE, and GOES studies show that about 10 percent of the CME kinetic energy often goes into accelerated particles. Uncertainties arise from limitations of knowledge of CME geometries and single-point sampling of particle intensities. STEREO and near-Earth spacecraft have enabled multipoint observations of cycle-24 SEP events, which indicate surprisingly broad longitudinal SEP distributions. Combined multipoint in situ and stereoscopic solar and CME studies by STEREO, SOHO, and SDO (to be augmented with radial coverage by Solar Orbiter and SPP) will provide more precise measurements of CME evolution, shock-acceleration efficiency, and its controlling properties (SHP actions 3a-c).
On a related note, recent SDO observations show near-simultaneous closely connected “sympathetic” eruptions over broad regions of the Sun (§10.3.3), which complicate eruption forecasts from active regions that may, somehow, be triggered remotely. Coordinated observations by SDO, STEREO, and other 1-AU resources during 2013-2019 (with 360° solar viewing) may reveal the nature of those connections.
The launch of the Fermi astrophysical observatory supplements RHESSI solar gamma-ray observations of eruptive events with extended temporal and energy coverage (to about 300 MeV). Combining those with STEREO and near-Earth SEP coverage can determine the escape efficiency of flare-accelerated particles from large solar eruptions (SHP action 3b).
9 J.G. Luhmann et al., Extended Missions: Engines of Heliophysics System Science, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 167; C.W. Smith et al., The Case for Continued, Multi-Point Measurements in Space Science, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 246; M.H. Israel et al., The Effect of the Heliosphere on Galactic and Anomalous Cosmic Rays, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 116.
TABLE 10.2 Solar and Heliospheric Space Missions
|Mission||Description||SHP Major Science Goals|
|Advanced Composition Explorer (ACE)||Studies elemental and isotopic composition of solar wind, solar energetic particles, and cosmic rays; provides real-time solar wind and magnetic-field data from L1||2, 3, 4|
|Geostationary Operational Environmental Satellites (GOES)||National Oceanic and Atmospheric Administration meteorologic satellites that provide real-time solar X-ray, solar energetic-particle, magnetic-field, and X-ray imaging data||3|
|Hinode||JAXA-led mission that measures the full solar vector magnetic field and coordinated optical, X-ray, and EUV images||1, 2, 3|
|Interstellar Boundary Explorer (IBEX)||Provides ENA all-sky images of heliospheric boundary and measures interstellar H, He, O, and Ne neutral gas at 1 AU||4|
|Ramaty High Energy Solar Spectrographic Imager (RHESSI)||Explorer mission that provides spatial and time-resolved X-ray and gamma-ray images of solar flares||2, 3|
|Solar Dynamics Observatory (SDO)||Provides full-disk Dopplergrams, vector magnetography, UV and EUV images, and EUV irradiance and spectra at high cadence||1, 2, 3|
|Solar Mass Ejection Imager (SMEI)||Multiagency mission led by the U.S. Air Force with an all-sky camera that images the corona and CMEs out to more than 1 AU||2, 3|
|Solar and Heliospheric Observatory (SOHO)||ESA-NASA mission providing solar wind and solar energetic-particle data from L1; can act as a backup for magnetograms and UV coronal images||1, 2, 3|
|Solar Terrestrial Relations Observatory (STEREO)||Provides coronagraph images, EUV, solar wind, interplanetary magnetic field, radio, and solar-particle coverage at increasing longitudinal separation from Earth||2, 3, 4|
|Wind||Provides solar wind, magnetic-field, plasma-wave, radio-burst, solar-particle, and anomalous cosmic-ray data from L1||3, 4|
|Voyager Interstellar Mission||The Voyagers provide magnetic-field, plasma, radio, suprathermal, anomalous, and galactic cosmic-ray data in the heliosheath; one or both may cross the heliopause||4|
Another key objective for the HSO is the study of the prolonged solar minimum and the variable heliosphere. During the long solar minimum of 2008-2009, many solar and interplanetary measures reached extremes for the space age, including a record-low interplanetary magnetic field (IMF) strength and solar wind dynamic pressure, with reduced solar wind He/H ratios and freeze-in temperatures. The weakened solar wind and IMF can be related to continuing high cosmic-ray intensities (see Figure 10.17) and to a smaller heliosphere.
At the same time, the Sun’s polar magnetic field has declined substantially, and the solar dipole is less pronounced. Sunspots are apparently weakening, and average CME mass is reduced. Those and other observations have prompted suggestions that we may be entering another Maunder minimum or at least a lower solar-activity level than seen for about 100 years. Researchers thus have a unique opportunity to track solar and interplanetary phenomena (SHP actions 1a-c) with the most powerful instrumentation of the space age while the Sun is apparently undergoing dramatic changes and providing clues to its past and future behavior. In addition, with 1-AU spacecraft currently spread in longitude, and by 2019 spread to near the heliopause, there is a unique opportunity to study how the heliosphere shields against cosmic rays in response to the enigmatic behavior of the Sun (SHP actions 4a and c). Can we afford to wait for opportunities like this to return?
10.4.7.1 Science Goals for the Advanced Technology Solar Telescope
The Advanced Technology Solar Telescope (ATST) is a ground-based, 4-m-aperture solar telescope whose first observations are expected in 2016. By far the largest optical solar telescope in the world, ATST will provide revolutionary observational resolution as small as 30 km. Comparing observations on this scale with equally resolved numerical models will transform physical understanding of many solar features from informed speculation to hard science. ATST will furnish enough intensity to feed a large array of sensitive and powerful instruments. As a general-purpose community facility, ATST will address a wide variety of ever-changing science goals over its decades-long lifetime.10
Within the photosphere, nearly all the Sun’s magnetic flux is in the form of small, dynamic elements. Constantly energized by convection, this magnetic sea directs upward flows of mass and energy to create the chromosphere, corona, and solar wind. Studying that fundamental process is a major initial ATST science goal. The key observations will include dynamic magnetic and velocity field measurements of small magnetic features at several heights in the solar atmosphere. Small magnetic features may contribute to the total solar irradiance. ATST observations will be used to define the spectral emission characteristics of a sample of those features as an important contribution to understanding TSI variations. Revolutionary observations will be made of larger magnetic features, such as sunspots and active regions, and of transient drivers of space weather, such as flares and erupting prominences.
ATST will also provide high-resolution observations in the infrared portion of the solar spectrum where molecular signatures appear. That capability will be used to study indications of extraordinarily cool molecular clouds in the upper photosphere. In addition, ATST can be operated as a coronagraph. The primary goal of early coronal observations will be to characterize the magnetic and changing velocity fields of coronal features above both active and quiet regions on time and spatial scales that have heretofore been beyond reach. The relative importance of heating of the corona by dissipation of wave motions excited from below will be a specific research target.
10.4.7.2 Science Goals for Ground-Based Solar Research
Important science results of the past decade were accomplished at ground-based facilities.11 A summary of the principal U.S. ground-based observatories and their observational emphasis is given in Table 10.3. Compared with space missions, these facilities can be far larger, more flexible and exploratory, and longer-lived. Accordingly, emphasis is on achieving high spatial resolution (as with the NST and the ATST), making unique measurements of physical processes at long wavelengths (as with E-OVSA and FASR), and collecting sufficient light flux to make high-time-resolution, high-precision measurements of
10 S.L. Keil et al., Science and Operation of the Advanced Technology Solar Telescope, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 130; S.L. Keil et al., Generation, Evolution, and Destruction of Solar Magnetic Fields, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 131; T. Ayers and D. Longcope, Ground-based Solar Physics in the Era of Space Astronomy, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 3.
11 T. Ayres and D. Longcope, Ground-based Solar Physics in the Era of Space Astronomy, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 3; A.A. Pevtsov, Current and Future State of Ground-based Solar Physics in the U.S., white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 218; K.P. Reardon et al., Approaches to Optimize Scientific Productivity of Ground-Based Solar Telescopes, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 224; S. McIntosh et al., The Solar Chromosphere: The Inner Frontier of the Heliospheric System, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 193.
TABLE 10.3 Dedicated Ground-Based Daily and General Research Solar Facilities
|Helioseismic far-side images||G|
|Photospheric longitudinal B fields||S||G||S|
|Photospheric vector B fields||S|
|Photospheric spectral line profiles||S|
|Photospheric sunspot B fields||S|
|Chromospheric longitudinal B fields||S||S|
|Chromospheric spectral line profiles||S|
|Chromospheric radio emission||G|
|Coronal Thomson scatter images||S|
|Coronal emission line images||S||S|
|Coronal radio emission||G||Y|
|General solar research||Y||Y||Y||Y|
NOTE: G, global network; S, single instrument; Y, yes; AFRL, Air Force Research Laboratory; AFWA, Air Force Weather Agency; BBSO, Big Bear Solar Observatory; MLSO, Mauna Loa Solar Observatory; MSO, Mees Solar Observatory; MWO, Mt. Wilson Observatory; NSO, National Solar Observatory; OVSA, Owens Valley Solar Array; SFO, San Fernando Observatory; WSO, Wilcox Solar Observatory.
magnetic and velocity fields; long-term synoptic observations; and novel frontier observations of various kinds (as with COSMO).
An important science goal is to continue comparison of highly resolved observations with numerical models to define the physical nature of photospheric features, such as sunspots, faculae, and cool molecular clouds. Another goal is to understand the physical behavior of magnetic fields in the chromosphere and corona—on both small and large scales—to support study of the flow, storage, and eruption of energy and mass in these poorly understood regions (motivation M1). The recent, mostly unexpected, behavior of the solar cycle and the inability to predict its course make synoptic studies of the magnetic field and of surface and interior mass flows that are related to the solar dynamo a high-priority goal (motivation M3). Ground-based observations are increasingly used in near-real-time data-driven models of the heliosphere and space weather. Two goals are to improve the quality of the measurements and to extend them upward into the chromosphere and corona (motivation M2).
