Magnetospheric Plasma

Abstract

This paper reviews the principal features of the distribution of plasma in the magnetosphere as inferred from observations of particles with energies in the keV range and below. Low energy electrons are found within the equatorial region of the magnetotail, where they form the plasma sheet, and throughout the outer magnetosphere, where they envelop the Earth with a complicated and as yet only partly explored structure. Within the plasma sheet, electrons typically have a number density of 0.1 to 1 cm⁻³ and a mean energy of 1 keV, while protons have the same number density and a mean energy higher by a factor of about 5; all these properties exhibit large temporal fluctuations. The sum of particle and magnetic pressures is roughly constant across the plasma sheet. The intense electron fluxes of the plasma sheet terminate at the so-called inner boundary of the plasma sheet, which is located at about 11 R _e in the evening side of the magnetosphere and approaches the plasmapause near the midnight meridian. This inner boundary is characterized by a rapid decrease of electron mean energy with decreasing distance and little or no change in number density. During substorms, the inner boundary moves closer to the Earth, while deep in the magnetotail the plasma sheet first becomes thinner and then expands. Low energy proton fluxes extend earthward as far as the plasmapause; between the inner boundary of the plasma sheet and the plasmapause, they become more intense and form the so-called ring current. Proton energy spectra show an increase of both energy and intensity with decreasing distance, while electron energy spectra show a large non-adiabatic increase of low energy fluxes at the inner boundary of the plasma sheet. Comparison between magnetotail electron densities and whistler measurements suggests that the observed plasma sheet particles may constitute the thermal particle population of the magnetotail. The plasma sheet appears to map into the auroral oval at ionospheric heights. The electron energy flux precipitated into the auroral oval is on the average 1/4 of the omnidirectional electron energy flux within the plasma sheet; the decrease of the precipitated flux with increasing latitude corresponds to the decrease of the plasma sheet flux with increasing distance from the neutral sheet.