In Earth's magnetosphere, plasma sheet is the region of closed field lines in the equatorial magnetotail (however, see also plasmoids). Plasma sheet is typically divided into plasma sheet proper, or central plasma sheet, and plasma sheet boundary layer (PSBL), the latter being at higher latitudes adjacent to the tail lobes. It is also divided into two parts by the cross-tail current sheet in the equatorial plane. However, as the plasma sheet extends from the magnetotail to the geostationary orbit, the effects of the current are not as important in the inner plasma sheet as further tailward, and the magnetic field geometry has a transition region between the inner dipolar form and outer taillike field.
Plasma sheet is a very important region for auroral physics, since the nighttime auroral oval maps to it (to what radial distances is an unanswered question; however, see isotropic boundary). The diffuse aurora originates from a region closer to Earth than the discrete aurora.
Particles and fields
Plasma sheet particles are hot, having energies in the keV range. Plasma density is a slowly varying function of time, average value being about 0.4 - 2 cm-3. It also correlates with the solar wind density. This and the fact that H+ dominates during low geomagnetic activity indicates that solar wind is - at least partly - providing the plasma. Ion temperature is about seven times the electron temperature (Baumjohann, Paschmann, and Cattell, 1989).
Because of the magnetospheric, large scale convection electric field, plasma is in continuous movement both toward Earth and toward the central cross-tail current region from tail lobes. At the inner plasma sheet boundary electrons and ions are on different convection paths, and electron plasma sheet does not always quite reach the geosynchronous orbit, while the ion plasma sheet does.
Electrostatic and electromagnetic waves in the distant plasma sheet were first described by Scarf et al. (1974). Observations by Gurnett et al. (1976) and Cattell et al. (1986) divide the observed waves into
- intense broadband electrostatic noise (BEN)
- extends from lower hybrid frequency to electron cyclotron frequency
- enhanced near the cross-tail current, but supressed within
- magnetic noise bursts
- relate to cross-tail current and intervals of tailward flows
- electrostatic electron cyclotron waves
- upper hybrid waves
Storms and substorms
During increased geomagnetic activity, O+ ion dominates in the inner magnetosphere. This is evident both in storm time ring current and substorm time plasma sheet (including late growth and the expansion phases; see, e.g., Daglis et al., 1994). Since these ions come from the ionosphere (see upward flowing ions), ionosphere may be able to control the evolution of dynamic geospace processes via transient and localized dominance. The major substorm phenomena, including magnetic field dipolarization and particle injections, occur within the inner plasma sheet. Another substorm related phenomena are the high speed flows observed both in the central and boundary layer plasma sheet.
It has also been suggested that the periods of superdense plasma sheet may be an important precondition for geomagnetic storms (Borovsky et al., 1997). These events occur about 2 or 3 times per month, and densities are up to 2 - 5 cm-3. The extra plasma may be peeled off from the plasmasphere.
Plasma sheet boundary layer, PSBL
Plasma sheet is separated from the tail lobes by the plasma sheet boundary layer (PSBL). It is probably located on closed field lines, and the densities are only little smaller than in central plasma sheet. PSBL is the location of, e.g., velocity-dispersed ion structures, VDIS, which may relate to some high latitude proton aurora. For more about PSBL, see Eastman et al. (1984).
- Baumjohann, W., G. Paschmann, and C. A. Cattell, Average plasma properties in the central plasma sheet, J. Geophys. Res., 94, 6597, 1989.
- Borovsky, J. E., M. F. Thomsen, and D. J. McComas, The superdense plasma sheet: Plasmaspheric origin, solar wind origin, or ionospheric origin?, J. Geophys. Res., 102, 22089-22097, 1997.
- Cattell, C. A., F. S. Mozer, R. R. Anderson, E. W. Hones, Jr., and R. D. Sharp, ISEE observations of the plasma sheet boundary, plasma sheet, and neutral sheet 2. Waves, J. Geophys. Res., 91, 5681-5688, 1986.
- Daglis, I. A., S. Livi, E. T. Sarris, and B. Wilken, Energy density of ionospheric and solar wind origin ion the near-Earth magnetotail during substorms, J. Geophys. Res., 99, 5691-5703, 1994.
- Eastman, T. E., L. A. Frank, W. K. Peterson, and W. Lennartsson, The plasma sheet boundary layer, J. Geophys. Res., 89, 1553-1572, 1984.
- Gurnett, D. A., L. A. Frank, and R. P. Lepping, Plasma waves in the distant magnetotail, J. Geophys. Res., 81, 6059, 1976.
- Scarf, F. L., L. A. Frank, K. L. Ackerson, and R. P. Lepping,, Plasma wave turbulence at distant crossings of the plasmasheet boundaries and neutral sheet, Geophys. Res. Lett., 1, 109, 1974.