A new science goal is to develop strategies and instrumentation to observe the earliest stages of an emerging active region at high spatial and temporal resolution. That cannot be done now except as a matter of blind-targeting good luck.
This section describes the imperatives proposed by the SHP panel for three groups: NASA, NSF, and multiagency. All imperatives are based in part on white-paper input. Of 288 white papers submitted by the community (for a list of titles, see Appendix I), about 150 were relevant to solar and heliospheric physics. Each was reviewed by at least two SHP panel members and many were also brought to the attention of the interdisciplinary working groups. The SHP panel is grateful to the community for its hard work,
creative ideas and suggestions, and insight into issues that affect the health of the solar and heliospheric physics discipline.
The IRIS, SPP, and Solar Orbiter missions are in development. The new NASA mission concepts and other initiatives proposed here are predicated on the assumption that NASA will complete the development and launch of those three missions, each of which will make critical progress toward achieving SHP science goals 1-3 outlined above (§10.1).
Sections 10.5.2.2-10.5.2.4 review three high-priority mission concepts that the SHP panel recommends for consideration during the coming decade. Two of the concepts—IMAP (§10.5.2.2) and SEE (§10.5.2.4)— underwent an independent cost and technical evaluation (CATE) that is described in Appendix E and briefly summarized in §10.5.2.1). Solar-C (§10.5.2.3) is an opportunity for NASA to participate in an international mission developed under the leadership of Japan. It is discussed below, but as an international mission it could not be reviewed in the same manner as other concepts and, in particular, it could not undergo an independent cost and technical review.
Sections 10.5.2.5-10.5.2.7 discuss one high-priority concept (L5) that should be considered for the following decade and two concepts—the Solar Polar Imager and the Interstellar Probe—that address high-priority goals but require new propulsion technology and possibly other new technologies.
10.5.2.1 Solar and Heliospheric Physics Panel Participation in the Cost and Technical Evaluation Process
At the SHP panel’s first meeting in November 2010, it reviewed 30 white papers (for a list of titles, see Appendix I) describing future mission concepts, including potential strategic and Explorer missions for this decade and beyond. The SHP panel’s second meeting included 19 invited talks on mission concepts, ground-based facilities, theory, modeling and data centers, and new technology. The panel selected four candidates for consideration by the steering committee for submission to the CATE process (SEE, IMAP, L5, and Reconnection and Microscale [RAM]) and reviewed two concepts for Solar-C.
The survey committee decided to include all four SHP-endorsed concepts in the CATE process; it also gave the SHP panel and the other panels uniform guidance that included a request to focus mission concepts around key objectives so as to minimize the required payload and cost. “Captains” later documented science objectives, measurement requirements, and mission and instrument concepts. Those “pre-CATE” activities resulted in schematic spacecraft and mission designs, refined science objectives, identification of potential risks, and estimated costs. Using that information, the survey committee selected IMAP and SEE for the full CATE process. The panels and the survey committee used the CATE results for final priority setting.
The IMAP, SEE, and L5 concepts are described below, as is an opportunity for NASA to participate in the Solar-C mission, which addresses objectives similar to those of the RAM concept.
10.5.2.2 Interstellar Mapping and Acceleration Probe
Our heliosphere, its history, and its future in the galaxy are key to understanding conditions on our evolving planet and its habitability over time. By exploring our global heliosphere and interactions, we
develop key physical knowledge of the interstellar interactions that influence our home system in its current state, the history and destiny of our solar system, and the habitability of exoplanetary star systems.
Outer heliospheric science is an exciting, rapidly developing field because of groundbreaking all-sky images of the heliospheric boundaries based on energetic neutral atoms (ENAs) from the IBEX mission and Cassini-INCA in concert with dual in situ heliosheath observations from the Voyagers and IBEX measurements of interstellar neutral H, He, O, and Ne flow.
The surprising ENA “ribbon” (§10.3.4) demonstrates the importance of the interstellar magnetic field in the interaction of the heliosphere with our galactic neighborhood. The physical processes that form ENA spectra and the ribbon are hotly debated because of complex interactions between solar wind, pickup ions (PUIs), and suprathermal particles. The big picture provided by IBEX, complemented by Voyager observations, shows that the asymmetry of the heliosphere (§10.3.4) is shaped by the surrounding galactic magnetic field and that the physical processes that control the interaction exist on relatively small spatial and temporal scales (months). IMAP provides the next “quantum leap” forward in understanding the heliosphere through substantial improvements in spatial, temporal, and energy resolution12 and much broader energy coverage than that of IBEX.
Observations from many spacecraft in the HSO contribute dramatically to understanding of SEP events, of the importance of suprathermal ions for efficient energization (§10.3.3), of the sources and evolution of solar wind (§10.3.2), of solar wind and SEP inputs into geospace, and of the evolution of the solar-heliospheric magnetic field (§10.3.1). Those observable phenomena are controlled by myriad complex and poorly understood physical effects that act on distinct particle populations (Figure 10.20). IMAP combines highly sensitive PUI and suprathermal-ion sensors to provide the species, spectral coverage, and temporal resolution to associate emerging suprathermal tails with interplanetary structures and physical processes (SHP action 3c).
IMAP orbits the inner Lagrangian point (L1) with comprehensive and highly sophisticated instruments to make the key observations that answer these fundamental questions:
• What is the spatiotemporal evolution of heliospheric boundary interactions?
• What is the nature of the heliopause and the interaction of the solar and interstellar magnetic fields?
• What are the composition and physical properties of the surrounding interstellar medium?
• How are particles injected into acceleration, and what mechanisms energize them throughout the heliosphere and heliosheath?
• What are the time-varying physical inputs at L1 into the Earth system?
The mission’s heliospheric focus highlights the importance of making IMAP ENA maps and maps of ACR and CGR particles concurrently with in situ Voyager measurements of the heliospheric boundary region (motivation M3). IMAP enables understanding of particle acceleration through unprecedented collection power and time-resolved measurements of suprathermal ions that originate in the solar wind, interstellar medium, and inner heliosphere; enables environmental monitoring that is critical for effective background evaluation and removal from ENA maps and interpretation of PUI distributions; enables comprehensive interplanetary monitoring in support of geospace interaction studies; and enables space weather observations at the ideal location, L1 (SHP action 3d and motivation M2).
Answering the fundamental IMAP questions requires:
• High-resolution mapping and time evolution of heliospheric boundaries;
12 D.J. McComas et al., Interstellar Mapping Probe (IMAP) Mission Concept: Illuminating the Dark Boundaries at the Edge of Our Solar System, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 188.
FIGURE 10.20 Four distinguishable components in the measured solar wind distribution (from Gloeckler et al., 2012): bulk solar wind (red); much hotter halo solar wind (blue); interstellar pickup ion H+, observable at 1 AU during the deep solar minimum (green); and suprathermal tail with a spectrum approximated by a power law with an exponential rollover. (A -5 slope in phase space density corresponds to a differential intensity with a slope of -1.5; see Section 10.3.4.) SOURCE: Adapted from G. Gloeckler, L.A. Fisk, G.M. Mason, E.C. Roelof, and E.C. Stone, Analysis of suprathermal tails using hourly averaged proton velocity distributions at 1 AU, pp. 136-143 in Physics of the Heliosphere: A 10 Year Retrospective, Proceedings of the 10th Annual International Astrophysics Conference (J. Heerikhuisen, G. Li, N. Pogorelov, and G. Zank, eds.), Volume 1436, American Institute of Physics, Melville, N.Y., doi:10.1063/1.4723601, 2012.
• Properties of interstellar neutral gas flow and its composition for H (including isotopes), He, O, and Ne (also to address big bang cosmology with the first in situ D/H observations) and properties of the outer heliosheath;
• PUI composition (implications for big bang cosmology and nucleosynthesis with a dedicated PUI instrument: He3/He4 and Ne22/Ne20 with better than 5 percent accuracy);
• Seed populations of energetic particles with high time resolution (several minutes) (Figure 10.21);
FIGURE 10.21 Suprathermal particles injected at the Sun provide seed populations for efficient energization at widely varied locations. SOURCE: G.M. Mason, J.E. Mazur, and J.R. Dwyer, 3He enhancements in large solar energetic particle events, Astrophysical Journal 525:L133-L136, doi:10.1086/312349, 1999. Reproduced by permission of the AAS.
• Underlying time variations of ubiquitous suprathermal ions;
• SEP composition, injection, and acceleration;
• Suprathermal and energetic-particle transport;
• ACR/GCR modulation and evolution with time; and
• L1 environmental monitoring and solar wind input for magnetospheric and atmospheric science.
IMAP spacecraft and instrument implementation is based largely on ACE with ENA imaging infused from IBEX. IMAP is a Sun-pointed spinner, with spin-axis readjustment every few days to provide all-sky maps every 6 months. The L1 placement avoids magnetospheric ENA backgrounds and allows continuous interplanetary observations. Mission goals are achieved with a 2-year baseline, including transit to L1, with possible extension to longer operation. IMAP combines the following measurement capabilities, for which no further development effort is required:
• High-resolution ENA maps. Two ENA cameras produce new ENA observations of the heliospheric boundary over an extended energy range (0.3-20 keV, 3-200 keV) with substantially improved sensitivity (-80 times the combined sensitivity and duty cycle of IBEX-Hi and CASSINI/INCA), spatial (10 times the IBEX-Hi angular resolution), and energy and time resolution (for example, oversampling of polar regions with few-day time resolution) compared with prior observations.
