E

Progress for Science Challenges in 2013 Heliophysics Decadal Survey

Chapter 2 provides a brief overview on a selection of science highlights and advances in research tools from the first half of the decade period covered by the 2013 decadal survey (NRC, 2013), and this appendix provides more details about these science highlights and how they relate to the 12 science challenges provided in the decadal survey. The decadal survey identified four key science goals and, subsequently, the science challenges that flow from these goals from the three primary branches of heliophysics—atmosphere-ionosphere-magnetosphere interactions (AIMI), solar wind-magnetosphere interactions (SWMI), and solar and heliospheric physics (SHP). These are illustrated in Table E.1. Substantial scientific and technical progress has been made in each of the science challenge areas, but for brevity, only some of the recent progress results are presented here as organized by the three heliophysics branches.

E.1 SOLAR AND HELIOSPHERIC PHYSICS

As mentioned in Chapter 2, scientists have made great advances over the past 6 years in understanding the dynamic solar magnetic field and how it shapes the whole of the space environment, ranging from the Sun to far beyond the planets. This appendix provides more details on selected findings for the highlights noted in Chapter 2.

• Close to the Sun, the Parker Solar Probe has set the record for closest approach to the Sun; its ongoing mission is to sample solar coronal particles and the solar electromagnetic field to understand coronal heating, solar wind acceleration, and the formation and transport of solar energetic particles.
• Far from the Sun, measurements by the Voyager spacecraft (that exited the heliosphere in 2012 and 2018, respectively), combined with Interstellar Boundary Explorer (IBEX) and Cassini data, transformed our knowledge of the outer boundary of the heliosphere, placing outer-heliospheric science solidly among the other fast-developing branches of heliophysics.
• Fine-scale High Resolution Coronal Imager (Hi-C) rocket imagery combined with global-scale Solar Dynamics Observatory (SDO) images of the solar corona revealed the small-scale signatures of the reconfiguration of the magnetic field and of Alfvén waves running through that magnetic field. Combined SDO,

• the Interface Region Imaging Spectrograph (IRIS), and Hinode observations have furthered our understanding of the mechanisms that extract energy from the magnetic field either in the form of heat or through explosive eruptions, and also of the mechanisms that transport energy between different wave types and physical domains.

• Technical advances in computational methods and infrastructure provide critically needed insights into both the source of the solar magnetism and the formation of, and explosive instabilities in, the globally connected solar atmosphere as discovered with SDO observations. These newly developed computer models can be applied to new data, but also retrospectively to archival data so that we can efficiently learn from decades-long historical archives.

• Machine learning and big data techniques are helping us move towards improved multi-day predictions of space weather, while increasingly realistic computer models in regular use at the NASA Goddard Space Flight Center Community Coordinated Modeling Center are now able to work across multiple physical regimes as they simulate quantities that can be directly compared to real-world observations.
• Detailed observations of space weather throughout the solar system helped planetary scientists and stellar astrophysicists understand the range of possibilities for stellar wind conditions and, thus, how these conditions influence exoplanets.

These exciting new results from both space-based missions and ground-based instruments demonstrate the importance of diverse and complementary observing capabilities, both remote and in situ, in order to understand fundamental physical processes. They also emphasize the importance of accessible, standardized archives and the need to develop community-usable numerical tools for modeling and analysis, sometimes using artificial intelligence, including data-driven full-system models that include the space weather forecasting systems.

Decadal Survey Challenge SHP-1:
Determine How the Sun Generates the Quasi-Cyclical Variable Magnetic Field that Extends Throughout the Heliosphere

Inside the Sun, hot, ionized gas (“plasma”) moves in a complex hierarchy from small-scale to globalscale flows to create and evolve magnetic fields in a process called the solar dynamo (Section 1.2). Where this field breaches the surface, it can form sunspots embedded in active regions, becoming part of the solar variability that drives space weather. Measurements of these subsurface movements are critical for understanding the dynamo and forecasting the 11-year solar cycle. Results from two methods are combined to infer plasma flows in the solar interior. One is to apply helioseismology—measuring the properties of sound waves running through much of the solar interior to deduce the conditions of the gas that they traverse. Another is tracking of magnetic elements as they move across the surface based on high-resolution magnetographs and (for sunspots) direct imaging. Some sound waves travel the Sun’s interior extensively. From this interior wave propagation, we can gain insight into the magnetic conditions on the far side of the Sun’s surface invisible from Earth by observing the multitude of waves on the Earth-facing side of the Sun. This enables predictions of the magnetism of the Sun’s far side about a week before it spins around to become the Earth-facing side (e.g., Arge et al., 2013; Kim et al., 2019). These more complete magnetic maps form the foundation for models of the solar wind sources as needed for space weather forecasts.

Using helioseismology, Zhao et al. (2013) detected an equatorward flow in the Sun’s interior between 0.83 and 0.91 solar radii, sandwiched between poleward flows below and above. This flow (illustrated in Figure E.1) may be a key ingredient of the dynamo as it can transport magnetic field lines both in the deep interior and near the surface. However, the rise of bundles of magnetic field to the solar surface was found to be much slower than expected from some models (Birch et al. 2016), meaning more theoretical work is needed to better understand how flux tubes rise through the surrounding gas.

In order to better understand the dynamo, these findings are combined with computer models that incorporate flow-field interactions as much as computing resources allow. Valuable progress has also been made through novel applications of earlier generations of ideas. One promising example is the coupling of geometrically simplified, axially symmetric dynamo models for the subsurface layers with surface flux transport models. Observed properties of the magnetic field emerging onto, and then moving across, the solar surface are used in machine learning methods to statistically describe the magnetic field that couples the interior and surface. Such modeling reproduces many of the properties of cycle-to-cycle variations and

shows promise in forecasting at least one sunspot cycle ahead (e.g., Lemerle and Charbonneau, 2017). This modeling work suggests much, perhaps most, of the cycle-to-cycle variations result from convective flows that nudge emerging sunspot regions away from their average orientation, thereby pushing the solar dynamo into an irregular mode (Karak and Miesch, 2017). Another source of cycle variability has been found in changes in the meridional circulation, which serves as a large-scale “conveyor belt” that transports the field (Upton and Hathaway, 2014). These findings emphasize the importance of continuous and continued observing of field and waves at the solar surface.

