Generation of electrostatic waves by discontinuous electron distributions

This paper presents studies of narrowband electrostatic waves (NEN) in the distant lobe region and electrostatic waves in the bow shock and slow mode shocks. Geotail measurements from the tail lobe region show a two‐humped feature in the low‐energy region of the parallel electron distributions correlated with lobe NEN emissions. Unstable waves were found in our linear dispersion and simulation studies initialized with the measured electron distributions. The wave energy density, wave frequency, and waveform were comparable with those from spacecraft observations. The electron contribution to bow shock and slow mode shock wave spectra was examined with the use of a parallel electron distribution model function that featured flat tops and single peaks drifting with speeds of the order of the effective electron thermal speed. The model assumed a possible discontinuity in the parallel distribution. Results from the model included waves with frequencies above the ion plasma frequency (ωpi) but well below the electron plasma frequency (ωpe) as well as broadband, higher‐frequency waves extending to ωpe. In addition, wave frequencies increased and growth rates and wave energy density decreased as the peak offset speed increased and the peak height decreased during a shock penetration. Dispersions from both linear theory and simulation were obtained in our studies. The unstable waves had a broad k range with kλD exceeding unity. For waves with frequencies well below ωpe, the frequency decreased further during the evolution of the electron distribution.