• High-resolution and high-sensitivity ISM flow collection. An ISM neutral atom camera and the first dedicated PUI sensor will take coordinated high-sensitivity observations of the interstellar gas flow through the inner solar system. The ISM neutral camera provides ISM flow observations of H, D, He, O, and Ne at 5-1,000 eV with pointing knowledge of 0.05° and over 10 times the combined sensitivity or duty cycle of IBEX-Lo, also extending the ENA maps to <0.3 keV. The PUI sensor provides distributions of interstellar H+, 3He+, 4He+, N+, O+, 20Ne+, 22Ne+, and Ar+ and inner-source C+, O+, Mg+, and Si+ over 100 eV to 100 keV/e with a combined sensitivity or duty cycle 100 times that of SWICS, also providing solar wind heavy-ion composition.
• High-cadence suprathermal-ion observations. Overlapping with the PUI sensor, a suprathermal-ion sensor provides composition (0.03-5 MeV/nuc) and charge state (0.03-1 MeV/e) for H through ultra-heavy ions (1-min cadence for H and He).
• Solar wind and interplanetary monitoring suite. This suite mitigates backgrounds for high-sensitivity ENA observations and provides societally important real-time solar wind and cosmic-ray monitoring. It measures solar wind ions (0.1-20 keV/e) and electrons (0.005-2 keV) every 15 s, the magnetic field at 16 Hz, and SEP, anomalous cosmic-ray, and galactic cosmic-ray electrons and ions (H-Fe) over 2- to 200-MeV/ nuc.
Solar-C is a Japan-led mission expected to include substantial contributions from the United States and Europe.13 It builds on the highly successful Yohkoh and Hinode collaborations with the United States’ most reliable partner. As with Yohkoh and Hinode, Japan will provide the satellite and launch. Almost all NASA funding would go to the U.S. science community for state-of-the-art instrumentation and data analysis. Hence, Solar-C presents an important opportunity to leverage NASA science funding.
The science objectives of Solar-C are to determine:
• How the energy that sustains the Sun’s atmosphere is created on small scales and transported into the large-scale corona and solar wind;
• How magnetic energy is dissipated in astrophysical plasmas; and
• How small-scale physical processes initiate large-scale dynamic phenomena, such as CMEs and flares, which drive space weather.
Achieving those objectives is a prerequisite for meeting SHP panel science goals 2 and 3. Solar-C is central to the science strategy for the next decade; therefore, the panel strongly endorses U.S. participation in the mission. As with Hinode, the data should be open to the full U.S. science community. Furthermore, a competitive Solar-C guest-investigator program, overseen by NASA, that follows the guidelines of the general guest-investigator program initiative would achieve maximum science benefit (§10.5.3.4).
To meet its three objectives, Solar-C will obtain highly precise spectroscopic and polarimetric measurements designed to determine the full-vector magnetic field accurately, especially in the chromosphere, and high-throughput measurements designed to resolve the plasma dynamics. Furthermore, spectroscopic measurements that seamlessly cover each temperature domain of the solar atmosphere—the photosphere, lower chromosphere, upper chromosphere, transition region, inner corona, and high-temperature flare— will be obtained to improve understanding of the entire chain of energy transport and dissipation. Finally, high-spatial-resolution measurements will be obtained for resolving elementary physical processes.
Solar-C can meet its measurement strategy with three strawman instruments designed to deliver an order-of-magnitude improvement over present measurement capabilities:
• A Solar UV-visible-IR telescope that will resolve and measure magnetic fields and gas dynamics in the lower atmosphere—from the photosphere through the upper chromosphere—with a diffraction-limited telescope that has an aperture 1.5 m in diameter.
• An EUV/FUV high-throughput spectrometer that will measure spectral lines in the FUV-EUV region from plasma in the upper chromosphere, transition region, lower corona, and flares simultaneously to
13 G. Doschek et al., The High-Resolution Solar-C International Collaboration: Probing the Coupled Dynamics of the Solar Atmosphere, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 60.
trace energy flow throughout the solar atmosphere and follow the energy released by such processes as magnetic reconnection and instabilities.
• An X-ray imaging (spectroscopic) telescope that will resolve and measure the plasma in the hot corona to improve our understanding of its elemental structure, origins, and dynamics.
The strawman instruments outlined above are only for the purposes of planning and costing the mission. Concrete plans for the instruments and for the roles of the international partners are urgently needed; consequently, it is a high priority of the SHP panel that, as with Hinode, NASA and its partners form a Science and Technology Definition Team for Solar-C as soon as possible. Although the NASA contribution has yet to be decided, the panel expects that NASA contributions would involve the most technically challenging elements, such as the focal-plane packages (cameras, detectors, and so on), which would afford the U.S. science community an opportunity to make critical advances in remote-sensing capabilities. The total cost to NASA through Phase E should be capped at $250 million.
Solar-C presents a unique opportunity for solar and space physics to make flagship-level science advances for the cost of an Explorer.
10.5.2.4 Solar Eruptive Events Mission
Major solar eruptive events, consisting of both large flares and fast massive CMEs, are the most powerful explosions and particle accelerators in the solar system (Figure 10.22). They produce the most extreme space weather, generating SEPs that pose a major radiation hazard for spacecraft and humans, intense photon emissions that disrupt GPS and communications (see Figure 10.13), and storms in the magneto-sphere that can cause power blackouts and disable satellites. Thus, understanding the fundamental physics of solar eruptive events is one of the most important goals of heliophysics.
Observations indicate that the flare energy-release particle-acceleration region is high above the flare X-ray loops (§10.3.3). The acceleration of fast CMEs is synchronized with the flare energy release, and this suggests that magnetic reconnection in the current sheet behind the CME both generates the flare and accelerates the CME. The CME-driven shock then accelerates SEPs. Despite much progress in developing this picture, fundamental physics questions remain:
• How is magnetic energy suddenly released to produce both a flare and a CME?
• How are CMEs accelerated to high speeds?
• How can flares accelerate electrons and ions so efficiently?
• How are escaping SEPs accelerated to such high energies?
• How can the magnetic energy for major solar eruptive events be accumulated in the corona?
To make major breakthroughs, a single-spacecraft SEE14 mission in low Earth orbit, with powerful new instruments and a roughly 10-m boom for optics and occulter (Figure 10.23), will provide, for the first time, the following detailed measurements of accelerated electrons and ions plus ambient plasma conditions in the energy-release particle-acceleration regions (SHP action 3a):
• Focusing Optics X-ray Spectroscopic Imager (FOXSI). Provides HXR (about 2 to over about 80 keV) imaging (about 7 arcsec) spectroscopy (less than about 1-keV full-width half-maximum [FWHM]) of accelerated electrons and hot thermal plasmas in the high-coronal-energy-release particle-acceleration region
14 R.P. Lin et al., Solar Eruptive Events (SEE) 2020 Mission Concept, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 162.
FIGURE 10.22 Images and energy budget of the major components of a SEE mission. This comprehensive picture required input from instruments on five spacecraft. SOURCE: A.G. Emslie, H. Kucharek, B.R. Dennis, N. Gopalswamy, G.D. Holman, G.H. Share, A. Vourlidas, T.G. Forbes, P.T. Gallagher, G.M. Mason, T.R. Metcalf, R.A. Mewaldt, R.J. Murphy, R.A. Schwartz, and T.H. Zurbuchen, Energy partition in two solar flare/CME events, Journal of Geophysical Research 109:A10104, doi:10.1029/2004JA010571, 2004. Copyright 2004 American Geophysical Union. Reproduced by permission of American Geophysical Union.
for most flares, even in the presence of intense footpoint emission, with about 30-100 times RHESSI’s dynamic range and sensitivity. FOXSI will also detect accelerated electrons in impulsive SEP events and type III radio bursts in the corona, as well as nanoflares.
• Energetic Neutral Atom Spectroscopic Imager (ENASI). Provides ENA imaging (about 0.1 R) spectroscopy of about 4- keV to about 30-MeV SEPs accelerated by CME-driven shocks at about 1.5-10 RS
FIGURE 10.23 Schematic spacecraft and instrument accommodation for the minimum SEE mission SOURCE: Courtesy of the Aerospace Corporation.
altitudes, including the SEP seed population down to about 4 keV, with about 1,000 times STEREO’s sensitivity. ENASI should also detect ENAs from ions accelerated in impulsive SEP events in the corona.
• Gamma-Ray Imaging Spectrometer (GRIS). Provides imaging of a few to hundreds of millions of electron-volt flare-accelerated ions through their gamma-ray line emissions with sufficient spatial resolution (about 7 arcsec), spectral resolution (a few thousand-electron-volt FWHM), and sensitivity to follow the evolution of the ion footpoints, even for normal-size flares.
• UV-EUV Imaging Spectrometer (EUVIS). Provides measurements of the ambient density, electron and ion temperatures, ionization states, composition, and flow and turbulent velocities in the flare energy-release particle-acceleration region with high-cadence imaging (less than about 10 arcsec) spectroscopy (l/Dl over 3,000) up to about 1.2 RS. EUVIS should also detect downward-going protons accelerated over 10 keV through their redshifted Lyman alpha emission.
• UV Coronagraph Spectrometer (UVCS II). Provides the same measurements as EUVIS but from about 1.2 to 10 RS.
• White-Light Coronagraph (WLC). Provides imaging of CME structure and evolution from 1.5 to 15 Rs.
SEE would operate autonomously in a store-and-dump mode (like RHESSI) with a large onboard memory. Some SEE measurements (such as HXR or ENA of near-Sun SEP intensities) may be good precursors of these major eruptions; near-real-time data could be downlinked for space weather warnings.
ATST, FASR, and COSMO can make crucial measurements of coronal magnetic fields in the energy-release particle-acceleration regions. SPP and Solar Orbiter would provide ideal complementary in situ SEP/CME measurements close to the Sun and solar imaging (coronagraph, heliospheric imager, and HXR).
The minimum SEE mission evaluated by the CATE process included only FOXSI, ENASI, and UVCSII, launched on a Taurus 3210. A EUVIS type of instrument and a WLC are part of Solar-C and Solar Orbiter, respectively. GRIS requires more mass and power than the other instruments combined; flying it on ultra-long-duration balloon flights at solar maximum should be investigated.