Decadal Survey Challenge SHP-2:
Determine How the Sun’s Magnetic Field Creates Its Dynamic Atmosphere

The last 6 years saw numerous advances in what we know of the coupling between the hot solar corona and the heliosphere, driven by both observations and numerical models that span the smallest observable scales on the Sun to the entire heliosphere. Scientists applied a systems approach, by driving numerical models with observational data, to study the corona and heliosphere as a whole.

Magnetic Alfvén waves are thought to play a role in heating the solar corona and in driving the solar wind. These waves are initially excited in the solar interior from where they travel outward along magnetic field lines. Different types of waves travel along distinct paths from the solar interior through the solar atmospheric regimes (Zhao et al., 2016, Morton et al., 2019). New computational results also show how Alfvén waves can transport energy into the solar chromosphere and corona, there to be converted into heat.

Properties of Alfvén waves, which are difficult to detect because they are faint and have small amplitudes compared to instrumental resolution, were quantified by combining state-of-the-art spectroscopy and advanced modeling (Okamoto et al., 2015; Goossens et al., 2014; van Ballegooijen et al., 2014; Lionello et al., 2014; van der Holst et al., 2014; Kerr et al., 2016; Arber et al., 2016; Cranmer and Woolsey, 2015; Soler et al., 2015). This new generation of models integrates small-scale processes, such as ion-neutral interactions and shocks, with the global-scale nature of the highly interconnected system on the Sun (e.g., Tadesse et al., 2011). Such models are critical for understanding the observations from high-resolution, high-cadence spectrographic observations from IRIS and future observations from DKIST.

High-cadence observations of the entire Earth-facing side of the Sun by SDO and multi-point observations of the sides and far side of the Sun by the Solar Terrestrial Relations Observatory (STEREO) spacecraft revealed that the corona is a highly interconnected system (Tadesse et al., 2011). Changes in one location, such as by an active region emerging onto the solar surface, can trigger the magnetic field to restructure itself elsewhere (Zhang and Low, 2001, 2002; Longcope et al., 2005), sometimes explosively in flares and coronal mass ejections (CMEs) (Fu and Welsch, 2016, Balasubramaniam et al., 2011; Schrijver and Title, 2011), or on large scales by creating and closing coronal holes (Karachik et al., 2010).

There has also been significant progress in computer simulations that reach from the solar interior to the corona (e.g., Amari et al., 2014; Fisher et al., 2015; Rempel et al., 2017; Wyper et al., 2017; Cheung et al., 2019). For example, the physical domain in the model developed by Cheung et al. (2019) encapsulates the solar convection zone, photosphere, chromosphere, transition region, and corona, beginning 7,500 km below the solar surface and extending up to 42,000 km above it. These comprehensive models help scientists understand the heating of the solar atmosphere as well as the mechanisms that trigger flares and CMEs.

Decadal Survey Challenge SHP-3:
Determine How Magnetic Energy Is Stored and Explosively Released

The explosive release of magnetic energy creates a variety of phenomena that include flares, CMEs, shocks and energetic particles, and magnetospheric (sub-storms) and aurorae at Earth. New observations and models of fast magnetic reconnection, which leads to a sudden change in the topology of the magnetic field, have shown that this universal and fundamental process occurs on scales both large and small all over the heliosphere and is similarly expected to occur in other planetary systems.

Magnetic reconnection in the solar atmosphere has long been elusive owing to the high speed and small spatial scale on which it occurs, but newer generations of instruments are now revealing (1) reconnection-driven heating (based on Hi-C sounding rocket images; see Cirtain et al., 2013), (2) outflows (seen in images taken by the SDO and the Ramaty High Energy Solar Spectroscopic Imager [RHESSI]; see Su et al., 2013), (3) shocks (using the Jansky Very Large Array [JVLA]; see Chen et al., 2015), (4) electron beams from the energy release sites (JVLA; see Chen et al., 2013), and (5) extended sources of microwave flare emissions (EOVSA; see Gary et al., 2018). An example of one of the largest flares in this solar cycle is given in Figure E.2.

Indirect observations of magnetic energy release also grew considerably, along with the advent of big data. In the last 5 years, modern solar and space physics instruments took more data than ever before (e.g., SDO, which takes 1.5 terabytes of data a day) and with higher data rates than ever before (e.g., the Magnetospheric Multiscale Mission (MMS), which takes data as fast as every millisecond). In order to effectively analyze such massive data volumes, scientists introduced machine learning to efficiently and affordably identify features and even to forecast events (LeCun et al., 2015), including solar flares (e.g., Bobra and Couvidat, 2015; Nishizuka et al., 2018; Panos et al., 2018), solar energetic particle (SEP) events

(e.g., Winter et al., 2015), and particle density enhancements in Earth’s magnetosphere (e.g., Zhelavskaya et al., 2016; Bortnik et al., 2016).

Decadal Survey Challenge SHP-4:
Discover How the Sun Interacts with the Local Galactic Medium and Protects Earth

Over the past 6 years, there have been several surprises about the boundary between the immense magnetic bubble containing our solar system and the surrounding interstellar medium. These discoveries are also important for astrophysics in general because our local heliosphere is the only astrosphere that we can study up close to learn about fundamental processes that are also likely to occur elsewhere.

