High latitude E-region irregularities: New results

Radar observations have shown that the phase velocity of small scale (a few meters and smaller) auroral two stream waves remains close to the ion acoustic speed. However, during periods of very large plasma drifts the electron temperature, and consequently, the ion acoustic speed in the auroral electrojet region is enhanced possibly by strongly driven two stream waves. Recent nonlinear theories can explain the large anomalous electron heating as well as the variation of the two stream phase velocity. However, the observations of plasma waves at angles larger than a few degrees from the perpendicular to the magnetic field remain unexplained. Radar interferometer measurements show that during very active periods strong echoes are often observed from highly localized and dynamic scattering regions. On these occasions, the two stream waves have phase velocities which can exceed 900 m/s, and the gradient drift waves have very broad spectra with mean phase velocities sometimes larger than 600 m/s. Furthermore, resonant (i.e., sharply peaked) auroral radar echoes, with Doppler shifts independent of the ambient cross field plasma drift are often present near the edges of auroral arcs. These waves have been associated with ion cyclotron waves driven by very large field aligned drifts in the upper E-region. This paper will review recent experimental results on the study of meter scale plasma waves in the auroral E-region, particularly during highly disturbed periods, and will also discuss the physical processes responsible for their generation and saturation.

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