To appreciate the complex structure and evolution of Earth’s home in space, one need only look at the striking image of the extended solar atmosphere, the corona, taken during the July 11, 2010, solar eclipse (Figure 2.1, left panel). Turbulent convection below the Sun’s visible surface is the engine that drives the extreme ultraviolet (EUV) and X-ray radiation and the solar wind. The solar magnetic field is churned and twisted by this subsurface convection and in turn produces the fine-scale structure of the solar corona. The right panel of Figure 2.1 shows magnetic lines of force from a physics-based prediction of coronal structure based on solar surface magnetic field measurements for the same event. The correspondence between the imaged corona and the simulated magnetic field structure is striking.
The corona is the source of both EUV radiation and the solar wind, an outward flowing plasma and entrained magnetic field with speeds in the range of 400 to 800 kilometers per second, or around a million miles per hour. Solar ultraviolet and X-ray radiation, for example from solar flares, reaches Earth directly in 8 minutes, where it is absorbed in the thermosphere, the uppermost portion of Earth’s atmosphere. This photon energy heats the thermosphere and produces the electrically conductive ionosphere within the thermosphere. The ionosphere is linked both with the neutral atmosphere below through waves generated in the troposphere near Earth’s surface that propagate upward through the atmosphere and with the magnetosphere above via electric currents and the flow of charged particles. In contrast with the EUV radiation, the solar wind does not impact Earth directly but instead encounters Earth’s dipolar magnetic field, which deflects the solar wind and channels electric currents and energetic particles to the polar regions, shielding the middle and equatorial atmosphere.
Earth is therefore best understood not as orbiting the Sun in isolation through a vacuum, but as a physical system intimately linked to the highly variable solar atmosphere that engulfs the entire solar system. The magnetized solar atmosphere, solar wind, and Earth’s magnetosphere, ionosphere, and atmosphere are connected through a chain of interactions that govern the state of our space environment. Furthermore, the Sun occasionally sends out powerful mass ejections, which are accompanied by shock waves that accelerate charged particles to very high speeds, up to nearly the speed of light. These disturbances in the
FIGURE 2.1 Left: White-light image of the solar corona out to 4 solar radii during the solar eclipse of July 11, 2010. Right: Predictive Science, Inc., prediction of the magnetic field during the July 11, 2010, eclipse, using observations of photospheric magnetic field and numerical simulation of magnetized fluid. SOURCE: Left: Courtesy of M. Druckmüller, M. Dietzel, S. Habbal, and V. Rušin; available at http://www.predsci.com/corona/jul10eclipse/jul10eclipse.html. Right: Courtesy of Predictive Science, Inc.
solar wind intensify the Van Allen radiation belts, drive the aurora and powerful electric currents on Earth, and violently churn the ionosphere and uppermost atmosphere.
There is a growing appreciation that solar systems are commonplace in the universe and that the physical processes active in Earth’s heliosphere are universal. Deepening understanding of our own home in space therefore informs humanity’s understanding of some of the most basic workings of the universe. As human exploration extends farther into space via robotic probes and human flight, and as society’s technological infrastructure is increasingly linked to assets that are affected by the space environment, a deeper and fundamental understanding of these governing processes becomes ever more pressing (see Chapter 3).
The principles governing the Sun-Earth system include the physics of plasmas and of neutral and ionized atmospheres; atomic and molecular physics; radiative transport; and relativistic particle acceleration. The problems in solar and space physics are the basis of some of the most daunting challenges in these fields. For example, the physical regimes of plasmas in the heliosphere range from the highly collisional environment of the Sun’s convective zone to nearly collision-free environments of the Sun’s outer corona, as well as the interplanetary medium and planetary magnetospheres. In each regime, different theoretical approaches must be used to describe the system, and no single theoretical treatment applies throughout this vast range of regimes. Moreover, the dynamics of the system are governed by processes that span a broad range of spatial and temporal scales and are often the product of nonlinear or chaotic processes. For convenience, the broad discipline of solar and space physics, often referred to as heliophysics, is divided into the following three areas, each of which is described in much greater detail in Part II, Chapters 8 through 10, of this report:
• Solar and heliospheric physics (SHP)—which covers the physics of the outer regions of the Sun and the solar wind and its expansion through interplanetary space;
• Solar wind-magnetosphere interactions (SWMI)—which deals with the interaction of the solar wind with magnetized bodies (principally Earth and other planets) and the resulting dynamics of their magnetospheres and the associated coupling to the underlying ionosphere or planetary surface; and
• Atmosphere-ionosphere-magnetosphere interactions (AIMI)—which concerns the dynamics of planetary ionospheres owing to solar, magnetospheric, and atmospheric drivers and coupling.
The work of the three decadal survey panels representing these areas provided the basis for distillation of the key science challenges discussed in this chapter.
The decade 2003-2012 was a time of significant progress in all areas of solar and space physics. Dramatic advances were made in establishing the relationships among solar activity, resulting interplanetary disturbances, the response of Earth’s space environment, and the dynamics of the outer boundaries of our solar system with interstellar space. The links between the solar dynamo, convection, active regions, flares, coronal mass ejections (CMEs), and disturbances in the interplanetary medium are now identified. Researchers have identified candidate mechanisms that accelerate ions and electrons to relativistic energies in the inner heliosphere. They know the interplanetary conditions that drive geomagnetic activity and storms and have identified the dominant dynamic characteristics of the coupled magnetosphere-ionosphere-thermosphere system. Finally, they have now begun to explore the outermost reaches of the Sun’s influence at the boundary between the heliosphere and interstellar space.
These developments occurred in coordination with advances in physics-based numerical simulations that provide the foundation for understanding phenomena in terms of underlying physical processes, yielding insights into the basic physics of the systems and attaining a measure of predictive capability. Researchers are now poised to answer questions concerning universal physical processes, advance understanding of the complex coupling and nonlinear dynamics of the heliosphere, and apply this understanding for mitigation of harmful impacts to Earth’s technological infrastructures. To show how the recommendations of this report follow from the flow of scientific discovery, a selection of the most salient discoveries and advances are presented below.1
The Solar Dynamo and Activity
Over the past decade, the solar dynamo, which is the source of the Sun’s magnetic field and the resultant dissipation that drives solar activity, continued as a high-priority focus of research. The results of this work also have important implications for understanding stellar dynamos. Solar activity reached normal levels in cycle 23, but the minimum between cycles 23 and 24 in 2008-2009 reached low levels not seen
1 For a more complete discussion of ongoing missions and their contributions, see, for example, NASA, “Senior Review 2010 of the Mission Operations and Data Analysis Program for the Heliophysics Operating Missions,” July 5, 2010, available at http://science.nasa.gov/media/medialibrary/2010/07/22/SeniorReview2010-MODAProgramPublic_V3.pdf. Also see, NASA, “Heliophysics: State of the Discipline,” in Heliophysics: The Solar and Space Physics of a New Era: Recommended Roadmap for Science and Technology 2009-2030, 2009 Heliophysics Roadmap Team Report to the NASA Advisory Council Heliophysics Subcommittee, May 2009, available at http://sec.gsfc.nasa.gov/2009_Roadmap.pdf.
for nearly a century. Cycle 24 had been predicted to be more active than cycle 23, and the unexpected deep minimum focused attention on the need to improve understanding of the solar dynamo. Ground-based and SOHO space-based measurements prior to the activity minimum revealed unusually low magnetic flux near the poles of the Sun, and these low flux levels suggest that solar activity at the maximum of the current solar cycle will be low relative to that of recent past cycles.
Poleward meridional flows in the solar convective zone may be responsible for concentrating solar magnetic flux at the poles. Observations of this flow were made possible with great improvements in space-based (SOHO and SDO) and ground-based (GONG) helioseismic measurements of the solar interior. These observations have revealed changes in zonal and meridional flows consistent with the low polar flux and have shown that solar active regions exhibit subsurface helical flows whose strength is closely related to flare activity. Helioseismic observations are needed to firmly establish if scientists have indeed found a key to understanding the engine of solar activity.
The deep solar minimum in 2008-2009 provided an opportunity to study the heliosphere under conditions not present since the dawn of the space age and—enabled by STEREO—to study it for the first time in a truly global fashion: cosmic-ray fluxes near Earth reached the highest levels on record; reduced heating of Earth’s upper atmosphere by solar ultraviolet radiation led to unprecedented low drag on satellites; and the radiation belts reached historically low levels of intensity. This enhanced galactic cosmic-ray flux was caused by reduced modulation in a historically weak solar wind with slower speeds, lower magnetic field, and historically low activity.