Voyagers 1 and 2 are currently exploring the local interstellar medium (LISM) outside the heliosphere, from its particle makeup to its turbulence (Burlaga et al., 2015). It turns out that the LISM is far from quiet and pristine (Gurnett et al., 2015) and is strongly influenced by the heliosphere. The galactic cosmic rays (GCRs) there are not isotropic, which might be due to the draped interstellar magnetic field (Rankin et al., 2019). Shocks measured in the LISM have different properties than those in the outer heliosphere, indicating that the interactions of charged and neutral particles are important. Voyager measurements have for the first time revealed how effectively the heliosphere shields us from GCRs—for example, 75 percent in the Voyager 1 direction (Cummings et al., 2016) (see Figure E.3).

Voyager 1 observations indicate that the heliopause (the surface between the solar wind on the inside and the interstellar gas on the outside) is not a perfect boundary (Parker, 1961), but is instead porous, possibly the result of reconnection, where turbulence may be important (Swisdak et al., 2013; Grygorczuk et al., 2014; Florinski, 2015; Schwadron and McComas, 2013). Comparison of data from the two Voyagers indicates that the conditions at the heliosphere’s flanks are substantially different, which is now being further investigated with models.

Anomalous cosmic rays are particles accelerated somewhere inside the boundary of the heliosphere. Until the Voyager observations, they were thought to be accelerated where the solar wind goes through a slow-down shock well ahead of the heliopause, but new data suggest that this acceleration may occur much closer to that interface.

The very shape of the heliosphere remains a mystery (Figure E.4). Older work by Baranov and Malama (1993) suggested a comet-like shape. More recent computer models that include magnetic fields (Opher et al., 2015; Drake et al., 2015) suggest, in contrast, that the tension force of the field could help shape the solar wind between the termination shock and heliopause into two jet-like structures. However, particle measurements with the Cassini spacecraft have led others to argue that the heliosphere is, instead, tailless (Dialynas et al., 2017).

Scientists also advanced their understanding of how the solar magnetic field affects the innermost heliosphere by using observational data from NASA’s IRIS and SDO to drive advanced models (Mikic et al., 2018; Yeates et al., 2018; Jin et al., 2017; Jin et al., 2018). These new models were validated by comparing their results to observations of a Sun-grazing comet deep within the solar corona (Downs et al., 2013), by Parker Solar Probe observations of the innermost heliosphere, and by space weather conditions near Earth.

Our knowledge of how a rocky planet like Earth responds to the magnetized solar wind provides a laboratory to study conditions at other planets, and even at exoplanets. For example, the in situ study of the solar wind at Mars by the Mars Atmosphere and Volatile Evolution (MAVEN) mission provides insights into these processes that extend beyond the parameter range seen at Earth: present-day Mars has no active dynamo, so Mars’s atmosphere is more directly exposed to the solar wind. Mars-orbiting observatories and Sun- and solar-wind observing spacecraft together reveal Mars’s atmospheric loss processes and their dependence on the solar extreme ultraviolet (EUV) irradiance (Dong et al., 2017). Measuring, modeling, and understanding these processes at Mars, Earth, and Venus help us understand how atmospheres would have responded billions of years ago, when EUV irradiance and CME activity would have been much stronger, at a phase during which Mars appears to have lost much of its atmosphere and oceans, and when life emerged on Earth.

Finally, in the last 6 years, data and models from solar and space physics helped characterize the space weather environment of exoplanets around other, relatively Sun-like (G-, K-, and M-type) stars. Models show how planets in extrasolar systems can experience extreme stellar wind regimes compared to those at present-day Earth (e.g., Garraffo et al., 2017). It is unclear whether some such planets can even retain their atmospheres—especially because their magnetospheres can change rapidly in structure (e.g., Cohen

et al., 2014; Cohen et al., 2015; Airapetian et al., 2017; Dong et al., 2017; Wood, 2018). In addition, Sun-like stars can release CMEs with a range of velocities and masses different from the present-day Sun (e.g.,Aarnio et al., 2012; Kay et al., 2016; Moschou et al., 2017; Alvarado-Gómez et al., 2018).

E.2 SOLAR WIND-MAGNETOSPHERE INTERACTIONS

The combination of solar EUV radiation and solar wind conditions, including transients related to stream structure and CMEs, produce a variety of conditions in Earth’s space environment. Since the publication of the decadal survey, significant progress has been made in understanding how these conditions come about, involving both the externally driven and the internally shaped processes by which the solar radiation and solar wind couple to a planetary magnetosphere, and how these processes transport mass and energy into and within the magnetosphere. Selected discoveries in the SWMI challenges are discussed in some detail here. The SWMI highlights mentioned in Chapter 2 are the following:

• MMS has observed how electrons are accelerated and heated even as they slip across the magnetic field in the process of magnetic reconnection.
• Waves excited by the solar wind flowing along Earth’s magnetosphere in the interface layer called the magnetopause have been discovered to play a substantive role in controlling how efficiently the magnetic fields in the solar wind and of the Earth reconnect. Where such waves are strongest depend both on the solar wind conditions and plasma and field conditions close to the magnetopause.
• The Van Allen Probes mission has changed our understanding of the structure of Earth’s Van Allen belts and how processes in these environments, including plasma waves, accelerate charged particles to ultra-high speeds. At times we observe three or more radiation belts. The inner edge of the outer belt for ultra-high energy electrons is unexpectedly sharp, and the inner belt is nearly void of high-energy electrons most of the time.
• The unusually weak recent solar cycle is providing important insights into how the inner-heliospheric conditions affect the propagation of CMEs, thereby changing the magnetic field and the dynamic pressure of heliospheric storm fronts as they reach Earth. The weak cycle has also given new insights into how the ionosphere responds to levels of lower-energy ionizing radiation.
• With spacecraft near Mercury, Venus, Earth, Jupiter, and Saturn, the evolution of solar eruptions traveling through the heliosphere could be observed and compared with simulation results. The analysis of many tails of comets over their observable trajectories is helping us understand solar-wind variability, specifically its turbulence, and how that evolves from near the Sun outward.
• Space-weather conditions at Mars were studied with particular emphasis on the solar wind coupled to that planet’s atmosphere with its weak, local magnetism.

