Copyright © National Academy of Sciences. All rights reserved.
Solar and Space Physics: A Science for a Technological Society
REPORT OF THE PANEL ON SOLAR WIND-MAGNETOSPHERE INTERACTIONS
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field lines, and both radio sounding and extreme ultraviolet (EUV) images revealed not only the dynamics
of the boundaries of the plasmasphere, but also actual global plasmaspheric density distributions. These
advances made it possible for the first time to observe the global evolution of the plasmasphere in response
to variable driving of the magnetosphere.
Energetic neutral atom (ENA) imaging established itself as a valuable tool in determining the globalscale configuration, dynamics, and composition of the ring current (e.g., see Figure 9.6 below). It was
found that the peak of the ring-current proton distribution during the main phase of magnetic storms could
lie not in the historically expected afternoon location, but in the early morning sector, revealing that the
coupling with the ionosphere can very strongly alter the behavior of magnetospheric plasmas. In addition,
the hydrogen component of the ring current builds up and decays gradually throughout a magnetic storm,
but the oxygen component rises and falls impulsively. These variations in oxygen are often correlated with
substorm injections, highlighting the coupling between the magnetosphere and ionosphere. These results
represent more than a huge leap forward in system-level knowledge of the ring current; they also offer a
tantalizing hint of the dynamic magnetospheric behavior that could be uncovered with higher-resolution,
continuous, and global imaging.
9.3.3 Processes
9.3.3.1 Magnetic Reconnection
The recent decade has witnessed substantial progress in understanding how magnetic reconnection
works. For example, increased computing power has allowed full-physics simulations describing the
essential physics and structure of the diffusion region, the key region where magnetic field lines break
and reform. The decoupling of ion and electron motions plays a key role, accelerating the energy release,
creating high-speed electron beams, and warping the magnetic field. These predictions have facilitated
the first direct detections of the ion diffusion region in the magnetosphere (Figure 9.3) and in the laboratory, as well as glimpses of the much smaller electron diffusion region. The observations in the vicinity of
the diffusion region revealed surprisingly that electrons can be accelerated by reconnection to hundreds
of kiloelectron volts.
The past decade has also witnessed surprises regarding the triggering and modulation of reconnection:
theoretical studies using fully three-dimensional simulations revealed that the added dimension facilitates
plasma instabilities that can disrupt the diffusion region, making reconnection highly turbulent. Observationally, reconnection seems to behave differently in different regions. On the dayside magnetopause,
as well as recently discovered in the solar wind, magnetic reconnection can be quite steady in time and
extended in space. In the magnetotail, however, reconnection is most often patchy and bursty, producing
narrow flow burst channels. Using multispacecraft observations, these reconnection-generated flow channels have now been demonstrated to initiate magnetospheric substorms.
9.3.3.2 Wave-Particle Interactions
A delicate balance between acceleration and loss caused by wave-particle interactions controls the
variability of radiation belt fluxes during geomagnetic storms. Understanding of magnetospheric plasma
waves and their role in radiation belt dynamics has increased significantly during the past decade. Statistical analyses of satellite wave data have led to the development of global models of the wave environment.
These have then been used to quantify the rates of energization and scattering loss to the atmosphere.
Time-dependent two-dimensional and three-dimensional models for the radiation belts and the ring cur-
Solar and Space Physics: A Science for a Technological Society
REPORT OF THE PANEL ON SOLAR WIND-MAGNETOSPHERE INTERACTIONS
217
field lines, and both radio sounding and extreme ultraviolet (EUV) images revealed not only the dynamics
of the boundaries of the plasmasphere, but also actual global plasmaspheric density distributions. These
advances made it possible for the first time to observe the global evolution of the plasmasphere in response
to variable driving of the magnetosphere.
Energetic neutral atom (ENA) imaging established itself as a valuable tool in determining the globalscale configuration, dynamics, and composition of the ring current (e.g., see Figure 9.6 below). It was
found that the peak of the ring-current proton distribution during the main phase of magnetic storms could
lie not in the historically expected afternoon location, but in the early morning sector, revealing that the
coupling with the ionosphere can very strongly alter the behavior of magnetospheric plasmas. In addition,
the hydrogen component of the ring current builds up and decays gradually throughout a magnetic storm,
but the oxygen component rises and falls impulsively. These variations in oxygen are often correlated with
substorm injections, highlighting the coupling between the magnetosphere and ionosphere. These results
represent more than a huge leap forward in system-level knowledge of the ring current; they also offer a
tantalizing hint of the dynamic magnetospheric behavior that could be uncovered with higher-resolution,
continuous, and global imaging.
9.3.3 Processes
9.3.3.1 Magnetic Reconnection
The recent decade has witnessed substantial progress in understanding how magnetic reconnection
works. For example, increased computing power has allowed full-physics simulations describing the
essential physics and structure of the diffusion region, the key region where magnetic field lines break
and reform. The decoupling of ion and electron motions plays a key role, accelerating the energy release,
creating high-speed electron beams, and warping the magnetic field. These predictions have facilitated
the first direct detections of the ion diffusion region in the magnetosphere (Figure 9.3) and in the laboratory, as well as glimpses of the much smaller electron diffusion region. The observations in the vicinity of
the diffusion region revealed surprisingly that electrons can be accelerated by reconnection to hundreds
of kiloelectron volts.
The past decade has also witnessed surprises regarding the triggering and modulation of reconnection:
theoretical studies using fully three-dimensional simulations revealed that the added dimension facilitates
plasma instabilities that can disrupt the diffusion region, making reconnection highly turbulent. Observationally, reconnection seems to behave differently in different regions. On the dayside magnetopause,
as well as recently discovered in the solar wind, magnetic reconnection can be quite steady in time and
extended in space. In the magnetotail, however, reconnection is most often patchy and bursty, producing
narrow flow burst channels. Using multispacecraft observations, these reconnection-generated flow channels have now been demonstrated to initiate magnetospheric substorms.
9.3.3.2 Wave-Particle Interactions
A delicate balance between acceleration and loss caused by wave-particle interactions controls the
variability of radiation belt fluxes during geomagnetic storms. Understanding of magnetospheric plasma
waves and their role in radiation belt dynamics has increased significantly during the past decade. Statistical analyses of satellite wave data have led to the development of global models of the wave environment.
These have then been used to quantify the rates of energization and scattering loss to the atmosphere.
Time-dependent two-dimensional and three-dimensional models for the radiation belts and the ring cur-