The extended solar minimum, prolonged period of low sunspot numbers, and record cosmic-ray intensity led to suggestions that the Sun might be entering an extended period of minimum activity such as that observed (in sunspot, 14C and 10Be data) during the Dalton minimum (1800-1820) or the Maunder minimum (1645-1715). Recent low activity was used to set a lower limit for total solar irradiance (TSI), a key factor in climate change. Measurements of TSI have consistently shown a cycle variation on the order of 0.1 percent but give conflicting results about its absolute value. These conflicts have recently been resolved, in favor of the lower values shown in Figure 3.1. It remains uncertain whether the TSI levels during the recent solar minimum are indicative of levels expected for a prolonged cessation of solar activity.
The past decade has seen spectacular advances in understanding of the structure of the solar magnetic field. Increases in processor speed and massively parallel computational techniques have enabled greater than 100-fold improvements in the spatial resolution of simulations. The resolution of observations has improved with data from the 0.5-m-aperture telescope of the Hinode satellite and through image processing techniques applied to ground-based data from the 1-m-class apertures and the new 1.6-m New Solar Telescope (NST). Researchers have now identified the main physical processes at work in sunspot penumbral filaments, bright umbral dots, bright faculae, and small-scale magnetic structures. Figure 2.2 is a side-by-side comparison of the results of a numerical simulation of a sunspot and an image from the NST; it shows astonishing correspondence in the background circulation pattern (granulation), the umbra fibrils, and the central spot.
Solar Wind Origins
New observations of the photosphere and lower corona have revealed significant information on the mechanisms of coronal heating, which is ultimately the driver of the solar wind. High-resolution chromospheric images from Hinode’s Solar Optical Telescope unveiled relentless dynamics and contorted structures. A new type of spicule (a radial jet of plasma) was discovered that may play a critical role in transferring mass and energy to the corona. The narrowband EUV images from the SDO Atmospheric Imaging Assembly have revealed that coronal loops cannot be in a steady state as previously believed.
FIGURE 2.2 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.
Furthermore, elemental fractionation signatures, identical to those in the quiet coronal loops, have been observed in slow solar wind.
The transition from the chromosphere to the solar wind is governed by the magnetic field of the corona. However, Hinode and SDO can measure the photospheric field but not the coronal magnetic field. Two advances of the past decade offer promise to fill this data gap: the first observations were made of the full chromospheric vector field on the disk, and the first maps were obtained of the coronal field above the solar limb using ground-based observations. Further advances in measuring the coronal field are crucial for understanding the origins of the solar wind and the driver of solar activity and its impact on Earth’s space environment.
Significant progress was made toward achieving closure between theory/models and observations. The first semi-realistic global-scale three-dimensional magnetohydrodynamic (MHD) numerical simulations of the corona were performed with spatial resolution sufficient to enable comparison with modern observations (e.g., Figure 2.1). Modeling the chromosphere, however, remains a significant challenge, because in this region the classical description of the transport of energy begins to break down and dynamically important spatial scales may not be resolved. Three-dimensional numerical simulations cannot yet address all the physical ingredients on scales larger than a few granules or one supergranule, but many of these challenges can be overcome in the coming decade if these efforts are adequately supported.
Explosive Release of Magnetic Energy
Flares and CMEs are the dominant sources of the solar energetic particles (SEPs) that threaten human spaceflight. Significant progress was made in understanding how magnetic energy is explosively released in flares. RHESSI hard X-ray (HXR) imaging-spectroscopy measurements revealed that accelerated electrons often contain ~50 percent of the magnetic energy released in flares and indicate that energy-release/ electron-acceleration is associated with magnetic reconnection. In large flares, HXR imaging of flare-accelerated ~30-MeV ions shows that these emissions originate from small foot points linked to magnetic loop structures rather than over an extended region, indicating that ion acceleration is also related to magnetic reconnection. The energy in >~1-MeV ions and that in >20-keV electrons appear comparable. Thus, understanding the remarkably efficient conversion of magnetic energy to particle energy flares is a significant challenge.
Major advances were also made in understanding photon energy release from flares. For the first time, flares were detected in TSI by the SORCE/TIM instrument showing that the total radiated energy and CME kinetic energy can be comparable. The SDO/EVE instrument discovered an EUV late phase in flares delayed many minutes from the X-ray peak. Global EUV observations by SDO/AIA and STEREO/EUVI revealed long-distance “sympathetic” interactions between magnetic fields in flares, eruptions, and CMEs likely owing to distortions of the coronal magnetic field.
The understanding of how CMEs and flares are produced and related has also progressed. CME velocity profiles below ~4 RS are in sync with flare-HXR energy releases. The magnetic flux-rope structure of models of CMEs is consistent with the observations of many events. Furthermore, shocks produced by fast CMEs can be identified in coronagraph images, suggesting that scientists are close to pinning down the sources of SEPs. Achieving a predictive capability for SEP energy spectra and transport variability is a greater challenge.
Structure and Dynamics of the Solar Wind
Major progress was made over the past decade in understanding solar wind structure and dynamics, a key to understanding the Sun’s influence on Earth’s geospace environment. The conceptual picture from Ulysses and ACE was that the sources of the slow and fast solar wind were at low-latitude and high-latitude regions of the Sun, respectively. Fast, slow, and transient (associated with CMEs) solar wind can now be identified and distinguished by ionic composition signatures (Fe charge states, Fe/O, O7+/O6+), and so the origins of solar wind parcels can be directly identified from in situ observations. Coronal mass ejections interact with these solar wind streams, leading to dynamic fluid interactions and also particle acceleration through a variety of processes. Microstructure of the solar wind, presumably related to structures in the corona, may now be analyzed with the most powerful set of in situ observations, sometimes using several observational platforms. The cascade of turbulence to short spatial scales and its ultimate dissipation are the likely source of energy for heating the expanding solar wind. Observations and models have produced major advances on this topic. Temperature anisotropies with respect to the local magnetic field of solar wind H+ and He2+ were shown to be limited by the mirror and firehose instabilities.2 These observations constrain the possible mechanisms of solar wind heating. Scientists have also discovered that magnetic reconnection between adjacent domains of opposing magnetic fields is ubiquitous in the solar wind but appears to involve little particle acceleration near heliospheric reconnection sites—a surprise, given the
2 See J.C. Kasper, A.J. Lazarus, and S.P. Gary, Wind/SWE observations of firehose constraint on solar wind proton temperature anisotropy, Geophysical Research Letters 29(17):20-1-20-4, 2002; B.A. Maruca, J.C. Kasper, and S.P. Gary, Instability-driven limits on helium temperature anisotropy in the solar wind: Observations and linear Vlasov analysis, Astrophysical Journal 748(2): 137, 2012.
efficiency of energetic particle production in flares. Unexpectedly, most of these reconnection sites have been found away from the heliospheric current sheet. The observations have also emphasized the importance of observations nearer to the Sun to enhance understanding of the roles of waves, wave turbulence, and reconnection physics in driving solar wind dynamics.
Solar Energetic Particles
New observations of solar energetic particles have yielded a number of surprises. Solar-cycle 23 produced 16 ground-level events in ground-based neutron monitors, which allowed researchers to establish that most large SEP events have a recent, preceding CME from the same active region. This finding indicates that the most intense events may involve the acceleration of particles in one or more flares that produce a seed population of energetic ions that can then reach very high energy through classical diffusive shock acceleration at the CME-driven shock. The measured enrichments by ACE of 3He and Fe in many large SEP events are consistent with this picture. Continuing observations from STEREO, ACE, and other platforms as well as upcoming Solar Orbiter and Solar Probe Plus missions will provide key measurements in the source regions of these events and data on their spatial extent and evolution so that the complex dynamics of SEP acceleration and transport to the geospace environment can be unraveled.
Exploring the Heliosphere’s Outer Limits
A series of groundbreaking discoveries were made as the Voyager spacecraft approached and crossed the termination shock (TS) and entered the heliosheath on their way to the heliopause, the outer boundary of the Sun’s domain in the universe. These measurements and results from the Interstellar Boundary Explorer (IBEX) and Cassini have significantly altered understanding of how the solar system interacts with the interstellar medium and have also quantitatively confirmed a number of scientific predictions about the heliospheric boundary region. The TS, which is where the solar wind can no longer maintain its supersonic velocity as it pushes against the interstellar medium, had long been accepted as the driver of anomalous cosmic ray (ACR) acceleration, but when the two Voyager spacecraft crossed the TS, neither found evidence that the local TS is the source of ACRs. The source of the ACRs is now a subject of fierce scientific debate. In addition, consistent with earlier theoretical predictions, most of the supersonic-flow energy did not heat the ambient solar wind but likely went into supra-thermals (not measureable with the Voyager instruments). The most recent observations may indicate the presence of an unexpected transition region in which the outward solar wind flow stagnates.