Decadal Survey Challenge SWMI-1:
Establish How Magnetic Reconnection Is Triggered and How It Evolves to Drive Mass, Momentum, and Energy Transport

Key Science Goal 2 of the decadal survey is to determine the dynamics and coupling of Earth’s magnetosphere, ionosphere, and atmosphere and their response to solar and terrestrial inputs. Magnetic reconnection plays an important if not dominant role in that coupling. At its basic level, magnetic reconnection converts magnetic energy into particle motion. It is responsible for the transport of plasma and mechanical energy over magnetic boundaries, thus a detailed understanding of this process is crucial for understanding how the solar wind interacts with our magnetosphere during different interplanetary magnetic field (IMF)

orientations and solar-wind (and resulting magnetosheath) conditions. The presence of current sheets leads to magnetic reconnection in many heliophysical settings.

A revolution in the understanding of magnetic reconnection recently came about through MMS, which uses the near-Earth environment as a laboratory to study the microphysics of magnetic reconnection using in situ measurements. The near-Earth environment is the only practical place in the solar system where we can study the microphysics of this universal process that occurs throughout the domains of heliophysics. MMS uses four identically instrumented spacecraft, flying in the tightest ever pyramid formation for a satellite constellation, to measure the electromagnetic field with unprecedented accuracy, sampling 100 times faster than previous missions.

MMS provided important new measurements on where and how electrons contribute to reconnection. For example, at the magnetopause, the boundary between the solar wind and Earth’s magnetosphere, MMS observed that the reconnection process is highly localized and can strongly energize electrons (Figure E.5). Reconnection briefly decouples electrons from the magnetic field and then accelerates them in the electric field aligned with the magnetic field as a consequence of the strong gradients in the reconfiguring field, while often also heating the electrons in that process (Burch et al., 2016; Burch and Phan, 2017; Chen et al., 2016; Graham et al., 2016, 2017; Torbert et al., 2018).

However, because the electron diffusion regions cover only a small volume, they are insufficient to explain the overall observed energization. The details of the cross-scale coupling facilitated by reconnection remains a challenge. Computer simulations have shown that, for strong non-adiabatic heating to occur in association with shock waves in the outflow of the reconnecting field, the energy in the field must far exceed the energy in the particles, which is unlikely in the magnetosheath (Ma and Otto, 2014). Such conditions can more readily occur in, for example, the magnetosheath transition layer to the magnetopause (known as the plasma depletion layer), or near the magnetopause in waves that are excited by the solar-wind flowing by Earth’s magnetic field, just as ocean waves are excited by strong atmospheric winds (via Kelvin-Helmholtz instability).

In recent years, advanced computer simulations have clarified better how reconnection is strongly driven at the subsolar magnetopause under southward IMF and also how the reconnection rate on the flanks may be significantly modified in the presence of large-scale nonlinear Kelvin-Helmholtz waves running along the magnetopause (Ma et al., 2014; 2017). NASA’s THEMIS (Time History of Events and Macroscale Interactions during Substorms) mission has shown that such waves are very frequent there, in fact for most IMF orientations (Kavosi and Raeder, 2015), although where their occurrence peaks around the magnetopause depends on the magnetic field in the solar wind (Henry et al., 2017). These Kelvin-Helmholtz waves appear not only able to facilitate reconnection (Eriksson et al., 2016) and energize electrons, but they also heat the ions (Moore et al., 2016; 2017).

The processes taking place within Earth’s bow shock and magnetosheath also affect the process of reconnection near the magnetopause. Recently, the THEMIS spacecraft detected that high-speed jets in the solar wind compressed the originally thick magnetopause current-sheet until it was thin enough for reconnection to efficiently occur (Hietala et al., 2018). The effect on reconnection is transient and high-speed jets are relatively rare (Plaschke et al., 2018).

Another MMS discovery is that low-latitude reconnection can lead to formation of higher-latitude magnetic bottle structures that contain significant populations of energetic electrons and ions, as well as oxygen ions of ionospheric origin (Nykyri et al., 2019), although the exact energetic particle sources and energization mechanisms need more study. Another surprise was that the MMS spacecraft encountered “electron-only” magnetic reconnection without ion coupling in Earth’s turbulent magnetosheath (Phan et al., 2018). MMS observations also revealed how a hot flow anomaly at the bow shock accelerates solar-wind ions to almost 1 MeV (Turner et al., 2018). This provides new insight into how foreshock transients may be important in the generation of cosmic rays at astrophysical shocks throughout the universe.

The ensemble of compact and evolving regions of magnetic reconnection regulates much of the transfer of energy and momentum from the solar wind to the geospace system as a whole. This results in feedback to the reconnection region, altering the conditions of the reconnection itself, although the details of that back-reaction remain under study (Borovsky and Birn, 2014; Lopez, 2016; Zhang et al., 2016).

Decadal Survey Challenge SWMI-2:
Identify the Mechanisms that Control the Production, Loss, and Energization of Energetic Particles in the Magnetosphere

In Earth’s inner magnetosphere, charged particles are accelerated to speeds approaching the speed of light, forming the highly dynamic region known as the Van Allen radiation belts. The variability of the radiation belts has been a long-standing mystery, and this region is important both as a laboratory for studying particle acceleration and due to its space weather impacts on the nation’s space assets.

