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Solar and Space Physics: A Science for a Technological Society
274
SOLAR AND SPACE PHYSICS: A SCIENCE FOR A TECHNOLOGICAL SOCIETY
essential for understanding our home in space. The lack of such measurements of the corona is one of
the greatest obstacles to advancing solar and heliospheric science. Hinode and SDO can determine the
full vector field accurately in the photosphere, but the plasma beta is high there, and so it is not possible
to extrapolate the magnetic field into the low-beta corona reliably. Two major advances during the past
decade promise to overcome the magnetic-field measurement obstacle: the first observations of the full
chromospheric vector field on the disk, and the first maps of the coronal field above the solar limb.
The prevalence of high-resolution extreme UV (EUV) narrowband images from instruments on the
TRACE (Transition Region and Coronal Explorer) and SDO (Solar Dynamics Observatory) spacecraft have
revealed that warm (1 million Kelvin) coronal loops are up to 3 orders of magnitude overdense and hence
cannot be in steady state as previously believed (Figure 10.9). Observations and theoretical models of the
structures imply that they are still unresolved. How coronal structures are heated is still unknown, but it
has been suggested that including the coupling between the chromosphere and corona will prove essential
in determining the heating mechanism.
Great progress has also been made in the past decade in achieving closure between observations
and predictions from theory or models. The first three-dimensional magnetohydrodynamic (MHD) ab
initio numerical simulation of the corona was successfully performed. The chromosphere now stands
as the modeling frontier with qualitatively more challenging simulations requiring radiation coupled to
MHD (R-MHD) and no assumption of local thermodynamic equilibrium. Fully three-dimensional R-MHD
numerical simulations spanning the upper convection zone through the corona—treated as a coherent
system—are starting to appear but cannot yet address all the salient physical ingredients on scales larger
than a few granules or one supergranule. This state of affairs should improve in the coming decade given
the expected rapid progress in numerical hardware and software.
10.3.3 Determining How Magnetic Energy Is Stored and Explosively Released
Major solar flares and associated fast coronal mass ejections (CMEs) are the most powerful explosions
and particle accelerators in the solar system, and they produce the most extreme space weather. In the past
decade, substantial progress was made in understanding how magnetic energy is stored and explosively
released on both large and small scales (motivation M1).
RHESSI hard X-ray (HXR) imaging-spectroscopy measurements have shown that accelerated electrons
often contain about 50 percent of the solar-flare energy release and provided strong evidence that energy
release-electron acceleration is associated with magnetic reconnection. In one occulted flare, HXR and
microwave measurements from the acceleration region, high above the thermal soft-X-ray loop tops (Figure 10.10), showed that essentially all electrons in this region were accelerated to over about 15 keV with
no detectable thermal plasma. The accelerated-electron and magnetic-field energy densities were comparable. In large flares, the energy in ions over 1 MeV and in electrons over 20 keV appears comparable.
RHESSI gamma-ray imaging of flare-accelerated ions of about 30 MeV shows emission from two small
footpoints rather than an extended region. In the largest flare, the footpoints straddled the flare-loop arcade
(Figure 10.11), indicating that ion acceleration is also related to magnetic reconnection.
For the first time, flares were detected in total solar irradiance (TSI) with the SORCE/TIM instrument.
