Electron energization in the topside auroral ionosphere: on the importance of ion-acoustic turbulence

Abstract

EISCAT observations show enhancements of electron temperatures up to 8000 K in the topside ionosphere during active auroral conditions. These temperature enhancements correlate with electron density enhancements in the altitude region 170–230 km, which indicate that auroral electron precipitation in the 100–500 eV range is associated with the electron heating. Electron densities in other altitude intervals show no such correlation and thereby indicate that auroral particles outside the energy range 100–500 eV are not the major regular cause for this bulk electron heating. We also present observations of ion acoustic turbulence in connection with the bulk electron heating events. Such turbulence will give a greatly enhanced Joule heating in the presence of field-aligned currents because of the resulting anomalous resistivity by the turbulently fluctuating electric fields. This heating will occur in addition to the classical collisonal heating by precipitating particles, and is probably a major heat source for the topside ionospheric electrons. In fact, the precipitating electrons in the 100–500 eV range may themselves be runaway suprathermal electrons produced by the ion-acoustic turbulence. An ion-ion two-stream instability is suggested to be the cause of the observed enhanced ion-acoustic fluctuations.