NASA’s twin Van Allen Probes, launched in 2012, have changed our understanding of the very structure of the radiation belts, with their high-resolution instruments and the use of a pair of probes that enables us to tell apart structures in space from evolution in time. Not only did the probes discover a third radiation belt, likely a remnant of a prior geomagnetic storm (Baker et al., 2013), they also found so-called “Zebra stripes” in the inner electron belt (explained as a consequence of a resonance with Earth’s rotation; Ukhorskiy et al., 2014, see Figure E.6). The inner boundary for ultra-relativistic electrons at about 2.8 Earth radii was found to be unexpectedly sharp (Baker et al., 2014), and, contrary to previous belief, there are typically no high-energy electrons in the inner radiation belt (e.g., Fennell et al., 2015; Claudepierre, et al., 2019).

Understanding particle acceleration is a key objective of the Van Allen Probes. The mission answered the long standing question of whether local acceleration by high-frequency plasma waves could cause observed rapid enhancements of high-energy electrons (Reeves et al., 2013). Acceleration caused by nonlinear wave-particle interactions has also been observed for the first time (e.g., Foster et al., 2017).

New processes important for the creation of Earth’s radiation belts have also been uncovered, including rapid spikes in the electric field that could be caused by highly nonlinear evolution of strong whistler mode waves. Such structures accelerate very-low-energy particles up to the kiloelectronvolt energy range, thus creating the seed population from which very-high-energy electrons are created (Mozer et al., 2013; Mozer et al., 2014). This and many other discoveries from the Van Allen Probes has led to next-generation radiation belt models (e.g., Sorathia et al., 2018) and improvements to space weather models (e.g., Yu et al., 2019).

The mission also revealed new insight into production and propagation of plasma waves (e.g., Li et al., 2016b; Agapitov et al., 2016; Malaspina et al., 2017), particle injections (e.g., Turner et al., 2015; Mitchell,

et al., 2018), and the plasma populations that coexist with the radiation belts in the inner magnetosphere (e.g., Gkioulidou et al., 2014). The importance of plasmaspheric drainage plumes on ultra-low frequency waves (e.g., Degeling et al., 2018) and on particle loss to the atmosphere (e.g., Li et al., 2019) has also been explored. Finally, significant progress has been made on understanding radiation belt particle loss using coincident measurements between Van Allen Probes and balloons (e.g., Blum et al., 2015; Li et al., 2014) and CubeSats (e.g, Blake and O’Brien, 2016; Breneman et al., 2017).

Decadal Survey Challenge 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

The progress on coupled community-accessible computer models has made them sufficiently realistic to be driven by, and in some cases to reproduce, observations (Figure E.7). For example, solar magnetograms are the basis for describing coronal and solar wind properties and activity. Additionally, solar ionizing emissions and solar wind time series determine simulated magnetosphere and ionosphere states, including ground-induced currents and total electron content (e.g., Boteler and Pirjola, 2017).

Of especially broad interest are the extreme ranges of space weather conditions and their consequences for the space environment of Earth in particular, although the results are also relevant for planetary science and astrophysics. While extremes are often thought of as high-intensity episodes, the last few solar cycles (23 and 24) have produced historically weak solar outputs and activity compared to earlier cycles of the space age. Conditions normally associated with both solar maximum—such as flares, CMEs, solar energetic particle events, and geomagnetic storms—and with solar minimum—such as enhanced GCR fluxes—have been modified in response to various combinations of the diminished solar EUV and solar wind fluxes and a weakened solar magnetic field. For example, McComas et al. (2013) describe observations of solar wind mass fluxes and interplanetary field diminished by approximately 30 percent during the cycle 23 maximum, with solar wind dynamic pressures reaching some of the lowest levels in the space age. Other studies of the solar wind properties (e.g., Kilpua et al., 2016; Tindale and Chapman, 2017) examined changes to the inferred sources of the slow solar wind at Earth orbit and the increased ease with which GCRs reach the inner heliosphere (e.g., Leske et al., 2013). The weak solar cycle also saw less impact from solar eruptions, possibly because CMEs could more readily expand near the Sun, lowering their impact at Earth (Gopalswamy et al., 2014), in part by weakening the magnetic field and in part by lowering the dynamic pressure of the events (Kilpua et al., 2014; Jian et al., 2018). At Earth, Solomon et al. (2013, 2018) found approximately 30 percent reductions in thermospheric density during solar minimum and approximately 15 percent reductions in global mean ionospheric electron content related primarily to the reduced solar EUV fluxes.

Decadal Survey 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

Space weather is usually associated with Earth’s space environment, but it is in its broader definition solar system–wide (Figure E.8). Solar activity affects each planet and solar system body in ways determined by the properties of that body (including its orbital distance from the Sun). Many investigations of the environments of other solar system bodies rely on information obtained by the Heliophysics System Observatory (HSO), including the Advanced Composition Explorer (ACE), Solar and Heliophysics Observatory, SDO, and STEREO, for example, to interpret space weather conditions at Mercury, Venus, and Mars, while at the same time these studies provide information about the radial evolution of events as they propagate through the heliosphere and into the interstellar medium. For example, Winslow et al. (2015) combined near-Earth observations with Mercury MESSENGER observations to establish a clear overall weakening of most leading shocks en route to Earth, consistent with an average deceleration of the ejecta drivers in that heliocentric distance range. Good and Forsyth (2016) combined data over a time span of 7 years from Mercury MESSENGER, Venus Express, STEREO, and ACE when in alignment along the path of solar erup-

tions to analyze both the differences in space weather at the innermost three terrestrial planets and the heliospheric distributions of events.

Heliophysics resources have also supported investigations of space weather conditions at Mars, which concern how the solar wind interacts with its atmosphere and have significance for ongoing planning for human missions. The MAVEN mission, which arrived in late 2014, is specifically instrumented to measure the local space environment conditions and their consequences, while the Mars Science Laboratory Curiosity rover has carried the Radiation Assessment Detector (RAD) around the surface since its landing in late 2012. RAD observes the Martian equivalent of ground-level enhancement events and Forbush decreases. Several significant flare and CME-related events, including widespread SEP-stimulated Martian auroras, were interpreted with the aid of the HSO observations (Hassler et al., 2014; Lee et al., 2018). These observations lend themselves to developing and testing models that can be used in forecasting space environment conditions at Mars and indeed throughout the inner heliosphere. In another study involving the observation of an interplanetary CME at Mars, Möstl et al. (2015) use STEREO observations to model the direction and expansion of the initial coronal event, concluding its non-radial propagation was significant.