A two-instrument SEE mission (FOXSI and ENASI) was deemed by the CATE process to fit in the mid-scale line (less than $480 million), and it provides tremendous new science. One or more of these new instruments, of appropriate size, would also be appropriate in an Explorer mission.
Regarding development status, a FOXSI instrument is scheduled for rocket flight in early 2012 and the balloon-borne GRIPS (Gamma-Ray Imager/Polarimeter for Solar flares) payload is being developed for a 2012 flight. UVCSII is based on proven SOHO UVCS technology. The ENASI instrument uses silicon semiconductor detectors that have flown successfully on STEREO/IMPACT LET and STE instruments and RHESSI modulation-grid imaging methods. EUVIS is based on the Hinode technology, and UVCSII is based on UVCS on SOHO. A 10-m extendable boom will be flown on the NuSTAR SMEX mission (planned for launch in 2012).15
10.5.2.5 L5 Mission Concept
The L5 mission concept would place a spacecraft carrying imaging and in situ instruments in about a 1-AU orbit near the L5 Lagrangian point (Figure 10.24).16 From that location, the mission could make major advances in helioseismology by probing for longitudinal variations in tachocline magnetic fields, observe emerging active regions before they affect Earth, study CME evolution with stereoimaging and in situ data, and make major advances in space weather forecasting. The spacecraft and payload could rely on STEREO heritage. A Doppler magnetograph and UV spectrograph would be added.
An important science objective for L5 is to address the question, How does the solar dynamo drive magnetic activity on the surface? Global helioseismology has had remarkable success in revealing the Sun’s internal radial structure and internal rotation. However, global helioseismology does not resolve longitudinal variations of the solar interior. Local helioseismic techniques (such as time-distance helioseismology) provide longitudinal information, but only in the upper third of the convection zone if viewed from a single vantage point.
Calculations predict longitudinal variations as signatures of the magnetic field at the tachocline. Simultaneous observations (Earth + L5) will detect both ends of long, deep, wave ray paths that penetrate to the tachocline (see Figure 10.24). Combining L5 and near-Earth observations will probe variations over a large longitude range. The relatively stable L5-Earth separation and increased solar-surface coverage also enable improved measurements of rotational and meridional flows. Active longitudes and persistent surface “hotspots” of magnetic activity are probably also associated with hotspots in the tachocline region and require longitudinal resolution.
Another science objective concerns the question, Can helioseismology forecast strong flare activity? Recent studies show that the strength and vorticity of subsurface flows around active regions are closely
15 This panel report was completed in late 2012. An update in June 2013 to the information above follows: FOXSI was launched successfully in November 2012 from the White Sands Missile Range in New Mexico; GRIPS is now planned to have its first test flight in September 2014; and NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR) began its 2-year mission on June 13, 2012, aboard a Pegasus XL rocket launched from Kwajalein Atoll in the Marshall Islands.
16 A. Vourlidas et al., Mission to the Sun-Earth L5 Lagrangian point: An Optimal Platform for Heliophysics and Space Weather Research, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 273.
FIGURE 10.24 Left: The five Lagrangian points in the Earth-Sun system include L5, a broad gravitational plateau about 60° east of central meridian, where relatively little energy is needed to maintain an orbit extending about 40° to 90° east of the Earth-Sun line. Right: If travel times along the indicated ray paths are measured, thermal anomalies and flows in the tachocline can be resolved in longitude for the first time. For stereohelioseismology studies, L5 would be better suited than Solar Orbiter because of its continuous duty cycle at a relatively stable location. SOURCE: Left, NASA/WMAP Science Team. Right, Courtesy of S.P. Rajaguru, Indian Institute of Astrophysics, Bangalore, India.
related to flare activity and have the potential to become a forecast tool (see §10.3.1). In addition, there are indications that large active regions may be detected enough before they emerge to provide useful forecasts. L5 can be coupled with GONG/SDO to attempt probing vorticity deeper below the surface. Most important, L5’s additional longitude coverage will allow active-region evolution studies for a much longer period than is now possible.
Instruments at L5 could also contribute to new advances in space weather forecasting. From L5, it is possible to observe and forecast Earth-directed CMEs with high precision, observe emerging and developed active regions about 4 days earlier than from Earth, forecast corotating interaction regions about 4 days before they cause geomagnetic storms, and measure photospheric magnetic fields over about 60° of additional longitude, improving models of coronal magnetic fields rotating toward Earth.
The pre-CATE L5 concept included the following instruments, all with excellent heritage:
• White-light coronagraph images, 2-225 RS;
• EUV images, full-disk, 4 wavelengths;
• Doppler-magnetograph, full-disk;
• Off-limb spectroscopy, 8 wavelengths, 2 and 3.5 RS;
• Hard X-ray imaging spectroscopy, full-disk, 64-150 keV;
• Solar and interplanetary energetic particles, 0.05- to 100-MeV/nuc, and electrons;
• Solar wind ion composition and electrons; and
Two science phases are envisioned: drift to L5 at about 38° per year with continuous collection of science data and orbit around L5, 45°-90° from the Sun-Earth line. A long extended mission is possible.
Goddard Space Flight Center has studied a similar concept called Earth-Affecting Solar Causes Observatory17 (EASCO), featuring about a 2-year low-thrust trajectory to L5 and using solar-electric propulsion, saving more than 200 kg compared with hydrazine.
In summary, the L5 mission concept promises important breakthroughs in both helioseismology and space weather (motivation M2). It would make major advances toward SHP panel science goals 1 and 3, including, in particular, actions 1b and 3d. The panel encourages NASA, NOAA, and the Department of Defense (DOD) to carry out an interagency study of an L5 mission (see §10.5.5.7).
10.5.2.6 Solar Polar Imager Mission
Current understanding of the Sun, its atmosphere, and the heliosphere is severely limited by a lack of good observations of the Sun’s polar regions. The Solar Polar Imager (SPI) mission concept,18 a NASA vision mission with strong international interest, would go into a 0.48-AU circular orbit with 60° inclination to conduct extended (many days per orbit) observations of the polar regions, enabling the determination of polar flows down to the tachocline, where the solar dynamo is thought to originate. The rapid 4-month orbit, combined with in situ and remote-sensing instrumentation, will enable unprecedented studies of the physical connections between the Sun, the solar wind, and SEPs.
Instrumentation could include a Doppler magnetograph, white-light coronagraph, EUV imager, UV spectrograph, TSI monitor, energetic-particle spectrometer, solar wind composition spectrometer, and magnetometer and could provide studies of the polar magnetic field over the solar cycle, the three-dimensional global structure of the corona, and solar wind, energetic-particle, and TSI variations with latitude. Most important, SPI would measure the temporal evolution of time-varying flows, differential rotation, and polar-region meridional circulation down to the tachocline, addressing SHP science goal 1. Finally, SPI would explore how space weather forecasts can benefit from a polar perspective (SHP science goal 3 and motivation M2).
Solar-sail propulsion is proposed to place SPI into its orbit. Recent advances demonstrate that solar sails are technically feasible and effective for maneuvering in the heliosphere. A technology-readiness plan is outlined in Section 10.5.2.8. The SHP panel strongly encourages NASA to develop the propulsion technology needed to launch SPI during the 2023-2033 decade.
10.5.2.7 Interstellar Probe Mission Concept
Recent in situ measurements by the Voyagers, combined with all-sky heliospheric images from IBEX and Cassini, have made outer-heliospheric science one of the most exciting and fastest-developing fields of heliophysics. The measurements have transformed knowledge of the boundaries of the heliosphere. The
17 N. Gopalswamy et al., Earth-Affecting Solar Causes Observatory (EASCO): A New View from Sun-Earth L5, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 99.
18 P. Liewer et al., Solar Polar Imager: Observing Solar Activity from a New Perspective, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 156.
Voyagers are now deep in the heliosheath, and one or both may cross the heliopause in the next decade. Although they have performed spectacularly, the Voyager instruments are 1970s-vintage and, for example, are unable to measure suprathermal heavy ions or interstellar-plasma elemental and ionic charge-state composition. The interstellar probe19 would make comprehensive, state-of-the-art, in situ measurements of plasma and energetic-particle composition, magnetic fields, plasma waves, ionic charge states, energetic neutrals, and dust that are required for understanding the nature of the outer heliosphere and exploring our local galactic environment.
Advanced scientific instrumentation for an interstellar probe does not require new technology, as the principal technical hurdle is propulsion. (Also required are electric power from a low-specific-mass radioactive power source and reliable, sensitive, deep-space Ka-band communications.) Advanced propulsion options, which could be pursued with international cooperation, should aim to reach the heliopause considerably faster than Voyager 1 (3.6 AU/year). Possibilities include solar sails and solar electric propulsion alone or in conjunction with radioisotope electric propulsion.20,21 The panel did not find either the ballistic or the nuclear electric power approach to currently be credible. In summary, to enable achievement of this decadal survey’s key science goals in the coming decades, the SHP panel believes high priority should be given by NASA toward developing the necessary propulsion technology for visionary missions like SPI and interstellar probe.