Energetic neutral atom (ENA) maps by IBEX and Cassini show an unpredicted “ribbon” of emissions from the outer heliosphere, apparently ordered by the local interstellar magnetic field (Figure 2.3). The ribbon evolves on timescales as short as 6 months, demonstrating that the heliosphere/interstellar-medium interaction is highly dynamic. The role of the interstellar magnetic field in shaping the outer heliosphere is stronger than was expected prior to the recent influx of new data. Models based on these observations suggest that the local interstellar magnetic field provides most of the pressure in the local cloud. The unexpected results from Voyager, IBEX, and Cassini observations demonstrate how little is really understood about the interactions of stars with their interstellar environments.
Advances in the physics of magnetospheres, their dynamics, and their coupling with the solar wind and ionospheres were made on a number of fronts. Global imaging and in situ observation networks revealed
FIGURE 2.3 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 energetic neutrals. 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: Courtesy of the Interstellar Boundary Explorer Mission Team.
unexpected dynamics associated with plasma convection, particle acceleration, and particle transport. Key advances were made on the underlying fundamental physical processes that govern the nonlinear dynamics of the system, including reconnection, wave-particle interactions, and turbulence. Observations and simulations of the dramatically different magnetospheres of Jupiter and Saturn provided key tests of current understanding and highlight the great variety of behavior exhibited by different systems.
These advances were enabled by combining a wide array of observations in concert with theory, laboratory plasma experiments, and revolutionary computational models. Critical observations were returned from instruments on suborbital rockets and balloons, and from an extensive ground-based network of radars, lidars, imagers, magnetometers, and riometers. Instrumental to these advances were spacecraft observations returned from new satellites launched during the decade or just before (e.g., Cluster, IMAGE, THEMIS, TWINS) as well as data returned from earlier missions and data collected by instruments flown on non-NASA satellites.
The global dynamics of the magnetosphere are controlled by the changing north-south component of the interplanetary magnetic field (IMF), which drives global circulation in the magnetosphere, as shown in Figure 2.4. Changes in the IMF and solar wind dynamic pressure produce storms, light up the aurora, and drive a host of other global responses.
Global imaging of heretofore invisible plasma populations of the magnetosphere was used to identify its large-scale response to this variable solar wind forcing. The plasmasphere, which is the region of cool-dense plasma that co-rotates with Earth, was imaged in the extreme ultraviolet. Observations revealed that strong storms strip off the outer part of the plasmasphere in plumes, which convect outward to the dayside magnetopause (Figure 2.5) and map to produce ionospheric density enhancements of the type shown in Figure 3.3.
The magnetospheric equatorial ring current is enhanced during geomagnetic storms, and it perturbs the strength of the magnetic field at Earth’s surface. Understanding its dynamics is crucial for establishing a predictive capability of the response of geospace to storms. The injections of ring current ions were imaged for the first time, establishing their configuration and composition. Numerical models and global ENA imaging revealed that the ring current is highly asymmetric during the main phase of storms, which suggests a strong coupling with the ionosphere. The peak of the ring-current proton distribution during the main phase of magnetic storms was shown to occur consistently in the early morning and not in the afternoon as had been expected. This can happen only if the ionosphere feedback fundamentally alters the electric field that is responsible for magnetospheric convection.
Fundamental Physical Processes: Magnetic Reconnection and Wave-Particle Interactions
The understanding of fundamental physical processes that govern system-level dynamics advanced on a number of fronts over the past decade. Substantial progress was made in understanding how magnetic reconnection works. The first quantitative predictions of detailed magnetic and plasma flow signatures were spectacularly confirmed with in situ observations. Similarly, sophisticated kinetic simulations finally yielded a consistent understanding of signatures of the onset of magnetic reconnection in the tail.
Increased computing power has facilitated simulations of the essential physics and structure of the diffusion region, where magnetic field lines3 reconnect and change their connectivity (Figure 1.4). It was
3 Field lines are a convenient construct for understanding magnetic field connectivity and topology.
FIGURE 2.4 The critical processes that drive the magnetosphere. To achieve a full understanding of the complex, coupled, and dynamic magnetosphere, it is important to understand how global and mesoscale structures in the magnetosphere respond to variable solar wind forcing, and how plasmas and processes interact within the magnetosphere and at its outer and inner boundaries, by using a combination of imaging and in situ measurements. SOURCE: Courtesy of Jerry Goldstein, Southwest Research Institute.
shown that at the small spatial scales where reconnection occurs, the decoupling of ion and electron motion as a result of their very different mass plays a key role in facilitating the rapid rate of reconnection seen in the observations. Ions become demagnetized in a much larger region than did the electrons, which changes the forces that accelerate particles away from the x-line compared with the usual MHD description. These ideas led to predictions that facilitated the first direct detection of the ion diffusion region (where the ions decouple from the magnetic field) in the magnetosphere and in the laboratory, as well as glimpses of the much smaller electron diffusion region (where the electrons decouple from the magnetic field). The observations in the vicinity of the diffusion region revealed surprisingly that reconnection can accelerate electrons to hundreds of kiloelectronvolts, potentially providing a seed population for subsequent acceleration in the inner magnetosphere to form the electron radiation belts. Discoveries were also made regarding the triggering and modulation of reconnection. Prior to around 2000, computational resources had simulations limited to two spatial dimensions. New capabilities to perform fully three-dimensional simulations revealed that the added dimension facilitates the growth of plasma instabilities that may break up the diffusion region, making reconnection highly turbulent.
FIGURE 2.5 Measurements by the EUV instrument on the IMAGE satellite. EUV images before a storm and after a storm, when the plasmapause reaches its minimum radial extent due to erosion by enhanced convection. SOURCE: Reprinted from M.K. Hudson, B.T. Kress, H.-R. Mueller, J.A. Zastrow, and J. Bernard Blake, Relationship of the Van Allen radiation belts to solar wind drivers, Journal of Atmospheric and Solar-Terrestrial Physics 70(5):708-729, 2008, copyright 2008, with permission from Elsevier.
Observationally, reconnection seems to behave differently in different regions. Although reconnection in the magnetotail and in the magnetosheath, where multiple reconnection sites have been identified, appears to be transient and turbulent, it can, at other times, be quite steady in time and extended in space at the dayside magnetopause and in the solar wind. Reconnection in the magnetotail produces bursts of narrow channels of high-speed flow. Multi-spacecraft observations have revealed that these reconnection-generated flow channels initiate magnetospheric substorms and drive the Earth-ward convection in the magnetotail; however, further multi-spacecraft studies may be necessary to complete the pattern of global magnetospheric circulation predicted four decades ago. Finally, observational analyses will benefit greatly from the inclusion of reconnection scenarios more general than the standard X-point picture, including more general geometries identified in both theory and simulations.
Wave-particle interactions (WPIs) have been established as key drivers of particle energy gain and loss in the radiation belts (Figure 2.6). Plasma instability theory, global simulations that include WPI processes, and wave observations have demonstrated that the mixing of energetic and low-energy plasmas drives instabilities distributed throughout the ring current and radiation belt. Satellite observations of radiation-belt electrons demonstrate that local acceleration due to WPIs may at times dominate acceleration due to diffusive radial transport. Statistical analyses of satellite wave observations were used to quantify the rates of energization and scattering. The results have been incorporated into time-dependent models of the radiation belts and the ring current. Scientists now know that storm-time particle dynamics are the result
FIGURE 2.6 Model-generated image showing the two main radiation belts, the outer belt and the inner belt. The model was developed at the Air Force Research Laboratory. Colors in the radiation belts indicate relative number flux. The auroral zone colors reflect precipitation to the atmosphere. Shown here are representative orbits for three Global Positioning System and one geosynchronous spacecraft. SOURCE: Courtesy of R.V. Hilmer, Air Force Research Laboratory.
of a delicate balance between acceleration and loss of relativistic particles mediated by waves produced by local plasma instabilities.
In the past decade there has been tremendous improvement in understanding of how the magnetosphere responds to storm-time disturbances as a coherent system of coupled, mutually interacting plasmas. Imaging and global simulations have played a central role by providing quantitative contextual information that ties together single-point observations and allows assessment of the global behavior implied by local observations. These advances were coupled with continuous measurements of the solar wind and IMF and numerous in situ observations in space and ground-based and remote sensing networks to yield discoveries of characteristic global responses. Researchers now realize that there are multiple nonlinear dynamic linkages whose consequences for coupled magnetosphere-ionosphere behavior are revealed only when they are integrated together in the global system. As a result the system exhibits characteristic nonlinear,
chaotic dynamics, or “emergent” behavior, that could never have been predicted without knowledge of the coupling physics.