Heliophysics imaging capabilities are also being exploited by cometary observers for both characterizing comets and cometary orbits (Ye et al., 2014) and for studying the phenomenology and nature of observed comet coma structure (Raouafi et al., 2015). The STEREO images are especially well-suited for determining the properties of near-Sun and Sun-impacting comets (Ye et al., 2014) and for relating structural features such as “high velocity evanescent clumps” to surrounding structure in the solar wind. Also harkening back to the original concept of comets as wind socks in the solar wind, deForest et al. (2015) tracked over 200 tail features in Comet Enke’s tail to explore turbulent motions in the solar wind.

This progress requires integrating observations and interpretations of everything from solar activity to atmospheric responses. Moreover, it requires the multipoint, multiperspective information available from the HSO to assemble the three-dimensional picture of the external conditions affecting different planetary locations. In short, it exercises all our scientific options in order to understand occurrences at a remote location due to the local space weather there. The work in this area also requires us to interpret the responses observed at the various solar system planets in terms of what we know in much more detail from our Earth experiences. Furthermore, this type of research in many ways provides the “ground-truth” for applications of our understanding to exoplanet-stellar wind interaction studies.

E.3 ATMOSPHERE-IONOSPHERE-MAGNETOSPHERE INTERACTIONS

The AIMI region starts roughly just above Earth’s stratosphere (50 km) and extends up to several thousand kilometers above ground. The AIMI region is impacted by the Sun, interactions with the magnetosphere, and also by processes occurring in the atmosphere below. Discovering the processes that govern the conditions at this interface between Earth and space is fundamental to understanding planetary atmospheres and exospheres, as well as for operational needs, including the protection of astronauts and spacecraft, of humans on the ground, and for radio and navigation signal situational awareness. Distributed observational capabilities increasingly enforce the realization that the geospace system, from below the ionosphere to the outer reaches of the magnetosphere, is a single connected system. Selected discoveries in the AIMI challenges are discussed in some detail below, and some of the many advances in understanding the AIMI, as mentioned in Chapter 2, are highlighted below.

• The weak recent solar cycle simplified the separation of solar influences from effects from the lower atmosphere, enabling improved understanding of the coupling processes between the AIMI region and the atmosphere below. Models are bridging the knowledge gap on the coupling between larger-scale instabilities and smaller-scale turbulence that is important in regulating the dynamics of geospace.
• The energy of precipitating particles and heat from solar radiation enhance the concentration of nitric oxide (NO) in the thermosphere, which in turn has brighter infrared emissions to cool the thermosphere back down efficiently.
• Joint analyses and comparisons of plasma-neutral interactions in the solar chromosphere and in the terrestrial ionosphere stimulated by the NASA Living With a Star research and analysis program has provided deeper insights into the similarities and differences between these environments and is leading to the sharing of insights between two communities previously working largely in isolation.
• Atmospheric waves generated by tides, terrain, and atmospheric instabilities have been observed and modeled as they travel upward, strengthening in the process. Waves are also generated in the dynamics of the polar vortex at stratospheric altitudes. All these wave phenomena can modify high-atmospheric properties, including ionospheric properties, far from the latitudes where they originally formed, which, in turn, couple to space weather phenomena further out.
• Drivers of long-term trends in upper atmospheric properties are better clarified using ever more sophisticated global circulation models to reveal the dynamic effects from solar variability, the cooling influence of anthropogenic methane and carbon dioxide, and even the top-down coupling of atmospheric changes resulting from the long-term change of the terrestrial magnetic field, of which the shift of the magnetic poles is one consequence.

Decadal Survey Challenge AIMI-1:
Understand How the Ionosphere-Thermosphere System Responds to, and Regulates, Magnetospheric Forcing over Global, Regional, and Local Scales

The combination of electromagnetic fields, impact ionization (auroras), and heating leads to complex three-dimensional flows within the ionosphere-magnetospheric system. New distributed observations and computer modeling of plasma transport have called into question the traditional paradigm of treating the ionosphere and magnetosphere as distinct regions. Instead, this new work suggests that the entire geospace system should be treated as an extension of Earth’s atmosphere to understand how the atmosphere-magnetosphere system regulates the entry, storage, and dissipation of solar wind power.

Recent research has revealed an important manifestation of this paradigm shift: observations (Varney et al., 2016a) and models (Lund et al., 2018) have suggested that outflow of ionospheric ions along magnetic field lines affect a particular pathway of energy release in the magnetosphere. So called “sawtooth oscillations” are quasi-periodic injections of energetic particles observed near geosynchronous orbit, similar to periodic substorms but more global in nature. The outflow of heavy ions is thought to regulate the rate of reconnection, thereby producing the sawtooth-shaped events (Varney et al. 2016b).

Electromagnetic fields, plasma gradients, and rapid plasma flow arise ubiquitously in the geospace system through instabilities, and in turn produce small-scale turbulent local conditions. But how the larger-scale instabilities and the small-scale turbulence affect one another in detail represents a critical gap in understanding. The technical advances of tools to quantitatively couple these processes has led to a new understanding of how the formation of turbulent cells in the lower ionosphere affects electric currents that couple the outer atmosphere to the magnetosphere (Dimant and Oppenheim, 2011; Liu, 2016a).