10.5.2.8 Solar-Sail Propulsion for Heliophysics Missions
Solar sails have long been envisioned as a simple, inexpensive means of propulsion that could provide access to and maintenance of unstable orbits that would otherwise require, if they were possible at all, large and expensive propulsion systems. Solar sails can use solar photons to propel inner-heliosphere spacecraft to high velocities (Δv > 50 km/s) and can provide low-thrust propulsion to maintain missions in non-Keplerian orbits that are not feasible by other means. Solar sails will enable a number of important heliophysics missions, including the Solar Polar Imager (§10.5.2.6), an interstellar probe (§10.5.2.7), and a solar wind monitor several times farther upstream than L1. All indications are that solar-sail propulsion (SSP) is technically feasible and very effective for maneuvering in the heliosphere.22
Recently, the NASA Office of the Chief Technologist (OCT) selected a small sail-technology demonstration mission for implementation in the near future. However, for future missions like the Solar Polar Imager, a critical follow-on step will be flight validation of a full-scale (about 150 × 150-m) SSP system. That could be accomplished by NASA’s Heliophysics Division’s investing about $50 million as “seed money” in the full-scale SSP development effort over the next decade by partnering with the OCT Technology Demonstration Missions program (or other technology program as appropriate). In addition, investing a modest part of the seed money (about 10 percent) to fund grants to NASA centers and universities for solar-sail mission design, trajectory analysis, and so on, would lead to new mission applications for heliophysics exploration.
19 R. McNutt et al., Interstellar Probe, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 195.
20 R. McNutt et al., Interstellar Probe, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 195.
21 L. Johnson et al., Solar Sail Propulsion: Enabling New Capabilities for Heliophysics, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 122.
22 R.P. Lin et al., Expansion of the Heliophysics Explorer Program, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 160; E. Moebius et al., NASA’s Explorer Program as a Vital Element to Further Heliophysics Research, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 205.
Its advocacy for this seed-money allocation emphasizes the high priority that the SHP panel places on developing SSP technologies to enable future expeditions to the inner and outer heliosphere and to the local interstellar medium.
10.5.3.1 Completion of the Development and Launch of the Heliophysics Flight Program Now in Development
SHP Imperative: The highest-priority objectives for heliophysics during the coming decade are to complete the development and launch of the IRIS and SPP missions and to fulfill U.S. contributions to the Solar Orbiter mission.
Justification: These new missions each offer unique opportunities for important breakthroughs in achieving SHP goals outlined in Section 10.1 (see §10.4.6.1-§10.4.6.3). IRIS is a small Explorer that will investigate how internal convective flows power solar-atmospheric activity. SPP will make mankind’s first visit to the solar corona carrying a payload of in situ and remote-sensing instruments to discover how the corona is heated, how the solar wind is accelerated, and how the Sun accelerates particles to high energy. The ESA-NASA Solar Orbiter mission has a remote-sensing and in situ payload that, after a NASA launch, will investigate links between the solar surface, corona, and inner heliosphere from as close as 62 RS and as high as 35° solar latitude. The SHP panel gives its unqualified endorsement to the mission concepts under development in October 2011 as long as they satisfy cost and schedule guidelines.
10.5.3.2 Expansion of the Heliophysics Explorer Program
SHP Imperative: In light of the success of heliophysics Explorers in addressing focused, high-priority science targets in all three disciplines, the SHP panel gives high priority to expanding flight opportunities in the heliophysics Explorer program and adding new cost-effective middle-size launchers to the manifest.23
Justification: The Explorer line contains a mix of small and middle-size Explorers and a stand-alone mission of opportunity (SALMON) component supporting participation in space programs of other agencies and nations. These missions have been highly innovative, extremely successful, and implemented on time and within budget. There is a large supply of new, cutting-edge ideas for Explorer missions in all heliophysics disciplines. Since 2001, 43 heliophysics-related Explorer missions have been proposed; 15 (35 percent) were rated category 1, but only 3 have been implemented. Ramping up the heliophysics Explorer program to about $150 million per year could enable a launch opportunity (Explorer or SALMON) every year.
Currently available (as of May 2012) launch vehicles are not adequate to support middle-size Explorers. Adding cost-effective low-end Atlas, Minotaur, or Falcon-9 launchers to the Explorer manifest would enable future MidEx mission opportunities.
23 R.P. Lin et al., Expansion of the Heliophysics Explorer Program, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 160; E. Moebius et al., NASA’s Explorer Program as a Vital Element to Further Heliophysics Research, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 205.
10.5.3.3 Development of Strategic Missions in the Explorer Mode
SHP Imperative: Given the Explorer program’s excellent record in delivering state-of-the-art science within or below cost and on schedule, the SHP panel gives high priority to extending the Explorer mission-development model to middle-size strategic missions.
Justification: The heliophysics Explorer program has an unmatched record of innovative implementation and high science return on investment while staying within cost and schedule guidelines. Adopting the Explorer model for strategic missions whenever possible implies that missions are competed, led by principal investigators (PIs), and cost-capped. Unlike for Explorers, the science would be restricted. To encourage innovation, considerable latitude should be allowed to achieve the science objectives. That approach should be possible for missions up to about $500 million; the Planetary Division already has PI-led missions of this size. That is consistent with the 2003 decadal survey recommendation that for strategic STP and LWS missions “NASA should (1) place as much responsibility as possible in the hands of the principal investigator, (2) define the mission rules clearly at the beginning, and (3) establish levels of responsibility and mission rules that are tailored to the particular mission and to its scope and complexity.”24
10.5.3.4 Recovery of an Effective NASA Grants Program
The heliophysics grants program is the foundation of the NASA science enterprise, but its effectiveness has been severely compromised in recent years by budget cuts in both research and analysis and new-missions guest-investigator (GI) programs and by the dearth of opportunities to develop innovative instrument concepts. In all of 2010, there were only 12 advertisements for heliophysics postdoctoral positions (see Appendix D, “Education and Workforce Issues in Solar and Space Physics”)—clear evidence that the typical PI grant size is no longer sufficient to support postdoctoral researchers. The imperatives described below are essential for recovering an effective NASA science grants program.
Establishment of Heliophysics Science Centers
Achieving the SHP panel’s science goals (§10.1) requires solving a number of major science problems. Many are sufficiently mature that important progress toward achieving closure between theory and observations can be expected. Examples are the following:
• Generation, emergence, and detection of active regions and other subsurface structures;
• Dynamic coupling of the ambient solar corona to the inner heliosphere;
• Magnetic reconnection in the Sun and heliosphere;
• Acceleration and transport of high-energy particles from the Sun;
• Origin and evolution of extreme solar storms and their impact on the geospace environment;
• Structure of the large-scale heliosphere and its interaction with the interstellar medium; and
• Complexity, nonlinearity, and cross-scale coupling through physical processes, such as turbulence, plasma-neutral coupling, and wave-particle interactions.
Making ground-breaking advances on these major problems requires teams that combine different expertise.25 The SHP panel therefore strongly supports the creation of new heliophysics science centers composed of teams of theorists, numerical modelers, and data experts working collectively to tackle the
24 NRC, The Sun to the Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics, 2003, pp. 19 and 157-158.
25 A. Bhattacharjee et al., Advanced Computational Capabilities for Exploration in Heliophysical Science (ACCEHS): A Virtual Space Mission, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 12.
field’s most compelling science problems. (See the section “Venture: Venture Forward with Science Centers and Instrument and Technology Development” in Chapter 4.)
NASA Individual-Principal-Investigator Grants Program
To achieve its broad science goals, the NASA science enterprise requires coordinated programs of missions, data analysis, theory and modeling, and technology development. Heliophysics individual-PI grants programs (SR&T, TR&T, and GI programs) are the core support of the division’s non-mission-hardware science and are the bedrock for developing new science understanding and mission concepts. Many transformational advances, ranging from the prediction of a solar wind to the development of far-side imaging, are based on research supported by the core grants program.
Although PI-grant programs provide unmatched science return on investment, they have recently suffered severe losses, and their funding is now less than 10 percent of total division funding. That investment is inadequate; fully realizing the returns of the HSO with its enormous data sets from multiple missions demands a higher level of PI-grant funding. Therefore, the SHP panel assigns high priority to the Heliophysics Division’s gradually increasing the investment in individual-PI-grant programs to 20 percent of division funding over the next decade.26
Furthermore, the SHP panel suggests that the increase be implemented primarily by increasing grant size rather than grant number. The present grant size of $100,000 to $150,000 per year is inadequate to support even individual-PI investigations and restricts the scope and depth of the science that such grants can address too much. It also leads to enormous inefficiencies for the community and NASA in the number of proposals that are written and reviewed. It is an imperative of the SHP panel that NASA work toward doubling the size of SR&T, TR&T, and GI grants to $200,000 to $300,000 per year.
Heliophysics Systems Observatory and MO&DA Support
The HSO relies on a distributed network of missions (implemented for specific science goals) to address strategic objectives that require multipoint or remote-sensing observations on long timescales.27 The distributed network provides comprehensive measurements of the whole Sun-heliosphere system, including Sun-Earth interactions, spanning a few years up to almost a 22-year Hale cycle. It fuels new system science by extending individual missions beyond their prime phase.
Resource allocation among extended HSO missions is determined through the senior-review process, which evaluates future scientific priorities for each mission. The present 5-year budget requests show flat or declining HSO funding. In addition to supporting existing HSO missions, the budget must accommodate new missions, such as RBSP (renamed the Van Allen Probes) and SDO, that finish their prime mission in or before FY 2015; this will inevitably lead to forced termination of or severe cuts in current HSO missions. As a consequence, key systems-science objectives are endangered, and essential legacy data sets may be foreshortened at a time when solar activity is apparently evolving in unexpected ways. Multipoint observations throughout the heliosphere and from the Sun to geospace regions need to be maintained to enable systems science. The SHP panel assigns high priority to augmenting MO&DA support by annual inflationary increases plus $5 million to $10 million per year to accommodate new missions so that senior-review decisions can be prudently based on strategic evaluations of existing and emerging assets.
26 J.A. Klimchuck, Maximizing NASA’s Science Productivity, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 135; C.M.S. Cohen et al., Protecting Science Mission Investment: Balancing the Funding Profile for Data Analysis Programs, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 45.
27 J.G. Luhmann, Guest Investigator and Participating Scientist Programs, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 166.
