Abstract
During the 1970s several advances have been made in the understanding of radar aurora. It has been shown that this phenomenon is due to the combined effects of the two-stream and gradient-drift plasma instabilities. These instabilities require relative streaming of the electrons and ions. Measurements at VHF frequencies have shown that the instabilities generally are excited when the streaming velocity is in excess of 300 to 400 m/s. At UHF frequencies streaming velocities in excess of 500 m/s appear to be required. Since radar aurora require electron-ion streaming, they are collocated with the auroral electrojets and many studies have used radar aurora to detect these currents. It has been found that the eastward electrojet generally is equatorward of the auroral arcs in the evening sector and that it is collocated with the region of downward field-aligned current. It has also been observed that the amplitude of signals backscattered from radar aurora is often proportional to the electrojet current density. Recent VHF studies have shown that Doppler data obtained from radar auroral backscatter can be used to measure the E-region electron drift velocity, the F-region plasma velocity, and the ionospheric electric field. This type of measurement is particularly valuable when it is made with dual auroral radar systems similar to STARE (Scandinavian Twin Auroral Radar Experiment). Over the past two years STARE has been used to study electric field patterns associated with electrojet and field-aligned currents, magnetospheric convection, the Harang discontinuity, Pc5 micropulsations, and the substorm expansion phase.
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