ISEE observations of the plasma sheet boundary, plasma sheet, and neutral sheet: 2. Waves

Abstract

The companion paper describes the dc electric and magnetic fields, E × B velocities, plasma flow, composition and ground magnetic activity for two periods in the magnetotail near 20 RE. The relationship of the plasma waves to the dc field characteristics is described herein. In addition, for two subintervals which had strong ground magnetic activity, we report comparisons of plasma waves in the magnetotail to characteristics of the dc fields and fluid parameters such as the plasma flow, β, density, temperature, and fpe/fce. Upper hybrid waves at multiples of the electron cyclotron frequency near and above the electron plasma frequency were observed for the first time in this region of the magnetosphere and occurred in regions of low β and temperature and high density. This contrasts with electrostatic electron cyclotron waves which occurred in high‐temperature, high‐density regions, as previously reported by Gurnett et al. (1976), and were suppressed when β ≲ 1. Both types of waves were associated with small dc electric fields, nonturbulent dc electric and magnetic fields, and plasma flows of ≲100 km/s. Broadband electrostatic noise (BEN) and magnetic noise bursts were observed in regions where the dc electric fields and the plasma flows were large, and the dc electric and magnetic fields were turbulent. The dependence of the amplitude and frequencies of BEN on the magnitude of the magnetic field, plasma flow, density, and β was very complex. Enhancements at the upper hybrid frequency and the electron cyclotron frequency were common. BEN was usually suppressed near the neutral sheet but the amplitude reduction varied. The fact that the magnetic noise was most intense during tailward flow and often was not suppressed near the neutral sheet suggests that it may provide the dissipation necessary for reconnection to occur.