Adequate MO&DA Budgets and Mission-Related Guest-Investigator Programs
The science return from missions depends critically on adequate funding to analyze returned data, to develop relationships of observations across the HSO, and to test theories and models that are required to answer the science questions that motivated these missions quantitatively. Confirmed missions need to have adequate Phase E research budgets that ensure it is possible to achieve mission success. In addition, the budget for all missions needs to include an adequate GI program (typically 3 percent of the total mission cost spread over the prime mission) administered by NASA as part of the mission success criteria. The GI programs enable the community at large to exploit new data further and relate them to other fields of heliophysics research.28
Heliophysics Instrument Development Program
Heliophysics is now exploring the boundaries of its domain, performing increasingly complex measurements, and preparing for truly predictive capabilities. Progress hinges on growing the capabilities of new instrumentation. High-priority items include the following:
• UV-blind ENA detectors that promise breakthroughs in resolution and efficiency;
• Detectors for MeV ENAs to open new windows on localized acceleration processes; and
• Large-format, high-efficiency array detectors and rapidly switching polarization modulators that can measure heating and acceleration processes in the solar atmosphere.
Achieving such capabilities requires increased support for new instrument concepts (recommended in the 2003 decadal survey29). Also needed are opportunities for low-cost rides into space to boost the technology readiness level and means of leveraging technology-development resources in the Office of the Chief Technologist.
Therefore, the SHP panel strongly advocates that current funding for instrument development within SR&T, LWS, and the Low Cost Access to Space (LCAS) program be consolidated into a comprehensive heliophysics instrument development program30 that includes funding to cross the “valley of death” gap (technology readiness levels 4-6). If the program grows to about 2 percent of the heliophysics flight program budget, or about $6 million in current-year funds, comparable savings can be achieved from flight programs.
Low-Cost Access to Space Program
The NASA Heliophysics Division LCAS program provides opportunities for flying space experiments on sounding rockets (100- to 1,000-km apogee altitude), balloon payloads, and CubeSats to address new science opportunities, develop new instruments and technology, provide complementary science and underflight calibrations for missions, and train the next generation of space scientists and engineers. The SHP panel endorses the NRC’s 2010 recommendation to increase funding support of NASA’s suborbital programs.31 In particular, funding for science-payload development, flight, and data analysis is inadequate and needs to be doubled.
28 A.J. Tylka, Heliophysics System Science and Funding for Extended Missions, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 269; J.G. Luhmann et al., Extended Missions: Engines of Heliophysics System Science, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 167.
29 NRC, The Sun to the Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics, 2003, p. 11.
30 E.R. Christian, Heliophysics Instrument and Technology Development Program (HITDP), white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 34.
31 NRC, Revitalizing NASA’s Suborbital Program: Advancing Science, Driving Innovation, and Developing a Workforce, The National Academies Press, Washington, D.C., 2010.
Funding support for NASA’s Wallops Flight Facility (and subcontractors) appears adequate to support 20 or more flights per year, double the current rate. The SHP panel encourages continued development of new capabilities within the NASA LCAS program. High-altitude sounding rockets with three or four vehicles can provide longer observing time and carry heavier payloads. Balloons provide the only practical flight opportunities (because of launch-vehicle issues) for very heavy scientific payloads, such as the Sunrise solar experiment and the GRIPS Gamma-Ray Imager/Polarimeter mentioned above. Ultra-long-duration balloons (the first successful flight was in 2009) promise to provide about 100-day flight times in the near future. CubeSats can provide even longer observing times.
Conclusions of the Panel on Solar and Heliospheric Physics Regarding NASA’s Grants Program
Achieving the key science goals of this decadal survey requires establishing an effective NASA heliophysics grants program by implementing or augmenting the following key elements:
• Creation of new heliophysics science centers composed of teams of theorists, numerical modelers, and data experts to tackle the most compelling science problems;
• A graduated increase in individual-PI grants programs to about 20 percent of division funding, primarily by increasing the size of individual grants, to realize the science returns from the space missions and develop future ones;
• An augmented heliophysics MO&DA program to enable systems science by using the HSO;
• Adequate Phase E research budgets for confirmed missions to ensure mission success and appropriately sized GI programs as part of mission success criteria to ensure mission science productivity;
• Formation of a consolidated heliophysics instrument development program to facilitate innovative instrument concepts and make future mission implementation cost-effective; and
• Increased funding for science-payload development, flight, and data analysis in the LCAS program to provide urgently needed flight and training opportunities.
The following initiatives are based mainly on more than 30 white papers dealing with current or potential NSF-funded facilities and programs (for a list of titles, see Appendix I).
10.5.4.1 Advanced Technology Solar Telescope Operations
SHP Imperative: The SHP panel attaches high priority to NSF’s providing base funding sufficient for efficient and scientifically productive operation of the ATST to realize the scientific benefits of the major national investment in it.
Justification: Starting in 2017, the world-leading ATST will provide the most detailed and most accurate measurements of the Sun’s plasma and magnetic field ever obtained.32 The measurements promise to revolutionize understanding of the Sun and heliosphere. Realizing the abundant scientific payoff from the major national investment in ATST will require adequate sustained funding from NSF for operation, development of advanced instrumentation, and research-grant support for the ATST user community. The National Solar Observatory (NSO) will close all existing facilities except for a synoptic program, but projections indicate that the current NSO base funding will be adequate only to realize a small fraction of
32 S. Keil et al., Science and Operation of the Advanced Technology Solar Telescope, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 130; K.P. Reardon et al., Approaches to Optimize Scientific Productivity of Ground-Based Solar Telescopes, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 224.
the scientific potential of the ATST. Additional annual funding of around 4-5 percent of the capital cost of the ATST will dramatically enhance its science yield.
10.5.4.2 Need for a National Science Foundation Midscale Projects Line
SHP Imperative: The SHP panel strongly supports the establishment within NSF of a competed funding program for midscale projects.
Justification: NSF does not have a means of funding midscale projects ranging from about $4 million to $135 million in FY 2010 dollars. Several previous bodies, including the 2010 decadal survey of astronomy and astrophysics (Astro2010),33 have recommended that NSF implement a competed funding program for midscale projects similar to NASA’s Explorer model. A midscale funding program would enable NSF to provide a more balanced and flexible response to scientific opportunities that are currently too large to allow funding by the Major Research Instrumentation program and too small to qualify for funding by the Major Research Equipment and Facilities Construction line. A midscale program would offer excellent return by exploiting new techniques and instrumentation more rapidly, by ensuring a broader scientific portfolio of exciting and timely programs, and by offering additional opportunities for training scientists, engineers, and students. Examples of solar facilities that could be funded by this new line are the Frequency-Agile Solar Radiotelescope (FASR) and the Coronal Solar Magnetism Observatory (COSMO). They are described in more detail below.
10.5.4.3 Frequency-Agile Solar Radiotelescope
The SHP panel assigns high priority to NSF’s funding of construction and operation of FASR34 to produce three-dimensional images of the solar atmosphere with high temporal and spatial resolution. Solar radio emission provides uniquely powerful sources of diagnostic information with the potential for transformational insights into solar activity and its terrestrial impacts. That is because a number of distinct emission mechanisms operate at radio wavelengths: thermal free-free emission is relevant to the quiet solar atmosphere, thermal gyroresonance emission is a key mechanism operative in solar active regions, nonthermal gyrosynchrotron emission from electrons with hundreds of kiloelectron volts 10 MeV plays a central role in flares and CMEs, and a variety of coherent emission processes—such as plasma radiation, familiar to aficionados of type II and type III radio bursts—make it possible to trace electron beams and shocks in the solar corona and heliosphere. Radio observations and the diagnostic information that they provide are highly complementary to next-generation observations at optical and infrared wavelengths provided by the ATST and COSMO on the ground, at EUV wavelengths by Solar-C, and at X-ray wavelengths by SEE in space.
FASR is a solar-dedicated radiotelescope that provides a unique combination of superior imaging capability and broad instantaneous frequency coverage and thus exploits the powerful diagnostics available at radio wavelengths. The potential value of such a facility has been recognized in high-priority recommendations of FASR in two previous decadal surveys conducted by the National Research Council. The 2003 decadal survey of solar and space physics recommended FASR as its highest-priority “small project”35 in recognition of the unique and transformative role that it will play in addressing basic research questions
33 NRC, New Worlds, New Horizons in Astronomy and Astrophysics, The National Academies Press, Washington, D.C., 2010, p. 28.
34 D.E. Gary et al., The Frequency-Agile Solar Radiotelescope, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 86; D.E. Gary et al., Particle Acceleration and Transport on the Sun: New Perspectives at Radio Wave lengths, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 87.
35 NRC, The Sun to the Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics, 2003, p. 54.
and its complementary role with respect to other important instruments being developed to address the Sun and heliosphere. The Astro2010 decadal survey characterized FASR as a “compelling” midscale project and recognized it, with ATST, as a core facility in the U.S. ground-based solar portfolio.36 An independent analysis of cost and technical readiness (CATE analysis) described FASR as “doable today.” FASR thus has broad constituencies in solar and space physics and in astronomy and astrophysics.
The major advance offered by FASR over previous solar radio instrumentation is its unique combination of ultra-wide-frequency coverage, high spectral resolution, and high image quality. FASR measures the polarized brightness temperature spectrum over a broad frequency range (50 MHz to 21 GHz, or 1.4-600 cm) along every line of sight to the Sun as a function of time. Radiation in this radio wavelength range probes the solar atmosphere from the middle chromosphere to well into the corona. In essence, FASR images the entire solar atmosphere in three dimensions once every second from the chromosphere through the corona while retaining the capability to image a restricted frequency range with time resolution as small as 20 ms. In so doing, FASR enables fundamentally new, unique observables, including:
• Quantitative measurements of coronal magnetic fields, both on the disk and above the limb, under quiet conditions and during flares;
• Measurements of tracers of energy release and the spatiotemporal evolution of the electron distribution function during flares;
• Imaging CMEs and the associated coronal dimming, “EIT waves,”37 and coronal shocks; and
• Imaging of thermal emission from the solar atmosphere from chromospheric to coronal heights, including the quiet Sun, coronal holes, active regions, and prominences.