The electrodynamic coupling between the magnetosphere and ionosphere modifies the simple dissipative response in dramatic ways. The interaction of the ring current with the ionosphere severely distorts inner magnetospheric convection, which feeds back on the ring current itself, skewing its peak toward dawn. Duskside flow channels arising from ionospheric coupling remain long past periods of peak solar wind driving. These studies shattered the notion that the inner magnetosphere is well shielded from the outer magnetosphere and is quiescent.
It is now established that storm-time acceleration and injection of the ring current depend on pre-storm loading of the magnetosphere from the solar wind. Spacecraft observations and numerical simulations reveal that solar-wind plasma entry into the magnetosphere is surprisingly efficient under “quiescent” conditions of a northward interplanetary magnetic field. This plasma in turn participates as a substantial element of the storm-time ring-current development when southward interplanetary magnetic fields couple with and energize the magnetosphere. The plasmasphere in turn controls whether ring current injections act to enhance or deplete energetic particle populations, overturning the decades-old idea of a passive, quiescent plasmasphere. The overlap of freshly injected, hot, ring current plasma with the dense plasmasphere produces local instabilities. The resulting waves scatter radiation belt particles, depleting the radiation belts. The predictions that cold dense plasma facilitates the acceleration of energetic electrons to relativistic energies were also confirmed by observations. Thus, it has been established that pre-injection dynamics are critical in establishing the state of the plasmasphere and governing the radiation belt storm response.
The ionosphere can also be a significant source of plasma for the magnetosphere over the past decade. The understanding of ionospheric ion outflow advanced significantly, and the conditions promoting extraction of ionospheric plasma to high altitudes and into the magnetosphere were established. Solar wind density and dynamic pressure increases were shown to lead to enhanced ionospheric outflow, but the greatest outflow rates were also closely correlated with the electromagnetic energy flux into the ionosphere. The energy flux from the solar wind yields intense ionospheric ion outflows supporting the theoretical predictions that this outflow requires a multistep process involving a combination of local heating by waves and electromagnetic forcing. It was also demonstrated that ionospheric outflow has dramatic consequences for the dynamic evolution of the magnetosphere. Outflows merge with plasmas of solar wind origin in the plasma sheet, creating a multi-species plasma that alters the dynamics of magnetic reconnection. Multi-fluid global simulations confirmed the major role that ionospheric outflow plays in the creation of periodic substorms or so-called sawtooth intervals.
The discoveries of preconditioning interactions and of efficient pathways for magnetosphere-ionosphere coupling, and the identification of the dynamics that emerge, provide the basis for a program of research to achieve a quantitative, predictive understanding of system behavior under extreme conditions.
Magnetospheres of Other Planets
The past decade saw many advances in understanding the structure, dynamics, and linkages in other planetary magnetospheres or systems with magnetospheric-like aspects. For the terrestrial planets, they range from insights on atmospheric loss at Mars and the identification of Venus lightning from high-altitude radio wave measurements to observations of magnetospheric dynamics at Mercury that reflect dramatically stronger solar wind-magnetosphere coupling than that at Earth. There have also been advances in theoretical understanding and observational tests of the impact of solar wind dynamic pressure variations on Jovian auroral emissions, and significant progress in understanding magnetospheric interactions with Jupiter’s satellites, especially Io. ENA imaging demonstrated that an extensive torus of neutral gas from Europa has
a significant impact on Jupiter’s magnetosphere. Io-genic plasma is transported outward by flux-tube interchange processes on the dayside but by centrifugal instabilities and plasmoid ejection in the evening and at night. Intense bursts of energetic particles are accelerated in regions ~200 Jupiter radii down the tail on the dusk flank. These discoveries demonstrate the great range of physical processes that the Jovian system exhibits, presenting an enormous opportunity for advancing understanding of magnetospheric dynamics.
A major highlight of the decade came from the extensive measurements of Saturn’s highly structured magnetosphere and satellite system by the Cassini spacecraft. Plumes of water gas and ice crystals emanate from rifts in the south polar region of Enceladus (Figure 2.7). Flux tube interchange in the middle magnetosphere followed by plasmoid release in the magnetotail was revealed as the primary transport mechanism for cold Enceladus plasma.
Solar wind pressure variations strongly modulate the activity in the outer magnetosphere, including Saturnian kilometric radio emission and the acceleration of energetic particles in Saturn’s ring current. These results remain a challenge to explain and demonstrate the critical role the study of these other systems has in advancing magnetospheric physics.
A broad range of national, international, and multiagency programs facilitated major advances in the science of Earth’s ionosphere and thermosphere and their interactions with the magnetosphere and the lower atmosphere. A major surprise is that the ionosphere-thermosphere system exhibits unexpected structuring during solar-quiet conditions. New Global Positioning System (GPS)-based assets from ground and space led to fundamental discoveries of dynamics of the global ionospheric density. Reactive feedback processes of thermospheric upwelling and intense ionospheric ion outflows were demonstrated to occur in new ways and were shown to have profound consequences for magnetospheric dynamics. The storm-time response of the system is now better characterized than ever before, and key gaps in understanding of the linkages between drivers and responses have been identified. Finally, tropospheric forcing from below was discovered to play a surprisingly strong role in the dynamics and structure of the ionosphere and thermosphere.
It is worth noting here the importance of international and cross-agency support that has made these scientific discoveries possible. For example, the COSMIC mission—a six-satellite joint U.S.-Taiwanese mission to improve understanding of both weather and space weather—carries instruments developed by JPL and the Naval Research Laboratory and was launched by the U.S. Air Force (USAF), and the data it collects are downloaded at NOAA and NASA facilities and processed at the NSF-supported National Center for Atmospheric Research (NCAR). The C/NOFS mission is another more recent example of scientifically productive cooperation between the USAF and NASA. Perhaps not surprisingly, several scientific discoveries involve physical processes that extend across the regions of interest of these agencies, and across nations.
Active Ionosphere During Solar Minimum
Gradual changes in solar activity, solar wind, solar EUV radiation, and Earth’s magnetic field play a significant role in defining the long-term variation in the geospace environment. The most recent solar minimum produced a prolonged period of low solar EUV fluxes and corresponding heating rates. At the same time, the thermospheric densities dropped to anomalously low levels, lower than any observed in the past four solar cycles. No numerical model has yet been able to predict or reproduce the density observations, which are thought to have resulted from some combination of low solar and geomagnetic activity, cooling from increasing greenhouse gas concentrations, and possibly additional chemical or dynamical
FIGURE 2.7 Saturn’s Enceladus-dominated, rotating magnetosphere. (a) Saturnian kilometric radio (SKR) emission periodicities. (b) Enceladus, its geysers, resulting plasma, and connection to Saturn’s ionosphere. (c) Return of energized plasma after tail plasmoid loss. These injections yield bright auroral displays in the same region as the SKR emissions. SOURCE: (a) D.A. Gurnett, J.B. Groene, A.M. Persoon, J.D. Menietti, S.-Y. Ye, W.S. Kurth, R.J. MacDowall, and A. Lecacheux, The reversal of the rotational modulation rates of the north and south components of Saturn kilometric radiation near equinox, Geophysical Research Letters 37:L24101, doi:10.1029/2010-GL045796, 2010. Copyright 2010 American Geophysical Union. Reproduced by permission of American Geophysical Union. (b) JHUAPL/NASA/JPL/University of Colorado/Central Arizona College/SSI. (c) Adapted from D.G. Mitchell, S.M. Krimigis, C. Paranicas, P.C. Brandt, J.F. Carbary, E.C. Roelof, W.S. Kurth, D.A. Gurnett, J.T. Clarke, J.D. Nichols, J.-C. Gérard, et al., Recurrent energization of plasma in the midnight-to-dawn quadrant of Saturn’s magnetosphere, and its relationship to auroral UV and radio emissions, Planetary and Space Science 57(14-15):1732-1742, doi:10.1016/j.pss.2009.04.002, 2009.
changes propagating upward from the atmosphere below. The extended epoch of low solar EUV and the reduced neutral densities at low-Earth-orbit altitudes led to the unexpectedly long mission life for the German Challenging Mini-Satellite Payload (CHAMP) satellite. But despite the extended solar quiet period, the ionosphere displayed a surprising array of dynamics, including complex density structures in the morning hours near local dawn that were documented by the USAF C/NOFS mission and NASA’s CINDI experiment and other space- and ground-based assets. It is now known that a quiet Sun does not correspond to a calm, benign ionosphere and that deleterious impacts on navigation and communications occur under these conditions in unexpected ways.
