Figure 14.6 offers another global view of the different plasma regions found in the magnetosphere, complementing Figures 14.4 and 14.5.
Figure 14.6: Schematic illustration of Earth's magnetotail, emphasizing the different plasma regions [Cravens, 1997]. Note the cusps, plasma mantle, tail lobes, plasmasheet, neutral sheet, ring current, radiation belts, plasmasphere, and auroral ovals.
We now describe these regions in some detail. Note that these regions are not all distinct; instead there may be smooth transitions from one region to another without clear boundaries.
Magnetic field lines leaving the cusp are magnetically connected to the solar wind, thereby permitting the solar wind's convection electric field to map across Earth's polar cap and causing convection of plasma there. The cusp maps to the auroral oval near noon, with the auroral oval generally separating the regions with closed and open terrestrial magnetic field lines.
Figure 14.7: Noon-midnight cross section of the magnetosphere and geomagnetic tail [Hughes, 1995]. Magnetic field lines are shown using solid lines while the dashed lines shows particles moving along the field subject to the drift for a solar wind convection field directed from dusk-to-dawn.
The situation is analogous to Figure 11.1 for the heliospheric current sheets and coronal streamers. The magnetotail acts as a reservoir for plasma and magnetic field energy that is released in so-called ``magnetic substorms'' (Lecture 15). The magnetotail is at least long and may be thousands of long. Jupiter's magnetotail, for instance, was observed by the Voyager spacecraft to sometimes stretch all the way to Saturn, that is for a distance AU.
The current sheet at the center of the magnetotail, sometimes called the ``neutral sheet'', is at the center of the plasmasheet. The current in the current sheet is called the ``cross-tail current'' (Figure 14.4 and 14.5) and it flows from dawn to dusk.
It is partly a matter of definition whether the tail lobes are distinct from the plasma mantle/magnetopause boundary layer, since any transition between the two is smooth. Figure 14.8 illustrates the conceptual difficulty posed by the mantle plasma as it (typically) convects toward the plasmasheet from the cusp and magnetopause and partly fills the tail lobe regions.
Figure 14.8: Schematic of plasma regions in Earth's magnetosphere [Wolf, 1995]. Note the smooth transition from plasma mantle to tail lobe, the plasma sheet boundary layer, plasma sheet, radiation belts and ring current, and the plasmasphere. Note that geosynchronous orbit often lies close to the boundary of the plasmasphere and the plasmasheet.
Note that the plasma pressure in the lobes and mantle is very small compared with the magnetic field pressure.
The plasmasheet is the scene of much geomagnetic activity, particularly to do with substorms. Most theories for substorms involve magnetic reconnection proceeding at a distant site approximately downtail from Earth and also at a near-Earth reconnection site near in the tail, from which energetic particles are injected into geosynchronous orbit. In quiet times the plasmasheet primarily contains plasma of solar wind origin but in active times plasma of ionospheric origin may dominate.
The ring current is carried by energetic electrons and ions that are undergoing gradient and curvature drifts around the Earth (as well as their bounce and gyro motions). Figure 14.9 shows the directions of these drifts and the resultant westwards direction of the current, which opposes the Earth's field in the region interior to the current but adds to the Earth's field in the exterior region.
Figure 14.9: Illustration of why the ring current flows westward [Brand, 1999]. The figure also indicates why the ring current can depress the north-south magnetic field at Earth's surface but increase the effective field in the outer magnetosphere.
Equation (2.30) shows why these drifts are energy dependent, so that the cold plasma population does not contribute significantly, and why the electron and ion currents add up. The ring current is diamagnetic, so that times of enhanced ring current (during geomagnetic activity like substorms) correspond to decreases in the field observed at the Earth's surface. One geomagnetic activity index, , measures the decrease in the surface magnetic field due to increases in the ring current.
The radiation belts have higher energies than the main ring current particles and are much more stable, having loss times (due to loss-cone effects, wave-particle scattering and collisions with ionospheric and plasmaspheric particles) that are much longer than the ring current particles (years rather than a few days). The radiation belts do not usually vary with geomagnetic activity. However, new radiation belts can be created due to the injection of unusually large amounts of energetic plasma into near-Earth orbit. Examples of this are the enhanced radiation belt formed after the atmospheric nuclear explosion Starfish and new belts formed by an unusually energetic solar shock a few years ago.
The plasmaspheric plasma corotates with Earth. This means that a large corotation electric field exists in the plasma, since the plasma is still collisionless. The outer boundary of the plasmasphere, the plasmapause is relatively sharp. Both the sharpness and location of the plasmapause vary with geomagnetic activity, being sharper and located closer to Earth during times of larger geomagnetic activity.