Average electric wave spectra in the plasma sheet: Dependence on ion density and ion beta

Using 4 months of tail data obtained by the ELF/MF spectrum analyzer and the plasma instrument on board the AMPTE/IRM satellite, more than 50,000 ten‐second‐averaged electric wave spectra were analyzed in order to establish typical spectra for periods of high and low ion density and high and low ion β. The general spectral slope of the spectra in the plasma sheet follows an f−2 law. Ion β has a stronger influence on the spectral form than the ion density. Highest average spectral densities are obtained in the low‐β plasma sheet boundary layer, where the spectrum is that of broadband electrostatic noise extending to frequencies near and above the upper hybrid frequency. Lowest wave intensities are encountered in the high‐β inner central plasma sheet. The outer central plasma sheet has generally low wave intensities and is dominated by electron cyclotron odd half‐harmonics and electron plasma wave emissions. Lower electron cyclotron harmonics are found in the high‐β cases in all three regions of the plasma sheet while higher odd half‐harmonics dominate the low‐β and low‐density inner central plasma sheet.