Coherent waveforms in the auroral upward current region

With the help of one-dimensional particle-in-cell simulations we show that observations of high-frequency electric wave spectra in the auroral upward current region can be reproduced. Using distribution functions suggested by the measurements, we find that in the nonlinear state, ion acoustic waves and electron two-stream (Langmuir) waves dominate the spectrum. In the absence of cold electrons, electron acoustic waves are not excited initially but appear only at a late time. They are the result of the formation of a two-temperature electron plasma by nonlinear interactions when all other instabilities have saturated. However, owing to the weakness of these electron acoustic waves, they are less important for the formation of electron holes and affect the particle dynamics much less than the two-stream instability.