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2
Solar and Space Physics: Recent Discoveries, Future Frontiers
SCOPE AND RELEVANCE OF THE DISCIPLINE
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). 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 sub-surface 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 400-800 km 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 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 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 our 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 further into space via robotic probes and human flight, and as society’s
technological infrastructure is increasingly linked to assets that are impacted by the space environment, a
deeper and fundamental understanding of these governing processes becomes ever more pressing (see
Chapter 3).
The underlying principles governing the Sun-Earth system include the physics of plasmas, 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
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FIGURE 2.1 Left: White light image of the solar corona out to 4 solar radii during the solar eclipse of 11
July 2011. Right: Predictive Science Inc. prediction of magnetic field during July 11 2011 eclipse, using
observations of photospheric magnetic field and magnetized fluid numerical simulation SOURCES: 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.
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 non-linear 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
separate chapters (8 through 10) of this report:
• Sun and heliosphere (SH)—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) )—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;
• Atmosphere-ionosphere-magnetosphere-interactions (AIMI )—concerns the dynamics of
planetary ionospheres due to solar, magnetospheric, and atmospheric drivers and coupling.
As discussed below, each of these panels organized their work around 4 scientific challenges, which are
shown in Table 2.1.
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TABLE 2.1 Solar and Space Physics Decadal Science Challenges
The Sun and Heliosphere
SH-1 Understand how the Sun generates the quasi-cyclical magnetic field that extends throughout
the heliosphere.
SH-2 Determine how the Sun's magnetism creates its hot, dynamic atmosphere.
SH-3 Determine how magnetic energy is stored and explosively released and how the resultant
disturbances propagate through the heliosphere.
SH-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.
Atmosphere-Ionosphere-Magnetosphere Interactions
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.
A DECADE OF HELIOPHYSICS DISCOVERY
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 between 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.
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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 and enable this science to achieve insight into the basic physics of the systems and to attain a
measure of predictive capability. Scientists in heliophysics are now poised to answer questions
concerning universal physical processes, advance understanding of the complex coupling and non-linear
dynamics of our home in the solar system, and apply this understanding to mitigate the societal impacts of
changes to our space environment by identifying and forecasting the threats posed to technological
infrastructures. Only a selection of the most salient discoveries and advances are presented here to
provide sufficient context to understand how the recommendations of this report follow from the flow of
scientific discovery.1
Sun and Heliosphere
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 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 our 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 that solar
activity at the maximum of the current solar cycle maximum will be low relative to 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 (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 sub-surface 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 space age and—enabled by STEREO—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 intensity levels. This enhanced GCR 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 may be entering an extended period of minimum activity such as
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
1
For a more complete discussion of ongoing missions and their contributions, see, for example, “Senior Review
2010 of the Mission Operations and Data Analysis Program for the Heliophysics Operating Missions.” Available
online at: http://science.nasa.gov/media/medialibrary/2010/07/22/SeniorReview2010-
MODAProgramPublic_V3.pdf. Also see, “Heliophysics: State of the Discipline,” in Heliophysics: The Solar and
Space Physics of a New Era: Recommended Roadmap for Science and Technology 2009–2030; available online at:
http://sec.gsfc.nasa.gov/2009_Roadmap.pdf.
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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 enabled
greater than 100-fold improvements in the spatial resolution of simulations. The resolution of
observations 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 shows side-by-side comparisons of the results of a numerical simulation of a sunspot (left) and
an image from the NST, which shows astonishing correspondence in the background circulation pattern
(granulation), the umbra fibrils, and the central spot.
FIGURE 2.2 Numerical simulation of a sunspot (left) and an actual photograph right from the New Solar
Telescope at Big Bear Observatory, NJIT. Detailed comparisons have elucidated the physics of such solar
features. SOURCE: Left: Left half courtesy of M. Rempel, High Altitude Observatory; right half courtesy
of F. Woeger, National Solar Observatory; from M. Rempel, “Numerical Simulations of Sunspots: From
the Scale of Fine Structure to the Scale of Active Regions,” paper presented at the 42nd meeting of the
American Astronomical Society Solar Physics Division, Las Cruces, N.M., 2011. Right: Courtesy of Big
Bear Solar Observatory.
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 steady state as previously believed.
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.
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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 achieving closure between theory/models and observations. The
semi- realistic first global-scale three-dimensional magnetohydrodynamic (MHD) numerical simulations
of the corona were performed with sufficient spatial resolution to compare with modern observations
(e.g., Figure 2.1). Modeling the chromosphere, however, remains a significant challenge since 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 super-granule, 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 show 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
appears comparable. Thus, understanding the remarkably efficient conversion of magnetic to particle
energy flares is a significant challenge.
