Average electric wave spectra across the plasma sheet and their relation to ion bulk speed

Using 4 months of tail data obtained by the ELF/MF spectrum analyzer of the wave experiment and the three‐dimensional plasma instrument on board the AMPTE/IRM satellite, we have done a statistical survey on the electric wave spectral density in the Earth's plasma sheet. More than 50,000 10‐s‐averaged electric wave spectra were analyzed with respect to differences between their values in the inner and outer central plasma sheet and the plasma sheet boundary layer as well as their dependence on radial distance and ion bulk speed. High‐speed flows are dominated by broadband electrostatic noise with highest spectral densities in the plasma sheet boundary layer, where broadband electrostatic noise also exists during periods of low‐speed flows. The broadband electrostatic noise has a typical spectral index of about −2. During low‐speed flows the spectra in the central plasma sheet show distinct emissions at the electron cyclotron odd half‐harmonic and and upper hybrid frequency. On average, such waves do not appear as distinct emissions in the plasma sheet boundary layer, where they are either absent or masked by the intense broadband electrostatic noise always observed there. Finally, wave intensities during episodes of fast perpendicular flows are higher than those associated with fast parallel flows.