Copyright © National Academy of Sciences. All rights reserved.
Solar and Space Physics: A Science for a Technological Society
SOLAR AND SPACE PHYSICS: RECENT DISCOVERIES, FUTURE FRONTIERS
43
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 flareaccelerated ~30-MeV ions shows that these emissions originate from small foot points linked to magnetic
loop structures rather than over an extended region, indicating that ion acceleration is also related to
magnetic reconnection. The energy in >~1-MeV ions and that in >20-keV electrons appear comparable.
Thus, understanding the remarkably efficient conversion of magnetic energy to particle energy flares is a
significant challenge.
Major advances were also made in understanding photon energy release from flares. For the first
time, flares were detected in TSI by the SORCE/TIM instrument showing that the total radiated energy and
CME kinetic energy can be comparable. The SDO/EVE instrument discovered an EUV late phase in flares
delayed many minutes from the X-ray peak. Global EUV observations by SDO/AIA and STEREO/EUVI
revealed long-distance “sympathetic” interactions between magnetic fields in flares, eruptions, and CMEs
likely owing to distortions of the coronal magnetic field.
The understanding of how CMEs and flares are produced and related has also progressed. CME velocity profiles below ~4 R S are in sync with flare-HXR energy releases. The magnetic flux-rope structure of
models of CMEs is consistent with the observations of many events. Furthermore, shocks produced by
fast CMEs can be identified in coronagraph images, suggesting that scientists are close to pinning down
the sources of SEPs. Achieving a predictive capability for SEP energy spectra and transport variability is a
greater challenge.
Structure and Dynamics of the Solar Wind
Major progress was made over the past decade in understanding solar wind structure and dynamics,
a key to understanding the Sun’s influence on Earth’s geospace environment. The conceptual picture from
Ulysses and ACE was that the sources of the slow and fast solar wind were at low-latitude and high-latitude
regions of the Sun, respectively. Fast, slow, and transient (associated with CMEs) solar wind can now be
identified and distinguished by ionic composition signatures (Fe charge states, Fe/O, O 7+ /O 6+ ), and so the
origins of solar wind parcels can be directly identified from in situ observations. Coronal mass ejections
interact with these solar wind streams, leading to dynamic fluid interactions and also particle acceleration
through a variety of processes. Microstructure of the solar wind, presumably related to structures in the
corona, may now be analyzed with the most powerful set of in situ observations, sometimes using several
observational platforms. The cascade of turbulence to short spatial scales and its ultimate dissipation are
the likely source of energy for heating the expanding solar wind. Observations and models have produced
major advances on this topic. Temperature anisotropies with respect to the local magnetic field of solar
wind H + and He 2+ were shown to be limited by the mirror and firehose instabilities. 2 These observations
constrain the possible mechanisms of solar wind heating. Scientists have also discovered that magnetic
reconnection between adjacent domains of opposing magnetic fields is ubiquitous in the solar wind but
appears to involve little particle acceleration near heliospheric reconnection sites—a surprise, given the
2 See J.C. Kasper, A.J. Lazarus, and S.P. Gary, Wind/SWE observations of firehose constraint on solar wind proton temperature
anisotropy, Geophysical Research Letters 29(17):20-1-20-4, 2002; B.A. Maruca, J.C. Kasper, and S.P. Gary, Instability-driven limits on
helium temperature anisotropy in the solar wind: Observations and linear Vlasov analysis, Astrophysical Journal 748(2): 137, 2012.
Solar and Space Physics: A Science for a Technological Society
SOLAR AND SPACE PHYSICS: RECENT DISCOVERIES, FUTURE FRONTIERS
43
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 flareaccelerated ~30-MeV ions shows that these emissions originate from small foot points linked to magnetic
loop structures rather than over an extended region, indicating that ion acceleration is also related to
magnetic reconnection. The energy in >~1-MeV ions and that in >20-keV electrons appear comparable.
Thus, understanding the remarkably efficient conversion of magnetic energy to particle energy flares is a
significant challenge.
Major advances were also made in understanding photon energy release from flares. For the first
time, flares were detected in TSI by the SORCE/TIM instrument showing that the total radiated energy and
CME kinetic energy can be comparable. The SDO/EVE instrument discovered an EUV late phase in flares
delayed many minutes from the X-ray peak. Global EUV observations by SDO/AIA and STEREO/EUVI
revealed long-distance “sympathetic” interactions between magnetic fields in flares, eruptions, and CMEs
likely owing to distortions of the coronal magnetic field.
The understanding of how CMEs and flares are produced and related has also progressed. CME velocity profiles below ~4 R S are in sync with flare-HXR energy releases. The magnetic flux-rope structure of
models of CMEs is consistent with the observations of many events. Furthermore, shocks produced by
fast CMEs can be identified in coronagraph images, suggesting that scientists are close to pinning down
the sources of SEPs. Achieving a predictive capability for SEP energy spectra and transport variability is a
greater challenge.
Structure and Dynamics of the Solar Wind
Major progress was made over the past decade in understanding solar wind structure and dynamics,
a key to understanding the Sun’s influence on Earth’s geospace environment. The conceptual picture from
Ulysses and ACE was that the sources of the slow and fast solar wind were at low-latitude and high-latitude
regions of the Sun, respectively. Fast, slow, and transient (associated with CMEs) solar wind can now be
identified and distinguished by ionic composition signatures (Fe charge states, Fe/O, O 7+ /O 6+ ), and so the
origins of solar wind parcels can be directly identified from in situ observations. Coronal mass ejections
interact with these solar wind streams, leading to dynamic fluid interactions and also particle acceleration
through a variety of processes. Microstructure of the solar wind, presumably related to structures in the
corona, may now be analyzed with the most powerful set of in situ observations, sometimes using several
observational platforms. The cascade of turbulence to short spatial scales and its ultimate dissipation are
the likely source of energy for heating the expanding solar wind. Observations and models have produced
major advances on this topic. Temperature anisotropies with respect to the local magnetic field of solar
wind H + and He 2+ were shown to be limited by the mirror and firehose instabilities. 2 These observations
constrain the possible mechanisms of solar wind heating. Scientists have also discovered that magnetic
reconnection between adjacent domains of opposing magnetic fields is ubiquitous in the solar wind but
appears to involve little particle acceleration near heliospheric reconnection sites—a surprise, given the
2 See J.C. Kasper, A.J. Lazarus, and S.P. Gary, Wind/SWE observations of firehose constraint on solar wind proton temperature
anisotropy, Geophysical Research Letters 29(17):20-1-20-4, 2002; B.A. Maruca, J.C. Kasper, and S.P. Gary, Instability-driven limits on
helium temperature anisotropy in the solar wind: Observations and linear Vlasov analysis, Astrophysical Journal 748(2): 137, 2012.