Global Density Structures and Reactive Feedback
Global GPS maps of ionospheric density showed, for the first time, large-scale dense plumes of plasma extending from middle latitudes to the auroral zone at the onset of magnetic storms (see Figure 3.3). During such events, plasmaspheric imaging of He+ ions by IMAGE showed corresponding structures in the inner magnetosphere, where plasma was sheared away from the plasmasphere and advected toward the magnetopause (see Figure 2.5). The plasmaspheric structure was never expected to appear in the ionosphere, and the discovery points to a process critical to enhancing auroral ion outflow during storms.
Localized structures in the neutral density were discovered by international geodesy programs. The CHAMP and NASA/German Gravity Recovery and Climate Experiment (GRACE) missions led to the discovery of localized neutral upwelling very near the poles associated with strong Joule heating that occurs during geomagnetically calm or moderate conditions. This result demonstrated the surprising range of conditions wherein neutral densities are sufficiently altered to modify the decay rates of satellites in low Earth orbit. Understanding of the generation of these localized densities is not yet mature enough to predict their occurrence.
Recent results from NASA’s FAST and IMAGE satellites revealed intense outflows of ionospheric ions during storms. The solar wind-magnetosphere interaction on the dayside, that is, magnetopause reconnection, is a copious source of electromagnetic energy that propagates along the magnetic field into the ionosphere at high latitudes near noon. This energy is converted to heat and momentum through ion-neutral interactions and promotes resonant heating of O+ that drives outflows. The O+ flows upward and is carried into the magnetotail by the reconnection-convection cycle. The resultant large O+ densities in the tail plasma sheet appear to change reconnection dynamics in the tail, leading to the ~3-hour planetary-scale (sawtooth) oscillations or quasi-periodic substorms in the magnetosphere. The influence of the O+ outflow on global dynamics is only one of a number of instances in which nonlinear reactive feedback leads to nonlinear dynamics.
Several of the geomagnetic storms driven by CMEs during solar cycle 23 were considered “great” storms that led to highly nonlinear dynamics. Ionosphere observations indicated the emergence of a daytime superfountain effect, lifting the ionosphere to new heights and increasing its total electron content by as much as 250 percent. Other extreme responses included very-large-amplitude traveling ionospheric disturbances and modifications in the equatorial plasma irregularities that impact communications.
During cycle 23, there were 89 great storms that drove the geomagnetic storm index Dst4 below −100, but only one, associated with the extremely fast CME launched by the spectacular X17 flare of October
4 The Dst (disturbance–storm time) index is used to define geomagnetic storms. Quiet times usually have a Dst of between +20 and −20 nanoteslas.
28, 2003, topped −400. Fortunately that same active region had rotated past Earth when the largest flare ever measured by spacecraft erupted on November 4 with an energy index of X28.
During these great storms, the atmosphere responded with dramatic changes in neutral composition, winds, temperature, and mass density. Thermosphere mass density at 400-km altitude increased by more than 400 percent and recovered to pre-storm levels exceptionally rapidly, indicating a strong overcooling mechanism. Although many of the responses of the atmosphere-ionosphere-magnetosphere (AIM) system to these storms have been documented, the mechanisms responsible for producing these effects are poorly understood—scientists have not been able to emulate these effects in simulations. In particular, scientists cannot yet predict the impacts of so-called superstorms, storms comparable in magnitude to the Carrington event of 1859 that had an astounding estimated Dst of −850.
One of the most exciting developments in recent years has been the realization that tropospheric weather and climate can strongly affect the upper atmosphere and ionosphere. Ultraviolet imaging of Earth by the NASA IMAGE and TIMED satellites in the 2000-2003 time frame provided an unprecedented new view of the equatorial ionosphere that revealed a large, longitudinal variation in density, with peaks over rainforests. In the same period, atmospheric models developed at NCAR were gaining new capability showing that atmospheric tides driven by tropospheric heat released in thunderstorms would propagate well above 100 km and potentially modify the ionosphere-thermosphere (IT) system. Large-scale changes in the structure of the ionosphere on seasonal timescales were also revealed, which also matched the seasonal changes in tropical weather conditions. Since its launch in 2006, the COSMIC mission has observed a number of ionospheric features that point to forcing from below: tidal influence on total electron content and the F region of the ionosphere; wave signatures in the ionosphere and plasmasphere; a geographically fixed (with the Weddell Sea) ionospheric anomaly; and complex structure in ionosphere F-region density potentially attributable to tropospheric storm systems. These results have been matched by extensive numerical modeling efforts (e.g., the Whole Atmosphere Community Climate Model; WACCM) focused on understanding how atmospheric waves and tides of tropospheric origin propagate through the lower and middle atmosphere, and with the upper-atmospheric general circulation models now also being driven by stratospheric lower-boundary forcing that mirrors the tropospheric inputs, or with input of fitted wave data at approximately 100 km, the boundary of space. Further, the signature of tropospheric forcing has subsequently been observed in upper-thermospheric composition and temperature.
These and other observations and model studies have unequivocally revealed that Earth’s IT system owes a considerable amount of its longitudinal, local-time, seasonal-latitudinal, and day-to-day variability to atmospheric waves that begin near Earth’s surface and propagate into the upper atmosphere. Current estimates indicate that waves propagating upward from the lower atmosphere contribute about as much to the energy transfer in the IT system as does forcing from above in the forms of solar EUV and UV radiation, precipitating particles, resistive heating, and winds driven by magnetospheric convection.
Thermospheric Climate Change
A systematic decrease by several percent per decade in thermosphere mass density is now evident in the record of satellite orbit decay measured since the beginning of the space age. An effect predicted in the 1980s, this change is thought to be largely in response to the increase in atmospheric CO2, which, although it acts to trap infrared heat in the lower atmosphere, acts as a radiative cooler in the upper atmosphere. Thermospheric cooling is therefore an unambiguous signature of a human-influenced change in
the upper atmosphere. Much work is now focused on understanding the impact of climate change on the thermosphere and ionosphere and on identifying signatures in the thermosphere that can be used to assist in monitoring and clarifying the sources and mechanisms of climate change.
The advances over the past decade focus attention on the challenges that are most pressing, both scientifically and practically. Significantly, recent progress includes the greatest advances to date toward achieving the predictive capability needed to safeguard the global technological infrastructure. Distilled from the science goals presented by the survey’s three interdisciplinary panels, the challenges identified below by the survey committee are major areas of ongoing inquiry that provided the context for development of the program of research advocated by this survey for the coming decade. Scientifically important in their own right, the frontiers of heliophysics are also important as a source of practical knowledge for maintaining the operability of the assets of our increasingly technological society. Despite the challenges in studying these systems, the experience of the past decade demonstrates that scientists and engineers in the field have achieved dramatic progress in advancing the state of knowledge, and progress in the areas outlined below can reasonably be expected to be equally impressive. Even though the committee anticipates that key components of these challenges will be resolved in the coming decade, some questions will undoubtedly remain open, and other challenges are likely to emerge.
The magnetic field of the Sun is, directly or indirectly, the driver of much of the dynamics of the heliosphere. Thus, understanding how the solar magnetic field is generated—the dynamo problem—is a key challenge. Despite the complexity of the solar magnetic field at multiple scales, as is evident, for example, in Figure 2.1, the global solar magnetic field exhibits an approximately 22-year periodicity, with the polarity of the field reversing every 11 years. Various solar phenomena, of which number of sunspots is the most familiar, exhibit this same 11-year cycle. Indeed, evidence for decadal-scale periodicities has been found in the luminosities of other Sun-like stars. Scientists know that the twisting and amplification of seed magnetic fields in the Sun’s convective zone, the outer one-third of the Sun, are the source of the solar magnetic field. The resulting solar dynamo has therefore become a prototype for understanding how magnetic fields are generated throughout the universe. Although researchers have shown that quasi-periodic reversals are a natural consequence of dynamo action, they have not yet established why the solar dynamo produces a field that reverses with a nearly regular 11-year period. Moreover, the deep and prolonged minimum in the present cycle, cycle 24, was totally unexpected, demonstrating that understanding of the solar magnetic field has not yet risen to the state of a predictive science. The following remains a primary challenge: SHP-1. Understand how the Sun generates the quasi-cyclical magnetic field that extends throughout the heliosphere.
The complex structure of the light emitted from the corona as shown in Figure 2.1 reflects the corresponding structure of the magnetic field since hot plasma making up the corona very quickly spreads out along lines of force of the magnetic field. Scientists can now calculate this complex magnetic structure with computational models using the ground-based measurements of the magnetic field at the Sun’s visible surface, or photosphere. However, the time variation of the complex magnetic field in the Sun’s tenuous outer atmosphere or corona, which often takes the form of explosive events, is not fully understood and remains at the frontier of heliophysics research. Probing the details of the solar magnetic field at multiple heights in its atmosphere, and at very high temporal and spatial resolution, is the goal of NSF’s Advanced Technology Solar Telescope (ATST).
