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Solar and Space Physics: A Science for a Technological Society
58
SOLAR AND SPACE PHYSICS: A SCIENCE FOR A TECHNOLOGICAL SOCIETY
tospheric 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 within the heliosphere and in astrophysical
settings throughout the universe. They are also highly relevant to understanding reconnection events in
tokomak plasmas and in laboratory-based reconnection experiments. The centrality of reconnection in such
diverse settings motivates the following primary challenge: SWMI-1. Establish how magnetic reconnection
is triggered and how it evolves to drive mass, momentum, and energy transport.
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 R E from Earth’s center (see Figure 2.6). The outer radiation belt therefore overlaps the orbit radius of
geostationary satellites (6.6 R E ) 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 in 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; renamed the Van Allen
Probes) 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.
At around 100-km altitude, the atmosphere starts to transition from being neutrally dominant to being
dominated by charged particles. The ionosphere, which is often thought to be the inner boundary of the
magnetosphere, overlaps with the thermosphere. At these altitudes, the neutral density is about 1,000
times larger than the ion density, but the electromagnetic forces on the ions are significantly larger than
the forces on the neutrals, so they become more and more important as the altitude increases. This region
of the atmosphere is quite thick, being a couple of hundred kilometers in altitude, in comparison to the
troposphere, which is only 10 km thick, but it pales in comparison to the vast space carved out by the
magnetosphere, which extends out 10 to 100 Earth radii. If one were to calculate the whole mass of all
of the particles in the magnetosphere, it would be about an order of magnitude less than the mass of the
ionosphere, even though the ionosphere is so much smaller. This is because the density of the ionosphere
is so much larger than the near-vacuum of the magnetosphere.
Magnetic field lines converge in the polar regions in 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 upwelling of ions 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 on both the dayside and the nightside. Figure 2.10 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 asso-
Solar and Space Physics: A Science for a Technological Society
58
SOLAR AND SPACE PHYSICS: A SCIENCE FOR A TECHNOLOGICAL SOCIETY
tospheric 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 within the heliosphere and in astrophysical
settings throughout the universe. They are also highly relevant to understanding reconnection events in
tokomak plasmas and in laboratory-based reconnection experiments. The centrality of reconnection in such
diverse settings motivates the following primary challenge: SWMI-1. Establish how magnetic reconnection
is triggered and how it evolves to drive mass, momentum, and energy transport.
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 R E from Earth’s center (see Figure 2.6). The outer radiation belt therefore overlaps the orbit radius of
geostationary satellites (6.6 R E ) 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 in 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; renamed the Van Allen
Probes) 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.
At around 100-km altitude, the atmosphere starts to transition from being neutrally dominant to being
dominated by charged particles. The ionosphere, which is often thought to be the inner boundary of the
magnetosphere, overlaps with the thermosphere. At these altitudes, the neutral density is about 1,000
times larger than the ion density, but the electromagnetic forces on the ions are significantly larger than
the forces on the neutrals, so they become more and more important as the altitude increases. This region
of the atmosphere is quite thick, being a couple of hundred kilometers in altitude, in comparison to the
troposphere, which is only 10 km thick, but it pales in comparison to the vast space carved out by the
magnetosphere, which extends out 10 to 100 Earth radii. If one were to calculate the whole mass of all
of the particles in the magnetosphere, it would be about an order of magnitude less than the mass of the
ionosphere, even though the ionosphere is so much smaller. This is because the density of the ionosphere
is so much larger than the near-vacuum of the magnetosphere.
Magnetic field lines converge in the polar regions in 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 upwelling of ions 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 on both the dayside and the nightside. Figure 2.10 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 asso-