ELECTRON GENERATION OF ELECTROSTATIC-WAVES IN THE PLASMA SHEET BOUNDARY-LAYER

Broadband electrostatic noise (BEN) is a common feature of the plasma sheet boundary layer in the Earth's magnetotail. Observationally, BEN has been shown to occur in conjunction with ion beams. Consequently, e xtensive research has been done investigating the possible ion beam-re lated instabilities that could generate the observed wave spectra, It has also been demonstrated that unstable electron distribution functio ns are sometimes measured in the plasma sheet boundary layer. We prese nt simultaneous observations of ion and electron distribution function s and electric field wave spectra measured by ISEE 1 and ISEE 2 in the Earth's magnetotail. For this study we have used particularly slow tr aversals of the boundary layer where the distinct electron and ion bou ndaries can be clearly resolved. As the spacecraft moved from the tail lobe toward the plasma sheet, the first indication of boundary layer plasma was seen in the electron distributions, followed some minutes l ater by the detection of boundary layer ions. The onset of large-ampli tude electrostatic waves at frequencies up to the electron plasma freq uency was coincident with the onset of the boundary layer electrons. T his evidence suggests that broadband electrostatic waves may often be generated by unstable electron distributions in the plasma sheet bound ary layer, particularly the high-frequency portion of the wave spectru m. The lower-frequency waves are found to be polarized perpendicular t o the background magnetic field, consistent with previous observations . The higher-frequency waves (near the electron plasma frequency) had perpendicular polarization when the amplitudes were low and parallel p olarization when the amplitudes were high. The observed changes in the electron distribution functions indicate that the plasma was not heat ed locally by the waves.