Radar observations of the onset of current driven instabilities in the topside ionosphere

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Abstract
Millstone Hill radar spectra exhibit pronounced spectral asymmetries in the presence of field‐aligned currents in the topside ionosphere. The enhanced, often intense, returns associated with the current‐driven growth of the ion acoustic mode exhibit the characteristics of hard‐target backscatter suggesting that many “satellite echoes” observed by UHF radars at high latitudes might be caused by this mechanism. A large current density (>100 µamp/m²) is calculated for the conditions observed, consistent with that seen in intense earthward‐directed current filaments. Our observations indicate that the ion acoustic mode grows toward instability with increasing altitude. Backscatter observed from the unstable region at higher altitudes is characteristically at the ion plasma frequency consistent with the unstable growth of the acoustic mode for large ratios of electron to ion temperature.