The electromagnetic power generated by a geomagnetic storm is dissipated as heat in Earth’s outer atmosphere, in much the same way a battery heats a resistor. This heating causes the atmosphere to expand, which has deleterious effects on satellite orbits through increased satellite drag and increased outgassing (Wiltberger, 2015). However, there is a stabilizing backreaction: researchers have found that in addition to heat, intense storms also increase the amount of NO, which acts as an efficient cooling agent (Weimer et al., 2015) The larger the geomagnetic storm, the greater the NO cooling (Knipp et al., 2017). In fact, the cooling may win out under extreme conditions, producing the counterintuitive effect of atmospheric contraction. This result has substantial implications on our understanding of how geomagnetic storms affect the tenuous atmosphere within which satellites orbit.

Bright visible auroras are produced by energetic particles flowing along magnetic field lines into the upper atmosphere. Auroras have long been exploited as diagnostic of less accessible magnetospheric processes, but there are still surprises. Recently a new, very different, auroral phenomenon has been discovered—by a large ad hoc network of citizen auroral watchers. This faint feature, known as STEVE (Strong Thermal Emission Velocity Enhancement) and shown in Figure E.9, appears to be caused by a high-speed plasma jet flowing perpendicular to Earth’s magnetic field, exciting optical emissions through pathways not yet identified. The discovery of STEVE (MacDonald et al., 2018; Gallardo-Lacourt et al., 2018) highlights the discovery potential of geospace facilities that may be realized in creative and cost-effective ways.

Decadal Survey 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

Ground-based NSF incoherent scatter radar observations have shown dramatic enhancements of the E-region electron temperature in the subauroral and auroral electrojet regions during geomagnetic storms.

These temperature enhancements are associated with electrojet turbulence introduced by instabilities from large velocity differences between the electrons following the magnetic field and the ions scattered off the field by collisions. Although long considered a local phenomenon, the extent of enhanced electric fields across the high latitudes during geomagnetic storms suggests that this phenomenon can influence the large-scale, high-latitude ionospheric current system. This is supported by modeling efforts. National Center for Atmospheric Research TIE-GCM (Thermosphere-Ionosphere-Electrodynamics General Circulation Model) simulations revealed that the phenomenon led to increased electron heating, reduced electron losses, and more than 30 percent enhanced conductivity (Liu et al., 2016). LFM-RCM¹ global simulations show a lowering of the cross-polar-cap potential, improvements in current descriptions in the auroral electrojet, and increased peak pressure in the inner magnetosphere (Wiltberger et al., 2017). These advanced studies reveal how local and detailed small-scale processes can have global consequences.

A NASA LWS focused science team effort in plasma-neutral interactions brought together solar chromospheric and terrestrial ionospheric researchers. The effort culminated in an extensive paper (Leake et al., 2014) describing the similarities and differences in coupling processes of ionized plasma to neutral gas in the weakly ionized, stratified, electromagnetically permeated regions of the Sun’s chromosphere and Earth’s ionosphere-thermosphere. Related phenomena in the two environments were compared and described in a unified way, significantly improving on previously used contrasting paradigms. This study typifies the collaborative and elucidating approach to understanding our heliophysics system.

___________________

¹ Lyon-Fedder Mobary (LFM)-Rice Convection Model (RCM).

A discovery using ground-based resonance lidars of metallic neutral layers in the thermosphere reaching altitudes of 200 km has changed the view of how minor species transport and plasma-neutral chemistry interact in the thermosphere (Figure E.10). Theory suggests that thermospheric neutral-iron layers are formed through direct recombination of iron ions with electrons during the dark polar night at thermospheric altitudes above 120 km, and furthermore, that geomagnetic activity may play a role. However, it is known that there is no permanent stationary presence of iron ions at such high altitudes because meteor ablation and sputtering are insufficient sources to counter gravitational sedimentation of these heavy ions. Instead, Chu and Yu (2017) conceived the dynamic life cycle of meteoric metals via deposition, transport, chemistry, and wave dynamics for thermospheric iron layers with gravity waves.

Decadal Survey Challenge AIMI-3:
Understand How Forcing from the Lower Atmosphere via Tidal, Planetary, and Gravity Waves Influences the Ionosphere and Thermosphere

The vertical transport of energy and momentum by atmospheric waves is a fundamental process in planetary atmospheres and—on Earth—links tropospheric weather with the space weather of the ionosphere and thermosphere. The modulation of input of energy into the troposphere and stratosphere due to Earth’s rotation excites a range of planetary-scale thermal tides, while surface topography, unstable flows, and cloud dynamics excite waves all the way down to scales of only a few kilometers, introducing a range of periods from several weeks to a few minutes. The vertically propagating waves grow exponentially with height into the more rarefied atmosphere where they change neutral density, temperature, and winds. These wave-

induced wind variations then collisionally couple into the ionosphere, involving instabilities and seeding of plasma bubbles. Over the past 5 years, understanding vertical wave coupling has advanced significantly, capitalizing on advances in numerical modeling and multi-instrument observations in a systems approach. Many of these observations have come from NASA’s Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED), Aeronomy of Ice in the Mesosphere (AIM), and Communication/Navigation Outage Forecast System-Coupled Ion Neutral Dynamic Investigation (C/NOFS-CINDI) missions. This has yielded numerous exciting scientific discoveries, four of which are highlighted below.

Small-scale gravity waves have been tracked all the way from their thunderstorm sources to the F-region of the ionosphere at 100-300 km altitude (Figure E.11), exhibiting effects even above that, through a combination of ground- and space-based assets—for example, from Aqua, AIM, Visible Infrared Imaging Radiometer Suite (VIIRS), Global Navigation Satellite Systems (GNSS), and Arecibo (Yue et al., 2014; Azeem et al., 2015; Hysell et al., 2018). Computer simulations, meanwhile, elucidated how these waves perturb winds, temperatures, and ion densities (Vadas et al., 2014, 2018). It has now been realized that gravity waves generated by tropical monsoons can propagate to the polar regions and affect the frequency of polar mesospheric clouds, an important indicator of climate change in the upper atmosphere (Thurairajah et al., 2017). On larger scales, combined TIMED, satellite drag, and numerical model studies have unequivocally revealed that the ionosphere-thermosphere responds strongly to the global El Niño weather phenomenon and to the stratospheric quasi-biennial oscillation (Liu, 2016b; Li et al., 2016a; Warner and Oberheide, 2014).