FASR’s panoramic view allows the solar atmosphere and physical phenomena therein to be studied as a coupled system. Its powerful and unique capabilities allow it to address such high-priority science questions as these:
• The nature and evolution of coronal magnetic fields
—What is the quantitative distribution of coronal magnetic fields?
—How do coronal magnetic fields evolve in time and space?
—How is magnetic energy stored?
• The physics of flares
—What is the physics of magnetic energy release?
—How and where are electrons accelerated?
—What are the relevant particle transport processes?
• The drivers of space weather
—How are CMEs initiated and accelerated?
—What is the origin of coronal shocks?
—How are solar energetic particles in the heliosphere accelerated?
• The physics of the quiet sun
—How are the solar chromosphere and corona heated?
—What is the origin of the solar wind?
—What is the structure of prominences and filaments?
36 NRC, New Worlds, New Horizons, 2010, p. 191.
37 So named because they were first discovered by the Extreme-ultraviolet Imaging Telescope (EIT) on the SOHO spacecraft.
BOX 10.2 FASR SPECIFICATIONS
|Angular resolution||20/nGHz arcsec|
|Frequency range||50 MHz to 21 GHz|
|Number of data channels||2 (dual polarization)|
|Frequency bandwidth||500 MHz per channel|
|Frequency resolution||Instrumental: 4,000 channels|
|Scientific: minimum (1%, 5 MHz)|
|Time resolution||About 1 s (full spectrum sweep)|
|20 ms (dwell)|
|Polarization||Full Stokes (intensity and linear polarization and circular polarization)|
|Number of antennas deployed||A (2-21 GHz): about 100|
|B (0.3-2.5 GHz): about 70|
|C (50-350 MHz): about 50|
|Size of antennas||A (2-21 GHz): 2 m|
|B (0.3-2.5 GHz): 6 m|
|C (50-350 MHz): log-periodic dipole array|
|Array size||4.25 km east-west x 3.75 km north-south|
|Absolute positions||1 arcsec|
|Absolute flux calibration||<10%|
FASR comprises three antenna arrays that are designed to observe meter (FASR C), decimeter (FASR B), and centimeter wavelengths (FASR A). The instrument exploits the long heritage of Fourier synthesis imaging developed over several decades in radioastronomy. The techniques are mature, and the technologies robust. High-level instrument specifications are given in Box 10.2.
10.5.4.4 Coronal Solar Magnetism Observatory
The SHP panel’s other high-priority candidate for the NSF midscale line is the COSMO large-aperture coronagraph and chromosphere and prominence magnetometer,38 which would complement the ATST and FASR. Until explosively released, most of the mass and energy that produce space weather events are stored in the coronal magnetic field. Coronal faintness is an observational challenge especially for sensitive magnetic and velocity measurements. Responding to a next-step suggestion of the 2003 decadal survey,39 COSMO is proposed as a suite of three instruments: a ground-based, 1.5-m-aperture solar coronagraph supported by a 12-cm-aperture white-light K-coronagraph (under construction) and a 20-cm-aperture chromosphere and prominence magnetometer. COSMO will be a facility for use by the solar physics research community. The facility will take continuous daytime synoptic measurements of magnetic fields
38 S. Tomczyk et al., COSMO, The Coronal Solar Magnetism Observatory, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 268; J. Burkepile et al., The Importance of Ground-Based Observations of the Solar Corona, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 25; S. McIntosh et al., The Solar Chromosphere: The Inner Frontier of the Heliospheric System, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 193.
39 NRC, The Sun to the Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics, 2003.
in the solar corona and chromosphere to support understanding of solar eruptive events that drive space weather and investigation of long-term coronal phenomena. The large emission-line coronagraph will be the largest refracting telescope in the world.
COSMO complements other facilities and missions by providing 300-days-per-year limb observations of the coronal magnetic field down to 1 Gauss over a 1° field of view. Long-term synoptic observations will directly address how the coronal magnetic field responds to the sunspot cycle and how the switch in polarity of the global field manifests itself in the heliosphere.
On a shorter timescale, COSMO will provide new information on the evolution of and interactions between magnetically closed and open regions that determine the changing structure of the heliospheric magnetic field and provide verification of extrapolations of the photospheric field.
COSMO will detect polarization signatures of nonpotential fields to afford insight into the roles of energy storage, magnetic reconnection, helicity creation and transport, and flux emergence in CME formation. It will also provide white-light images of CMEs in the low corona needed to determine basic properties of CMEs in early stages of formation. Particles accelerated by CME-driven shocks have the highest particle energies and pose the greatest space weather hazards. COSMO can detect compressions and distortions in the field that are due to the formation and passage of a CME and associated waves. Density compressions resulting from shock formation can produce intensity enhancements in white-light coronal images. COSMO has the potential to detect shock and CME locations simultaneously.
COSMO will provide routine observations of prominence, filament, and chromospheric magnetic fields and prominence flows. Those observations will constrain prominence densities and determine how prominence and coronal magnetic fields interact, how and where magnetic energy is stored (for example, by flux and helicity transport), and how it is released (for example, by instabilities, reconnection, or dissipative heating). COSMO will provide the first routine high-time-cadence measurements of coronal magnetic fields in and around flares and associated CMEs as seen above the solar limb. The high-time-cadence observations can help to determine when, where, and how magnetic energy is released by CMEs and by dissipative processes that result in flares.
10.5.4.5 National Science Foundation Small-Grants Program
SHP Imperative: To support the community effort required to analyze, interpret, and model the vast new solar and space physics data sets of the next decade, the SHP panel assigns high priority to NSF’s adopting the policy of doubling the size of its small-grants programs.
Justification: The coming decade should see major advances in solar and space physics with new ground-based and space-based instrumentation coming on line, new computational technology becoming available, and new personnel in the nation’s universities and other institutions. Funding will be needed to achieve the science promise of the new capabilities and researchers. Core support for new researchers, especially in universities, comes from NSF small-grants programs, which include the CEDAR, GEM, SHINE, and base grants programs and associated postdoctoral and young-faculty programs. The NSF small-grants program is inadequate to cover the science requirements of existing facilities and personnel. Whether measured by publications, data analyzed, or students educated, increased investment in the small-grants program is by far the most effective strategy for NSF to use to achieve its science and education goals.
10.5.4.6 Broadening the Definition of the National Science Foundation’s Solar-Terrestrial Research Program to Include Outer-Heliosphere Research
SHP Imperative: New scientific discoveries about the outer heliosphere may have important effects on the history and future of our planet, and this justifies broadening the scope of NSF’s Solar-Terrestrial Research program.40
Justification: NSF’s Solar-Terrestrial Research program is in the Geospace Section of the Atmospheric and Geospace Sciences Division. To be funded, proposals to the program must demonstrate direct relevance to solar-terrestrial science or be transformational in advancing understanding of the outer solar system. Even high-quality proposals in outer-heliospheric research may not be funded if they are not perceived to be directly relevant to changing terrestrial conditions. Outer-heliosphere research is not funded in other NSF programs and is “falling between the cracks” in NSF. Recent discoveries suggest that solar interactions with the local interstellar medium may play a much larger role in influencing terrestrial conditions than previously thought. In addition, this exciting and rapidly emerging field is providing new insights into processes occurring in the Sun, solar wind, planetary magnetospheres and atmospheres, and the galaxy. All that suggests that the definition of solar-terrestrial research in NSF should be broadened to include solar wind interactions with the local interstellar medium.
10.5.5.1 Ground-Based Solar Observations
SHP Imperative: The SHP panel gives high priority to NSF’s and other agencies’ support (where it is appropriate to their missions) of continuation of ground-based observations of the Sun.41
Justification: The Sun’s scientific and societal importance stimulates a broad array of research approaches. Much of the current understanding of the Sun originates in observations made on the ground. Continuing ground-based observations serve crucial roles in studies of long-term variations of the Sun, in attaining the highest spatial resolution of solar features, in measuring wavelengths from the infrared to the radio, in exploring new ideas, in testing numerical models, in developing new instruments, and in verifying space-based results. National and university-based solar observatories are supported mainly by NSF, and support from mission-oriented agencies is sometimes available when specific mission objectives align with ground-based capabilities. In spite of an outstanding record of scientific accomplishments, support for ground-based observations and for research based on them has eroded. The SHP panel believes that agencies, especially NSF, should work to reverse that trend.
40 G.P. Zank, Role of the National Science Foundation ATM/GEO in Promoting and Supporting Space Physics, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 284.
41 T. Ayres and D. Longcope, Ground-based Solar Physics in the Era of Space Astronomy, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 3; F. Hill et al., The Need for Synoptic Optical Solar Observations from the Ground, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 108; A. Pevtsov, Current and Future State of Ground-based Solar Physics in the U.S., white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 218; A.G. Kosovichev, Solar Dynamo, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 141.
10.5.5.2 Laboratory Astrophysics and Calibration Facilities
SHP Imperative: NASA, the National Oceanic and Atmospheric Administration (NOAA), and NSF are encouraged to continue their support of laboratory facilities for accurate measurement of atomic properties and for instrument calibrations as needed for several research programs and missions.42
Justification: Understanding the Sun and heliosphere requires observing them remotely. Diagnosing these observations requires both theoretical knowledge of radiation across the electromagnetic spectrum and well-calibrated instrumentation. Heliophysics research relies heavily on measurements of atomic and molecular collision physics, especially for highly charged ions that dominate X-ray and EUV production. Proper interpretation of observations requires cross sections for calculating excitation, direct and dielectronic recombination, single-charge and multiple-charge exchange, lifetimes, and other values. And calibration facilities (for example, at the National Institute of Standards and Technology) are required for accurate calibration and characterization of NASA and NOAA instrumentation before flight. Without such facilities, the science return from missions and ground-based remote-sensing observations can be substantially limited. The SHP panel assigns high priority to continued support of these facilities by NASA, NOAA, and NSF, preferably through budget line items for laboratory facilities.
