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, extensive research has been done investigating the possible ion beam-related instabilities that could generate the observed wave spectra. It has also been demonstrated that unstable electron distribution functions are sometimes measured in the plasma sheet boundary layer. We present simultaneous observations of ion and electron distribution functions 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 traversals of the boundary layer where the distinct electron and ion boundaries 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 later by the detection of boundary layer ions. The onset of large-amplitude electrostatic waves at frequencies up to the electron plasma frequency was coincident with the onset of the boundary layer electrons. This evidence suggests that broadband electrostatic waves may often be generated by unstable electron distributions in the plasma sheet boundary layer, particularly the high-frequency portion of the wave spectrum. The lower-frequency waves are found to be polarized perpendicular to 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 polarization when the amplitudes were high. The observed changes in the electron distribution functions indicate that the plasma was not heated locally by the waves.