Polar vortex dynamics in the stratosphere cause massive holes in the ionosphere over North America lasting 2 to 4 weeks (Frissell et al., 2016), most likely through a coupling by waves that have been tracked through the mesosphere (Harvey et al., 2018). Dramatic stratospheric warmings—a break-up of the polar vortex in the stratosphere—carved a hole in the nighttime ionosphere over North America (Goncharenko et al., 2018) and are linked to changes throughout the entire atmosphere (Pedatella et al., 2018). The formation of the massive hole is more consistent with winds near 300 km altitude, instead of near 100 km as suggested by previous studies, which provides new insights how the polar vortex can impact space weather.

Thermosphere-ionosphere general circulation modeling driven by NASA TIMED and Modern-Era Retrospective analysis for Research and Applications (MERRA)-2 data at its lower boundary predict that the tidal spectrum modulated by planetary waves causes the thermosphere at orbital altitudes to oscillate by tens of meters per second with planetary wave periods from 2 to 16 days. These oscillations subsequently modify F-region electron densities by 50 percent, with significant implications for satellite drag and radio propagation (Forbes et al., 2018; Gan et al., 2017). First estimates of the tidal weather in the E region based on NASA TIMED data (Oberheide et al., 2015; Dhadly et al., 2018) revealed that the day-to-day variability is larger than previously thought and can approach 100 percent of the monthly average values.

Significant progress in multi-platform data assimilation for the mesosphere and lower thermosphere has been made (Pedatella et al., 2014, 2016a; Siskind et al., 2015). Coupled with ionospheric models (McDonald et al., 2018), the new models perform much better in nowcasting the state of the neutral and ionized atmosphere, an important milestone towards space weather predictability. Using the new data assimilation research testbed, measurements from ground level up to 110 km have been used in the Whole Atmosphere Community Climate Model (WACCM) (Pedatella et al., 2014; 2016a), and new versions of this model including the ionosphere (WACCM-X, v2; Liu et al., 2018) capture the near-space imprint of multiscale wave dynamics from the lower atmosphere from first-principles—a major milestone (Figure E.12).

Decadal Survey Challenge AIMI-4:
Determine and Identify Causes for Long-Term (Multi-Decadal) Changes in the AIMI System

The decadal survey recognized the continued long-term cooling of the thermosphere as “a remarkable planetary change attributable, at least in part, to human society’s modification of the atmosphere” (NRC, 2013, p. 27). The decadal survey identified the increasing lifetime of orbital debris caused by a less dense atmosphere as an important practical consequence. It recommended to protect long-term observations and to conduct research on understanding how the long-term changes are embodied in or transmitted through the AIMI system.

Solar cycle 24 had an extremely low level of solar activity compared to the three preceding ones, which resulted in uncommon impacts on conditions of the upper atmosphere and altered atmospheric coupling from above (magnetosphere-ionosphere coupling, for example, [Xu et al., 2015; Liu et al., 2015], and below (troposphere-stratosphere coupling into the mesosphere and above). Interestingly, the reduced solar activity has simplified identifying signatures of couplings from below, advancing our understanding of them (Jones et al., 2014; Goncharenko et al., 2018; Pedatella et al., 2018) and providing a major push for further modeling development (Liu et al., 2016; Pedatella et al., 2016a).

In contrast, studies of effects on the AIMI region by CME-induced geomagnetic storms declined because of the low frequency and relative weakness of the driving events. This led to an increased interest in, and thereby understanding of, the effects of high-speed solar-wind streams. These streams push into the slower solar wind ahead, creating fronts that, while less intense than CME fronts, can deposit more energy into Earth’s magnetosphere because the streams are sustained longer.

Based on Sounding of the Atmosphere using Broadband Emission Radiometry (SABER)/TIMED observations of infrared emissions from carbon dioxide and nitric oxide, Mlynczak et al. (2015, 2016) developed a thermosphere climate index to represent properties of the thermosphere. This index also represents the global infrared power radiated from Earth’s thermosphere since 1947, which proved surprisingly constant over the past 70 years. From that, it is inferred that the geoeffective energy input from the Sun in the form of ultraviolet photons and particle precipitation is also relatively constant over a solar cycle.

The capabilities of numerical models for simulating thermospheric density variations on timescales from decades to days have much improved over the past 5 years (Bruinsma et al., 2018), which is important, for example, for characterizing the lifetime of orbital debris. The new generation of whole atmosphere models shows an anthropogenic thermospheric cooling of 2.8 K per decade and a 3.9 percent per decade decrease in mass density for solar minimum conditions (Solomon et al., 2018). Exospheric hydrogen densities are important for assessing atmospheric evolution through planetary escape and ring current decay during geomagnetic storms. Nossal et al. (2016) found that the hydrogen response to methane is relatively independent of solar activity but that the impact of carbon dioxide is highly dependent on it. Greenhouse gas emissions will thus not only lead to a long-term trend in the exospheric hydrogen but also to an increased solar cycle variability of this important species.

Using WACCM-X simulations, Cnossen et al. (2016) predicted that long-term changes in Earth’s magnetic field directly impact the ionosphere and thermosphere via changes in ion-neutral interactions and also the atmosphere below through a top-down coupling, with polar surface temperature changes of up to about 1.3 K between 1900 and 2000. Because Earth’s magnetic field has been changing rapidly since 2000 (Chulliat et al., 2015), this finding is important for global climate modeling.

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