The geomagnetic field resembles a quasi-blunt object in a supersonic flow field in terms of its interaction with the solar wind stream. The earth's field is compressed on the sunward side and distended on the anti-sunward side, giving rise to a characteristic shape resembling a comet. Within the magnetosphere, solar wind particles are generally excluded, being deflected by the severely distorted geomagnetic field. A collisionless bow shock is formed upstream of the magnetospheric boundary (or magnetopause), and the region between the shock boundary and the magnetosphere is termed the magnetosheath. The magnetosheath is the region of closest approach for the deflected solar wind particles. Within the magnetosphere the motion of plasma is governed by the earth's magnetic field. Since ion-neutral collisions are not insignificant within the ionosphere, and since this may restrict geomagnetic control of plasma motion, we do not regard the ionosphere as part of the magnetosphere. Moreover, since the geomagnetic field vanishes beyond the magnetopause, to be replaced by the Interplanetary Magnetic Field or IMF, the magnetosheath is actually not part of the magnetosphere either.
Solar wind particles are typically denied entry to the ionosphere because of the geomagnetic field interaction just mentioned. However, there are some exceptions. Particles may gain entrance through the polar cusp regions. During energetic particle events, this process is enhanced and polar cap absorption (PCA) is the result. Also, because of magnetic merging of the IMF with the geomagnetic field, magnetosheath plasma may be temporarily captured by the plasma sheet (see Figure 2-20).
Another region of interest is the plasmasphere, which serves as a reservoir for ionospheric replenishment during the night and acts as a sink for electrons during the daytime. A very important property of the plasmasphere is its closed field lines. The plasmasphere contains the Van Allen radiation belts. The poleward boundary of the plasmasphere (called the plasmapause)
maps into an ionospheric region called the high latitude trough. Electron concentrations are relaxed in this region. Poleward of the plasmapause, the geomagnetic field lines are no longer closed, but are stretched out well into the magnetotail. This region of open field lines is called the plasma sheet and it has important implications for telecommunication systems at high latitudes. Disturbances within the plasma sheet produce enhanced auroral activity.
Boundary - / Layer
Figure 2-20: A depiction of the magnetospheric regions. From Hill and Wolf .
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