Active regions are locations where these explosive events are concentrated. There, magnetic energy is released in the form of ejected plasma, electromagnetic radiation, and heat that energize the local plasma. Figure 1.3 shows a series of active regions seen in EUV light from the Solar Dynamics Observatory (SDO) that form a chain across the upper half of the Sun. These arrays of loops emerge from the churning solar atmosphere below and are embedded in plasmas with temperatures of around 107 K. The photosphere, by comparison, is relatively cold at 6,000 K. The mechanisms that produce the hot corona of the Sun and other stars still defy definitive explanation, and determining how this occurs is a high-priority science goal of NASA’s Solar Probe Plus (SPP) mission and also of the Solar Orbiter ESA/NASA joint mission.
How the corona is generated and what physical processes heat the coronal plasma and control its dynamics are not yet understood, thereby defining the second major challenge: SHP-2. Determine how the Sun’s magnetism creates its hot, dynamic atmosphere.
An important result of recent research is the discovery of the critical role that magnetic reconnection plays in modulating the energy flux from the Sun. The turbulent flows of the Sun’s surface twist and distort the coronal magnetic fields, thereby increasing their energy. The magnetic energy accumulates over days, weeks, or perhaps longer. When adjacent magnetic fields pointing in opposite directions become sufficiently strong, the magnetic fields explosively annihilate each other during magnetic reconnection (see Figure 1.3). The released magnetic energy drives high-speed flows, heats the local plasma, and contributes in complex ways to accelerating particles to relativistic energies, producing the intense bursts of energized particles that characterize solar flares. This process occurs almost continuously in the active regions in the corona (see Figure 1.3). As a result, the corona and heliosphere are filled with high-energy radiation, both particle and electromagnetic (UV, X rays, and gamma rays).
The strongest of these reconnection events propel CMEs into the solar wind, and the CMEs steepen into shocks that accelerate ions and electrons to high energy. Figure 2.8 shows a numerical simulation of a CME, illustrating the scale of the ejected field and plasma. When directed Earth-ward, CMEs generate large geomagnetic storms and intense energetic particle events in near-Earth space. The energetic particles from these shocks pose significant threats to human and robotic space exploration.5
The success of simulations in reproducing many of these observations testifies to the maturity of scientific understanding of these significant events. However, even though it is now possible to predict where on the Sun a CME will originate, it is not yet possible to predict CMEs’ timing, speed, energy, or momentum, nor is there full scientific understanding of how a CME converts so much of its energy into particle radiation. The planned SPP and SO missions will provide crucial information related both to the reconnection process and to CME initiation. These issues present a third challenge: SHP-3. Determine how magnetic energy is stored and explosively released and how the resultant disturbances propagate through the heliosphere.
The heliopause, where the Sun’s extended atmosphere ends and the galactic medium begins, is a region that is rich in unique and unexplored physics. It is also the boundary that, in part, controls the penetration of high-energy galactic cosmic rays into near-Earth space. Interstellar neutrals are crucial to the outer heliosphere because they stream into the heliosphere unimpeded by the heliospheric magnetic field and dump energy into the solar wind. They are the dominant energy source of the outer heliosphere. A revolution in understanding of the outer heliosphere is unfolding as the Voyager spacecraft provide the first in situ data from this region and NASA’s Interstellar Boundary Explorer (IBEX) and Cassini missions use energetic neutral atoms to remotely sense processes occurring in the same region (see Figure 2.3).
During the next decade, the Voyager spacecraft are expected to exit Earth’s heliosphere, entering interstellar space. For the first time, operating spacecraft will enter into our local galaxy and gather local
5 National Research Council, Space Radiation Hazards and the Vision for Space Exploration: Report of a Workshop, The National Academies Press, Washington, D.C., 2006.
FIGURE 2.8 In this ultra-high-resolution numerical simulation of a reconnection-initiated CME and eruptive flare, the white contours indicate high current densities. Note the vertical flare current sheet below the erupting plasmoid. The plasmoid undergoes a sudden acceleration coincident with the onset of the flare (reconnection in this sheet). 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.
measurements from the interstellar medium—a truly historic event. The coming decade will therefore provide critical understanding of the heliospheric boundary regions and the processes that shape the interaction of the heliosphere with its local galactic medium. This motivates a fourth science challenge: SHP-4. Discover how the Sun interacts with the local interstellar medium.
While the broad view of how reconnection takes place and drives convection in the magnetosphere is now well established, the underlying physics of magnetic reconnection in the collisionless regime of the magnetosphere is not yet understood well enough to enable prediction of when, where, and how fast this process will occur and how it contributes to mass, energy, and momentum transport. NASA’s Magne-
tospheric Multiscale Mission (MMS) is designed to carry out in situ measurements in the magnetosphere to establish the mechanisms that control how magnetic field lines reconnect. The results are expected to have profound implications for understanding reconnection within the heliosphere and in astrophysical settings throughout the universe. They are also highly relevant to understanding reconnection events in tokomak plasmas and in laboratory-based reconnection experiments. The centrality of reconnection in such diverse settings motivates the following primary challenge: SWMI-1. Establish how magnetic reconnection is triggered and how it evolves to drive mass, momentum, and energy transport.
Magnetic reconnection in the magnetotail drives convection that carries energetic particles toward Earth, where they are injected and trapped in orbits around Earth to form the extraterrestrial ring current, a region of relatively high energy ions and electrons that is most intense near the equator at distances of 3 to 7 RE from Earth’s center (see Figure 2.6). The outer radiation belt therefore overlaps the orbit radius of geostationary satellites (6.6 RE) where the vast majority of communications and Earth-monitoring spacecraft reside. These satellites can be damaged by energetic radiation belt electrons whose flux is strongly enhanced during intense solar activity and the resultant storms in the magnetosphere. Understanding charged particle acceleration, scattering, and loss, which control the intensification and depletion of the radiation belts, is therefore a priority of solar and space physics.
The high variability of the radiation belts is evident in Figure 2.9, which shows a near-equatorial satellite view of energetic electron fluxes. The acceleration of particles in the radiation belts is believed to arise from a combination of compression as particles move from the weak magnetic field region in the distant magnetotail into the region of high magnetic field near Earth and the interaction with intense waves generated in the radiation belts themselves. NASA’s Radiation Belts Storm Probes (RBSP; renamed the Van Allen Probes) mission is designed to determine the mechanisms that control the energy, intensity, spatial distribution, and time variability of the radiation belts. To understand the response of the magnetospheric system to driving by the solar wind, the following challenge must be addressed: SWMI-2. Identify the mechanisms that control the production, loss, and energization of energetic particles in the magnetosphere.
At around 100-km altitude, the atmosphere starts to transition from being neutrally dominant to being dominated by charged particles. The ionosphere, which is often thought to be the inner boundary of the magnetosphere, overlaps with the thermosphere. At these altitudes, the neutral density is about 1,000 times larger than the ion density, but the electromagnetic forces on the ions are significantly larger than the forces on the neutrals, so they become more and more important as the altitude increases. This region of the atmosphere is quite thick, being a couple of hundred kilometers in altitude, in comparison to the troposphere, which is only 10 km thick, but it pales in comparison to the vast space carved out by the magnetosphere, which extends out 10 to 100 Earth radii. If one were to calculate the whole mass of all of the particles in the magnetosphere, it would be about an order of magnitude less than the mass of the ionosphere, even though the ionosphere is so much smaller. This is because the density of the ionosphere is so much larger than the near-vacuum of the magnetosphere.
Magnetic field lines converge in the polar regions in the ionosphere. The magnetospheric convection cycle described above maps to middle and high latitudes in the ionosphere where the resulting flows transport and mix plasma and the more dense neutral gas. Ionospheric conductance facilitates field-aligned currents that produce resistance to the convection flows to the magnetosphere. The closure of these currents in the ionosphere drives neutral-gas winds and expels ions upward along the magnetic field.