For the X17 October 28, 2003, flare, the TSI showed both impulsive (HXR-like) and gradual (soft-X-ray-like)
components with a peak increase of about 100 ppm. The total radiated energy (over about 10 32 ergs) and
associated-CME kinetic energy (about 10 33 ergs) were comparable. In addition, the SDO/EVE instrument
has discovered an EUV late phase in flares, a second enhancement delayed many minutes after the X-ray
peak. In addition, global EUV observations with SDO/AIA and STEREO/EUVI have revealed long-distance
“sympathetic” interactions between magnetic fields in flares, eruptions, and CMEs during August 1-2, 2010,
Solar and Space Physics: A Science for a Technological Society
274
SOLAR AND SPACE PHYSICS: A SCIENCE FOR A TECHNOLOGICAL SOCIETY
essential for understanding our home in space. The lack of such measurements of the corona is one of
the greatest obstacles to advancing solar and heliospheric science. Hinode and SDO can determine the
full vector field accurately in the photosphere, but the plasma beta is high there, and so it is not possible
to extrapolate the magnetic field into the low-beta corona reliably. Two major advances during the past
decade promise to overcome the magnetic-field measurement obstacle: the first observations of the full
chromospheric vector field on the disk, and the first maps of the coronal field above the solar limb.
The prevalence of high-resolution extreme UV (EUV) narrowband images from instruments on the
TRACE (Transition Region and Coronal Explorer) and SDO (Solar Dynamics Observatory) spacecraft have
revealed that warm (1 million Kelvin) coronal loops are up to 3 orders of magnitude overdense and hence
cannot be in steady state as previously believed (Figure 10.9). Observations and theoretical models of the
structures imply that they are still unresolved. How coronal structures are heated is still unknown, but it
has been suggested that including the coupling between the chromosphere and corona will prove essential
in determining the heating mechanism.
Great progress has also been made in the past decade in achieving closure between observations
and predictions from theory or models. The first three-dimensional magnetohydrodynamic (MHD) ab
initio numerical simulation of the corona was successfully performed. The chromosphere now stands
as the modeling frontier with qualitatively more challenging simulations requiring radiation coupled to
MHD (R-MHD) and no assumption of local thermodynamic equilibrium. Fully three-dimensional R-MHD
numerical simulations spanning the upper convection zone through the corona—treated as a coherent
system—are starting to appear but cannot yet address all the salient physical ingredients on scales larger
than a few granules or one supergranule. This state of affairs should improve in the coming decade given
the expected rapid progress in numerical hardware and software.
10.3.3 Determining How Magnetic Energy Is Stored and Explosively Released
Major solar flares and associated fast coronal mass ejections (CMEs) are the most powerful explosions
and particle accelerators in the solar system, and they produce the most extreme space weather. In the past
decade, substantial progress was made in understanding how magnetic energy is stored and explosively
released on both large and small scales (motivation M1).
RHESSI hard X-ray (HXR) imaging-spectroscopy measurements have shown that accelerated electrons
often contain about 50 percent of the solar-flare energy release and provided strong evidence that energy
release-electron acceleration is associated with magnetic reconnection. In one occulted flare, HXR and
microwave measurements from the acceleration region, high above the thermal soft-X-ray loop tops (Figure 10.10), showed that essentially all electrons in this region were accelerated to over about 15 keV with
no detectable thermal plasma. The accelerated-electron and magnetic-field energy densities were comparable. In large flares, the energy in ions over 1 MeV and in electrons over 20 keV appears comparable.
RHESSI gamma-ray imaging of flare-accelerated ions of about 30 MeV shows emission from two small
footpoints rather than an extended region. In the largest flare, the footpoints straddled the flare-loop arcade
(Figure 10.11), indicating that ion acceleration is also related to magnetic reconnection.
For the first time, flares were detected in total solar irradiance (TSI) with the SORCE/TIM instrument.
For the X17 October 28, 2003, flare, the TSI showed both impulsive (HXR-like) and gradual (soft-X-ray-like)
components with a peak increase of about 100 ppm. The total radiated energy (over about 10 32 ergs) and
associated-CME kinetic energy (about 10 33 ergs) were comparable. In addition, the SDO/EVE instrument
has discovered an EUV late phase in flares, a second enhancement delayed many minutes after the X-ray
peak. In addition, global EUV observations with SDO/AIA and STEREO/EUVI have revealed long-distance
“sympathetic” interactions between magnetic fields in flares, eruptions, and CMEs during August 1-2, 2010,