10.5.5.3 Enhanced Space Weather Monitoring and Modeling
SHP Imperative: To optimize space weather capabilities, the SHP panel recommends that all future solar and heliospheric space missions and ground-based facilities consider including capabilities for delivering space weather data products. It would be a benefit for NASA to develop multiagency missions whose primary purpose is space weather monitoring. An interagency clearinghouse and archive of space weather data would benefit forecasters, researchers, and model-builders.
Justification: During the past decade, the direct application of solar and heliospheric physics to the protection of life and our high-technology society has demonstrated that the nation’s investment in the field is returning tangible benefits. The SHP panel recognizes the great benefit that would be realized if studies of new NASA missions considered adding space weather measurements and real-time data-transmitting capabilities where appropriate. Examples of possible missions include IMAP, SEE, and L5. The technical and financial effects of adding space weather instrumentation could be addressed by partnering with other NASA directorates or other agencies. Furthermore, the SHP panel recognizes a benefit in NASA consideration of partnering with other agencies to fly missions whose primary purpose is space weather operations but that also have science objectives, such as an L5 mission (§10.5.2.5).
Currently, ground-based neutron monitors and solar optical telescopes and radiotelescopes provide unique real-time data. In the future, an expansion of SOLIS to a three-site network and completion of FASR and COSMO would provide key data. The panel encourages ground-based facilities to assess data products that could improve space weather capabilities. Finally, to expand the availability of space weather data to forecasters, researchers, and model-builders, the SHP panel encourages development of an interagency clearinghouse for real-time and near-real-time data and an active national archive of current and past space weather data.
42 F.G. Eparvier, The Need for Consistent Funding of Facilities Required for NASA Missions, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 69; A. Chutjian et al., Laboratory Solar Physics from Molecular to Highly-Charged Ions, Meeting Future Space Observations of the Solar Plasma and Solar Wind, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 39.
10.5.5.4 Continuity of Real-Time Solar Wind Data from L1
SHP Imperative: Maintaining the continuity of real-time solar wind data from L1 is essential for the preservation and improvement of current space weather forecasting capabilities.
Justification: Observations of the solar wind and the Sun from 1 million miles upstream from Earth (orbiting the L1 Lagrangian point) have proved to be extremely valuable. The ACE mission has returned solar wind observations in real time since 1998, and the NOAA Space Weather Prediction Center uses them to drive 5 of its 11 space weather watches and warnings with a lead time of up to 60 minutes. The real-time data are also downloaded about 1 million times a month by about 25,000 unique customers, including deep-sea drilling, surveying, mining, and airline companies. NOAA, NASA, and the U.S. Air Force are refurbishing the DSCOVR spacecraft to provide operational real-time data to partially replace the data supplied by the aging ACE spacecraft. IMAP will be able to replace and expand on the suite of real-time space weather data provided by ACE and DSCOVR.
10.5.5.5 Preserving Key Solar-Heliospheric Data Sets
SHP Imperative: Agencies that engage in solar-heliospheric research or use solar and heliospheric data products (such as NASA, NOAA, NSF, and DOD) are encouraged to initiate an internal or external study that would identify key long-term data records and recommend approaches to ensure that they are continued and archived.43
Justification: A number of long-term solar and heliospheric data sets are essential for tracking the evolution of the solar-heliosphere system on timescales ranging from minutes to millennia. Examples include neutron-monitor records of cosmic-ray variations, TSI measurements, and solar wind properties upstream of Earth. Some of the data sets are often in danger of being discontinued because of lack of funding or changes in agency priorities.
10.5.5.6 Laboratory Plasma Physics
SHP Imperative: NASA and NSF are encouraged to continue supporting laboratory plasma-physics research at current or higher levels because it complements space- and ground-based measurements in efforts to understand basic heliophysical processes.44
Justification: Facilities that approximate solar and space plasma environments in the laboratory have long been part of the scientific investigation of space. Next-generation experiments have been emerging to provide experimental capabilities to study detailed processes directly relevant to space and solar plasmas. Some fundamental plasma processes are difficult, if not impossible, to measure and confirm with spacecraft but are potentially within reach through laboratory experiments. Examples include studies of magnetic reconnection and associated particle acceleration, solar coronal loops, and large-scale dynamo experiments at the Madison Dynamo Experiment facility. Moderate funding for laboratory heliophysics in
43 J.M. Ryan et al., Ground-Based Measurements of Galactic and Solar Cosmic Rays, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 235; P. Pilewskie, The Total and Spectral Solar Irradiance Sensor: Response to the NAS Decadal Strategy for Solar and Space Physics, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 221; F. Hill et al., The Need for Synoptic Solar Observations from the Ground, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 108.
44 B. Brown et al., An Experimental Plasma Dynamo Program for Investigations of Fundamental Processes in Heliophysics, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 18; H. Ji et al., Next Generation Experiments for Laboratory Investigations of Magnetic Reconnection Relevant to Heliophysics, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 120.
support of space-plasma and solar research, preferably through budget line items for laboratory facilities, would be beneficial.
10.5.5.7 Interagency Planning for a Future L5 Mission
SHP Imperative: The SHP panel assigns high priority to the development by NASA, NOAA, and DOD of plans for a mission at the L5 Lagrangian point to conduct helioseismology studies and develop advanced capabilities to forecast space weather.
Justification: From L5, it is possible to forecast the arrival of Earth-directed CMEs with high precision, to observe emerging or developed active regions and corotating interaction regions about 4 days earlier than from Earth, and to measure photospheric magnetic fields over about 60° additional longitude, improving magnetic-field models of regions rotating toward Earth.45 In addition, stereohelioseismology studies combining L1 and near-Earth observations have the potential to enable even longer solar-activity forecasts (§10.3.1 and §10.5.2.5).
The SHP panel encourages NASA, NOAA, and DOD to study means of achieving an L5 orbit for minimal cost and to define the spacecraft, instrument, and mission requirements needed to optimize the scientific and operational opportunities from a future L5 mission.
The SHP panel has interests that are connected to those of the Panel on Atmosphere-Ionosphere-Magnetosphere Interactions (AIMI) and the Panel on Solar Wind-Magnetosphere Interactions (SWMI). Solar photon radiation and energetic particle outputs are primary energy inputs into and thus drivers of dynamic processes in Earth’s atmosphere, ionosphere, and magnetosphere and those of other planets. The Sun varies on timescales of minutes (flares, CMEs, and SEPs), days (27-day solar rotation), years (11-year solar-activity cycle), and even longer. There are corresponding responses throughout the heliosphere, including at Earth and other planets. Successful space weather modeling and forecasting of magnetospheric and ionospheric events include continuous tracing through interplanetary space and monitoring of conditions upstream of Earth. Consequently, most SHP panel initiatives also support AIMI and SWMI goals. Robust observational and modeling programs of solar outputs and interactive collaborations among the SHP, AIMI, and SWMI communities are critically important for current and future heliophysics research and space weather operations.
Solar-irradiance variations in the near-ultraviolet, visible, and near-infrared have direct forcing on Earth’s climate change because of energy deposition in the lower atmosphere and at the surface and oceans. The solar-energy input and other indirect forcings, such as top-down coupling of solar ultraviolet heating and photochemistry in the stratosphere, and the possible influence of galactic cosmic rays, affect both global climate changes and regional changes that are due to subtle changes in the atmosphere, ocean circulation patterns, and cloud cover. Solar-irradiance observations and reconstruction and modeling of long-term variations, such as improved solar-dynamo models, are important support for the Earth science
45 A. Vourlidas et al., Mission to the Sun-Earth L5 Lagrangian Point: An Optimal Platform for Heliophysics and Space Weather Research, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 273; N. Gopalswamy et al., Earth-Affecting Solar Causes Observatory (EASCO): A New View from Sun-Earth L5, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 99.
goal of understanding natural influences on climate change. The SHP panel encourages enhanced collaboration among the solar, heliosphere, Earth science, and climate-change communities, in particular between the NASA Heliophysics Living With a Star targeted research and technology and NASA Earth science radiation and stratosphere programs.
Understanding the Sun as a star, understanding how solar structure and dynamics are related to those of other stars, and understanding relationships between the heliosphere and astrospheres are examples of how solar, stellar, galactic, and heliospheric physics research efforts benefit each other. Basic plasma processes that include particle acceleration, reconnection, and turbulence in the heliosphere also occur in other astrophysical environments. Studies of stellar evolution, through asteroseismology—the study of a star’s internal structure by analyzing the frequency spectra of its oscillations—and other means, and improved understanding of the range of variability of Sun-like stars also benefit Sun-climate studies of long-term solar variations, such as the Maunder minimum.46 It is highly desirable to continue and improve collaborations between heliophysics and astrophysics programs in NSF.
The magnetosphere represents a boundary in space plasmas created through the interaction of Earth’s magnetic field and solar wind. Comparable boundaries exist around planets, their moons, asteroids, comets, the heliosphere, and astrospheres. Detailed study of plasma boundaries mutually benefits understanding of boundaries surrounding Earth, planets (including exoplanets), moons, comets, the heliosphere, and astrospheres surrounding other stars. It is highly desirable to continue and improve collaborations among heliophysics, planetary science, and astrophysics programs to elevate and generalize understanding of planetary and astrophysical boundaries in space.
46 K. Schrijver et al., The Solar Magnetic Dynamo and Its Role in the Formation and Evolution of the Sun, in the Habitability of Its Planets, and in Space Weather around Earth, white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 240.