Major advances were also made 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 due 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 of SEP energy spectra and transport
variability is a greater challenge.
Structure and Dynamics of the Solar Wind
Major progress was made 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,
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+) so that 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, is
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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 is the likely source
of energy to heat 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 fire hose instabilities. 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 efficiency of
energetic particle production in flares. Unexpectedly, most of these reconnection sites are found away
from the heliospheric current sheet. The observations 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 (SEPs) yielded a number of surprises. Solar-cycle
23 produced sixteen ground-level events in ground-based neutron monitors, which allowed us to establish
that most large SEP events have a recent, preceding CME from the same active region. This 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 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, which is
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 our 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 super-sonic 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 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 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 demonstrates how
little is really understood about their interactions of stars with the inter-stellar environments.
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FIGURE 2.3 The unexpected ribbon seen in 0.9 to 1.5 keV 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.
Solar Wind-Magnetosphere Interactions
Advances in the physics of magnetospheres, their dynamics and coupling with the solar wind and
ionospheres were made on a number of fronts. Global imaging and in situ observation networks revealed
unexpected dynamics associated with plasma convection, particle acceleration and 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
our 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 suborbital, ground-based in situ and remote sensing networks (including magnetometers,
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radars, images, and riometers). Spacecraft observations instrumental to these advances were returned both
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 from instruments flown on non-NASA satellites.
Global Dynamics
The global dynamics of the magnetosphere is 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 (Magnetosphere Schematic). 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 inward to produce ionospheric density enhancements of the
type shown in Figure 3.5.
FIGURE 2.4 Illustration of 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 using a
combination of imaging and in situ measurements. SOURCE: Courtesy of Jerry Goldstein, Southwest
Research Institute.
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FIGURE 2.5 Measurements by the EUV instrument on the IMAGE satellite. EUV images before the
storm and after the 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.
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 energetic neutral atom (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 lie consistently in
the early morning and not in the afternoon as had been expected. This can only occur 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. Substantial progress was made 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 the onset signatures 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 lines2 reconnect and change their connectivity (Figure 1.3). It
was 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 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 keV, potentially providing a seed population for
2
Field lines are a convenient construct to understand magnetic field connectivity and topology.
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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 the last decade,
computational resources had limited simulations 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.
Observationally, reconnection seems to behave differently in different regions. While
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 revealed that these
reconnection-generated flow channels initiate magnetospheric substorms and drive the Earthward
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 (WPI) 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 WPI 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 in time-dependent
models of the radiation belts and the ring current. Scientists now know that storm-time particle dynamics
are the result of a delicate balance between acceleration and loss of relativistic particles mediated by
waves produced by local plasma instabilities.
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 GPS and one geosynchronous spacecraft. SOURCE: Courtesy R.V.
Hilmer, Air Force Research Laboratory.
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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. Here, magnetic
energy is released in the form of ejected plasma, electromagnetic radiation and heat and energize the local
plasma. In Figure 1.2 are a series of active regions seen in extreme ultra violet light (EUV) from the Solar
Dynamics Observatory (SDO). They 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 key scientific goals 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: SH2. 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 corona 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 (Figure 1.2). As a result, the corona and heliosphere are
filled with high-energy radiation, both electromagnetic (UV, X-rays and gamma rays.
The strongest of these reconnection events propel CMEs into the solar wind, which 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 Earthward, 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.4
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 their timing, speed, energy, or
momentum, nor is their 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 us with a third challenge: SH3.
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 our understanding of the outer heliospheric is unfolding as the Voyager spacecraft
provides 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
(Figure 2.3).
4
National Research Council, Space Radiation Hazards and the Vision for Space Exploration: Report of a
Workshop, The National Academies Press, Washington, D.C., 2006.
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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, C.R. DeVore, and S.K. Antiochos, The
mechanisms for the onset and explosive eruption of coronal mass ejections and eruptive flares,
Astrophysical Journal, 2012, submitted.
During the next decade, the Voyager spacecraft are expected to exit the heliosphere, entering
interstellar space. For the first time, operating spacecraft will leave our home in space, enter into our local
galaxy, and gather local 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: SH4. Discover how the Sun interacts with the local interstellar medium.
Solar Wind-Magnetosphere-Interactions
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 predict when, where, and
how fast this process will occur and how it contributes to mass, energy and momentum transport.