During magnetic storms the intense upwelling of ions from the ionosphere into the magnetosphere is so strong that ionospheric O+ can dominate the high-altitude ion pressures. This alters magnetospheric dynamics by modifying magnetic reconnection on both the dayside and the nightside. Figure 2.10 shows simulations of the magnetospheric response to changes in the IMF which, when O+ outflow is properly included, results in the repeated onset of magnetic reconnection events that intensify the aurora and asso-
FIGURE 2.9 Energetic electron variability as measured during the 14-month Combined Release and Radiation Effects Satellite (CRRES) mission lifetime extending past geosynchronous orbit, 6.6 Earth radii (RE) from Earth’s center (22,000 miles above sea level), where spacecraft remain overhead as Earth rotates, a heavily populated orbit; data from July 1990 to October 1991, the maximum of solar cycle 22. A new radiation belt with energy greater than 13 million electron volts was created on a timescale of minutes in response to a strong interplanetary shock caused by a coronal mass ejection. This new radiation belt persisted until 1994. No solar wind observations preceding the arrival of the shock were available during this event due to absence of an L1 measurement (M. Blanc, J.L. Horwitz, J.B. Blake, I. Daglis, J.F. Lemaire, M.B. Moldwin, S. Orsini, R.M. Thorne, and R.A. Wolfe, Source and loss processes in the inner magnetosphere, Space Science Review 88(1-2):137-206, 1999). SOURCE: Reprinted from M.K. Hudson, B.T. Kress, H.-R. Mueller, J.A. Zastrow, and J.B. Blake, Relationship of the Van Allen radiation belts to solar wind drivers, Journal of Atmospheric and Solar-Terrestrial Physics 70(5):708-729, 2008, copyright 2008, with permission from Elsevier.
FIGURE 2.10 Multifluid MHD simulation results of substorm initiation without (left-hand panels) and with (right-hand panels) O+ outflow from the ionosphere. The colors indicate the densities of the two species in the simulations. The left-hand panels show only H+, the only species in the simulation, whereas the right-hand panels show the ionospheric O+, which is added to the H+. The red lines in each panel show magnetic field lines in the region of interest. In the upper panels, both simulations show a plasmoid release at 2 hours 50 minutes into the simulation, as indicated by the looped field lines beyond ~20 Earth radii. In both simulations, this plasmoid will depart rapidly downtail. In the lower panels (~2 hours later), the magnetosphere has stabilized in the simulation without O+, while the result with O+ shows a second plasmoid release in the region accessible to the O+. The addition of O+ as a distinct fluid with a significant contribution to the mass density makes the magnetosphere repetitively unstable. SOURCE: M. Wiltberger, W. Lotko, J.G. Lyon, P. Damiano, and V. Merkin, Influence of cusp O+ outflow on magnetotail dynamics in a multifluid MHD model of the magnetosphere, Journal of Geophysical Research—Space Physics 115:A00J05, 2010.
ciated ionospheric currents. These events inject plasma stored in the geomagnetic tail Earth-ward. This plasma acts as the seed population for the radiation belts and drives the plasma waves that are responsible for the scattering and loss of radiation belt electrons. In addition, storm-time ionospheric heating and convection produce large changes in the neutral and plasma densities that alter ionospheric conductances on a global scale.
Since the feedback of the ionosphere and thermosphere as a source of plasma and dissipation for the magnetosphere has such profound effects, the evolution of the ionosphere and magnetosphere must be studied as a globally coupled system. Thus, a key challenge is as follows: SWMI-3. Determine how coupling and feedback between the magnetosphere, ionosphere, and thermosphere govern the dynamics of the coupled system in its response to the variable solar wind.
Earth’s magnetosphere is a prototype of a universal plasma system: an object with a global magnetic field that is subjected to an externally flowing plasma and forms a magnetosphere. Five other planets in Earth’s solar system have magnetospheres: Mercury, Jupiter, Saturn, Uranus, and Neptune. Ganymede, one of Jupiter’s satellites, also has its own tiny magnetosphere embedded within Jupiter’s giant one. Although planetary systems exhibit analogous structures, the contrasting dynamics, boundary conditions, and magnetic fields make their detailed study of unique importance for testing theories and models.
Jupiter’s moon Io, deep within the enormous Jovian magnetosphere, is a copious source of neutral gas, which, upon ionization, is a dominant drag force on the rapidly co-rotating magnetic field of the planet. Similarly, the moons of Saturn, particularly Titan and Enceladus, are major sources of plasma that affects the dynamics of Saturn’s magnetosphere. A key enigma of the Saturnian system is the source of the regular, 10-hour 46-minute periodicity in Saturn’s radio emissions, which differs from its rotation period by 6 minutes. This difference, discovered in data from the Cassini and Voyager spacecraft, remains unexplained. The magnetospheres of Uranus and Neptune are largely unexplored but present unique cases that will likely further challenge scientific understanding. Finally, the tiny magnetosphere of Mercury is an extreme example of a magnetospheric system because it possesses no ionosphere. In such a situation the coupling processes that operate are radically different. Thus, these other systems present a suite of vastly different configurations. The opportunity to test current theories and models on these widely varying systems motivates a fourth challenge: SWMI-4. Critically advance the physical understanding of magnetospheres and their coupling to ionospheres and thermospheres by comparing models against observations from different magnetospheric systems.
Understanding ionosphere-thermosphere interactions is a major area of inquiry, especially during geomagnetic storms. The intense energy input from the magnetosphere, reaching up to terawatts, typically occurs in regions spanning less than 10 degrees in latitude but during storms is redistributed throughout the polar regions and down to middle latitudes over timescales from tens of minutes to hours. High-latitude heating (mainly below 200-km altitude) causes N2-rich air to upwell. Strong winds driven by this heating transport N2 equatorward. The mixing with ambient atomic oxygen produces dramatic changes in the ratio between atomic oxygen and molecular nitrogen. This global response was first discovered more than a decade ago, but researchers still cannot explain why it takes several hours for the global thermosphere to “inflate” after the high-latitude heating begins.
The ionospheric plasma also experiences major reconfigurations during storms as magnetospheric convection drives the mixing of low- and high-density regions of the ionosphere. Figure 3.3 shows an example of a plasma plume extending over thousands of kilometers that formed during the main phase of a geomagnetic storm. Redistributions of plasma by large-scale electric fields also occur in the middle and lower latitudes. At the onset of a storm, electric fields penetrate from the polar region and lift the equatorial ionosphere, depleting the equatorial density and producing anomalously high ionospheric densities on field lines that connect the high-altitude equator with ionospheric latitudes north and south of the equator. Convection in the polar regions also drives large-scale thermospheric winds that in turn carry ionospheric plasma across the polar regions to lower latitudes.
The storm response of the ionosphere and thermosphere produces structures over a wide range of time and spatial scales. To understand the storm-time behavior of this system, researchers must address the following science challenge: AIMI-1. Understand how the ionosphere-thermosphere system responds to, and regulates, magnetospheric forcing over global, regional, and local scales.
An important element of the dynamics of the IT system is the transfer of energy and momentum between the plasma and neutral components of the system and the role that electric and magnetic fields serve in accentuating and sometimes moderating this interchange. The pathways through which ions and neutrals interact are of course fundamental to space physics, given that they occur at all planets with atmospheres, at comets, and within the magnetospheres of Jupiter and Saturn. For example, in Earth’s ionosphere at an altitude from 100 to 130 km the collisions between ions and electrons and neutrals enable current to flow across the local magnetic field, which facilitates closure of currents flowing along magnetic fields from the magnetosphere. The proper description of these cross-field currents requires the development of an accurate model of the plasma “conductivity,” yet the dynamics of ionospheric conductivity are among the most poorly quantified parameters of the IT system. Earth’s equatorial region is a rich laboratory for the investigation of plasma-neutral coupling in the presence of a magnetic field. The behavior can be extraordinarily complex: plasma-neutral collisions and associated neutral winds drive turbulence that cascades to very small spatial scales and regularly disrupts communications. The chemical interaction of a variety of ion species further complicates the dynamics.
A different suite of interactions occurs at middle latitudes. Spontaneous airglow emissions at 6,300 Å exhibit waves propagating to the south-west. They are thought to originate as neutral density waves at high latitudes which then interact with the mid-latitude ionosphere to create the structures, but their occurrence is curiously unrelated to levels of magnetic activity.
Thus, plasma-neutral coupling plays a critical role in ionospheric dynamics across the full range of latitudes. Researchers must therefore address the following challenge: AIMI-2. Understand the plasma-neutral coupling processes that give rise to local, regional, and global-scale structures and dynamics in the AIM system.
Numerous recent observations and simulations show that the IT system owes much of its longitudinal, local-time, seasonal, and even day-to-day variability to meteorological processes in the troposphere and stratosphere. The primary mechanism through which energy and momentum are transferred from the lower atmosphere to the upper atmosphere and ionosphere is through the generation and propagation of waves. The absorption of solar radiation (e.g., by tropospheric H2O and stratospheric O3) excites a spectrum of thermal tides. Figure 2.11 shows the spatial structure in daytime convective clouds that is believed to introduce longitudinal structure in the ionosphere, seen in Figure 2.11 in ultraviolet emissions. Surface topography and unstable shear flows excite planetary waves and gravity waves extending from planetary to very small (~tens to hundreds of kilometers) spatial scales and having periods from tens of days down to minutes. Convective tropospheric weather systems radiate additional thermal tides, gravity waves, and other classes of waves.
