Abstract

Vela satellite observations of electrons and protons with E > 100 ev have shown that a sheet of plasma with enhanced energy density stretches across the earth's magnetotail from the dusk to the dawn boundaries of the magnetosphere. The plasma has been observed at geocentric distances between 15.5 and 20.5 earth radii; this plasma sheet probably extends from the night-side termination of the radiation belt to beyond 31 RE in the antisolar direction. Near the midnight meridian at ∼17 RE the sheet is often ∼4–6 RE thick, while toward the dusk and dawn boundaries the sheet flares out to about twice that thickness. The plane of symmetry of the sheet lies above or below the solar magnetospheric equatorial plane depending on whether the geomagnetic dipole axis tilts toward or away from the sun. The plasma is located in the vicinity of the ‘neutral sheet’ region of magnetic field reversal; inside the plasma sheet the measured kinetic energy densities of the electrons are comparable to the expected magnetic field energy densities, while outside the sheet the kinetic energy densities are much lower. The energy spectrums are quasi-thermal in character, having average energies extending from ∼200 ev to above 12 kev; omnidirectional fluxes extend to above 109 cm−2 sec−1. Both rapid (100–200 sec) and slower changes occur in the average energies of the plasma sheet electrons, resulting in large variations in the flux of energetic electrons (E > 45 kev). The plasma sheet boundaries, which are frequently in motion, are clearly defined by large changes in the electron flux. The average energy of the electron population is higher on the dawn side of the magnetotail than the dusk side, resulting in a more frequent appearance of energetic electrons on the dawn side.