NASA’s Magnetospheric 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 throughout the heliosphere
and in astrophysical settings throughout, the universe. They are also highly relevant in understanding
reconnection events in tokomak plasmas and in laboratory-based reconnection experiments. This
motivates the following primary challenge: SWMI-1. Establish how magnetic reconnection is
triggered and how it evolves to drive mass, momentum, and energy transport.
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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. (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 from 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)
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.
The ionosphere is the inner boundary of the magnetosphere. Magnetic lines of force converge in
the polar regions at low altitudes where the dipolar field of Earth dominates the fields produced by
external current systems. Although the ionospheric volume is tiny compared to the magnetosphere, the
mass of ions between 100 and 300 km altitude exceeds the total mass of the magnetosphere by roughly an
order of magnitude. In turn, the mass of neutral gas in this altitude range, the thermosphere, exceeds that
of 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 ion upwelling 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 both on the dayside and on the nightside. Figure 2.10
(Magnetotail Simulation) 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 associated ionospheric currents. These events inject plasma stored in
the geomagnetic tail Earthward. 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 our 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 our theories
and models.
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FIGURE 2.9 Energetic electron variability as measured during the 14-month Combined Release and
Radiation Effects Satellite (CRRES) 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 (Blanc et
al., 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.
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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 2h50m 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 figures (~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:
Wiltberger, M., Lotko, W., Lyon, J.G., Damiano, P., and Merkin, V., 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.
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 differ 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
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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.
Atmosphere-Ionosphere-Magnetosphere Interactions
Understanding ionosphere-thermosphere (IT) 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 time scales 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 O produces the 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 3.5 shows an
example of a plasma plume that extends over thousands of kilometers that forms 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 produce 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 which 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 to accentuate and sometimes moderate this interchange. The pathways through which ions and
neutrals interact are of course fundamental to space physics, as they occur at all planets with atmospheres,
comets, and within the magnetospheres of Jupiter and Saturn. For example, in Earth’s ionosphere from
100 to 130 km altitude 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 is 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
6300A 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
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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 here in ultraviolet
emissions. Surface topography and unstable shear flows excite planetary waves (PW) and gravity waves
(GW) extending from planetary to very small (~10’s to 100’s of km) 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 Na layer
observations revealing amazing wave structures at the base of the thermosphere, illustrating the rich
spectrum of dynamics that occurs. Although the presence and importance of waves is not in dispute, the
relevant coupling processes operating between the neutral atmosphere and ionosphere involve a host of
multi-scale 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 (Left) Mean 1984-2009 January Daytime Convective Cloud Amount in percentage from
ISCCP-D2. Blue indicates 10-15%, yellow/green indicates approximately 8%, and red indicates 0-4%.
Right: 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: Left: 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.). Right: England, S. L., Zhang, X., Immel, T. J.,
Forbes, J. M., and Demajistre, R., The effect of non-migrating tides on the morphology of the equatorial
ionospheric anomaly: seasonal variability, Earth, Planets and Space 61:493-503.
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FIGURE 2.12 High resolution Na lidar observations breaking gravity waves at the base of the
thermosphere. A 6-meter, zenith-pointing telescope comprising a spinning mercury mirror was coupled to
a Na 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
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, Geophys. Res. Lett. 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 the 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 inter-molecular 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.
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 will affect the prevalence of the resulting ionospheric irregularities. Secondly, continued cooling of
the thermosphere will reduce satellite drag, thereby increasing orbital debris lifetimes, and lower the
effective ionospheric conductivity. The latter will alter global currents in the magnetosphere-ionosphere
system and therefore fundamentally alter magnetosphere-ionosphere coupling. The committee, therefore,
identifies the following scientific challenge: AIMI-4. Determine and identify the causes for long-term
(multi-decadal) changes in the AIM system.
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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 altiudes 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, Geophys. Res. Lett. 35:L05101,
doi:10.1029/2007GL032809, 2008. Copyright 2008 American Geophysical Union. Reproduced by
permission of American Geophysical Union.
RISING TO THE CHALLENGES OF THE COMING DECADE: OVERVIEW
The survey committee’s four top-level science goals for the coming decade were presented in
Chapter 1. Achievement of these goals requires addressing the science challenges, which are also shown
in Chapter 1, for each of the sub-disciplines of solar and space physics. In turn, this 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 the 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 (“science targets”). New missions, as described in
Chapter 4, can be carefully chosen to address the most pressing of these science targets. 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 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 which they reflect. Reaching scientific closure and
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advancing our predictive understanding therefore depends 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 threats to a society that is increasingly
reliant upon technologies that are vulnerable to solar and geospace activity.
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