Those waves that propagate vertically grow exponentially with height into the more rarified atmosphere. Some of the waves spawn additional waves and turbulence. Figure 2.12 shows sodium layer observations revealing amazing wave structures at the base of the thermosphere, illustrating the rich spectrum of dynamics that occurs. Although the presence and the importance of waves are not in dispute, the relevant coupling processes operating between the neutral atmosphere and ionosphere involve a host of multiscale dynamics that are not understood at present. This leads to another major scientific challenge: AIMI-3. Understand how forcing from the lower atmosphere via tidal, planetary, and gravity waves influences the ionosphere and thermosphere.
FIGURE 2.11 Top: Mean 1984-2009 January daytime convective cloud amount in percentage from ISCCP-D2. Blue indicates 10-15 percent, yellow/green indicates approximately 8 percent, and red indicates 0-4 percent. Bottom: Average ionospheric equatorial densities derived from TIMED GUVI observations of 135.6-nm OI emissions showing unexpected wave structure in ionospheric densities on the same longitude scales as the tropospheric pressure waves. The double-banded structure is due to the neutral wind dynamo at the magnetic equator which transports equatorial plasma north and south of the equator. SOURCE: Top: The International Satellite Cloud Climatology Project (ISCCP) D2 data/images (described in W.B. Rossow and R.A. Schiffer, Advances in understanding clouds from ISCCP, Bulletin of the American Meteorological Society 80:2261-2288, 1999) were obtained in January 2005 from the ISCCP website (available at http://isccp.giss.nasa.gov and maintained by the ISCCP research group at the NASA Goddard Institute for Space Studies, New York, N.Y.). Bottom: S.L. England, X. Zhang, T.J. Immel, J.M. Forbes, and R. Demajistre, The effect of non-migrating tides on the morphology of the equatorial ionospheric anomaly: Seasonal variability, Earth, Planets and Space 61:493-503.
FIGURE 2.12 High-resolution sodium lidar observations of breaking gravity waves at the base of the thermosphere. A 6-meter, zenith-pointing telescope comprising a spinning mercury mirror was coupled to a sodium lidar system and revealed amazing detail in MLT instability structures, identified as Kelvin-Helmholtz billows evident at the base of the sodium layer, at a temporal resolution of 60 milliseconds and a spatial resolution of 15 meters. SOURCE: T. Pfrommer, P. Hickson, and C.-Y. She, A large-aperture sodium fluorescence lidar with very high resolution for mesopause dynamics and adaptive optics studies, Geophysical Research Letters 36:L15831, doi:10.1029/2009GL038802, 2009. Copyright 2009 American Geophysical Union. Reproduced by permission of American Geophysical Union.
The release of greenhouse gases (e.g., CO2 and CH4) into the atmosphere is changing Earth’s surface climate by warming the lower atmosphere; these gases are also changing geospace climatology by cooling the upper atmosphere. In the lower atmosphere, the opacity of greenhouse gases to infrared radiation traps energy by capturing the radiant infrared energy from Earth’s surface and transferring it to thermal energy via collisions with other molecules. In the thermosphere, however, where intermolecular collisions are less frequent, greenhouse gases promote cooling by acquiring energy via collisions and then radiating this energy to space in the infrared. This well-understood role of CO2 as an effective radiator of energy in the upper atmosphere has produced a systematic decrease in thermospheric mass density by several percent per decade near the 400-km altitude. This systematic decrease follows from the record of satellite orbit decay measured since the beginning of the space age (Figure 2.13).
There are two other consequences of climate change for the ionosphere and thermosphere. First, changes in tropospheric weather patterns and atmospheric circulation may alter the occurrence of ionospheric instabilities triggered by tropospheric gravity waves propagating into the upper atmosphere. This change will affect the prevalence of the resulting ionospheric irregularities. Second, continued cooling of the thermosphere will reduce satellite drag, thereby increasing orbital debris lifetimes, and will lower the effective ionospheric conductivity. The latter change will alter global currents in the magnetosphere-ionosphere system and therefore fundamentally alter magnetosphere-ionosphere coupling. The survey committee, therefore, identifies the following science challenge: AIMI-4. Determine and identify the causes for long-term (multi-decadal) changes in the AIM system.
FIGURE 2.13 Long-term mass density variations as determined from satellite drag observations normalized to 400-km altitude demonstrating a consistent long-term cooling of the thermosphere consistent with increased CO2, which at these altitudes cools the atmosphere by providing a mechanism to radiate energy at infrared wavelengths—the same property that traps heat lower in the atmosphere. SOURCE: J.T. Emmert, J.M. Picone, and R.R. Meier, Thermospheric global average density trends, 1967-2007, derived from orbits of 5000 near-Earth objects, Geophysical Research Letters 35:L05101, doi:10.1029/2007GL032809, 2008. Copyright 2008 American Geophysical Union. Reproduced by permission of American Geophysical Union.
Achievement of the survey committee’s four key science goals (see Chapter 1) for the coming decade requires addressing the 12 science challenges, discussed above and also listed in Table 2.1, for the three subdisciplines of solar and space physics. In turn, addressing science challenges requires optimal use of existing assets, as well as initiation of new programs that will drive future discovery. Chapters 4, 5, and 6 outline the survey committee’s recommendations for the upcoming decade and discuss how they may be implemented by NSF and NASA. The survey committee’s recommendations were informed by a recognition that the interconnected nature of the science of solar and space physics requires a research effort that spans the entire front of science challenges. New missions, as described in Chapter 4, can be carefully chosen to address the most pressing of these science challenges. It will be evident, however, that in the foreseeable future nearly half of the science challenges are not targeted by any new heliophysics mission.
The survey committee views the Explorer line as a critical asset for broadening the field of inquiry to include questions not addressed by upcoming or recommended missions. In addition, the rich array of existing assets of NASA, NSF, NOAA, and DOD, as well as the use of non-science space platforms, also facilitates scientific discovery in solar and space physics provided that these assets are adequately supported and that research and analysis efforts are sustained. The central importance of L1 in situ observations of the
TABLE 2.1 Solar and Space Physics Decadal Science Challenges
|The Sun and Heliosphere|
|SHP-1||Understand how the Sun generates the quasi-cyclical magnetic field that extends throughout the heliosphere.|
|SHP-2||Determine how the Sun’s magnetism creates its hot, dynamic atmosphere.|
|SHP-3||Determine how magnetic energy is stored and explosively released and how the resultant disturbances propagate through the heliosphere.|
|SHP-4||Discover how the Sun interacts with the local interstellar medium.|
|Solar Wind-Magnetosphere Interactions|
|SWMI-1||Establish how magnetic reconnection is triggered and how it evolves to drive mass, momentum, and energy transport.|
|SWMI-2||Identify the mechanisms that control the production, loss, and energization of energetic particles in the magnetosphere.|
|SWMI-3||Determine how coupling and feedback between the magnetosphere, ionosphere, and thermosphere govern the dynamics of the coupled system in its response to the variable solar wind.|
|SWMI-4||Critically advance the physical understanding of magnetospheres and their coupling to ionospheres and thermospheres by comparing models against observations from different magnetospheric systems.|
|AIMI-1||Understand how the ionosphere-thermosphere system responds to, and regulates, magnetospheric forcing over global, regional, and local scales.|
|AIMI-2||Understand the plasma-neutral coupling processes that give rise to local, regional, and global-scale structures and dynamics in the AIM system.|
|AIMI-3||Understand how forcing from the lower atmosphere via tidal, planetary, and gravity waves influences the ionosphere and thermosphere.|
|AIMI-4||Determine and identify the causes for long-term (multi-decadal) changes in the AIM system.|
interplanetary medium in particular motivates the continuation of these observations to support a broad range of research in solar and space physics.
Finally, the nearly explosive growth in the ability to model complex phenomena in solar and space physics with realistic numerical simulations suggests that the field is on the cusp of greatly expanded predictive power and fundamental understanding. The advanced state of theory and simulation also provides a powerful opportunity to couple efforts in this area with observations, which will always remain limited in key aspects, to realize the full potential of the observations and their implications for understanding the underlying physical processes that they reflect. Reaching scientific closure and advancing predictive understanding therefore depend critically on robust support for theory and modeling across the spectrum of science challenges.
In summary, the program of solar and space physics research recommended in this report is specifically designed to make the most effective use of the nation’s resources in a program that maximizes scientific advances and furthers understanding of the space weather threats to a society that is increasingly reliant on technologies that are vulnerable to solar and geospace activity.