SuperDARN observations of medium-scale gravity wave pairs generated by Joule heating in the auroral zone

It is known that Joule heating caused by currents associated with enhanced electric fields in the auroral ionosphere can generate gravity waves. Some modeling studies found that each short-lived (similar to 10 min) electric field enhancement in the ionosphere leads to the generation of a single gravity wave pulse. Observational evidence of such single wave pulses has been reported in the ground-scatter echoes seen by the SuperDARN HF radars. In this paper, we present new SuperDARN observations which show that, under certain conditions, each electric field enhancement can generate a pair of gravity waves. On December 2, 1998, the IMF B-z had a perturbation with a period of similar to 100 min and caused oscillations in the ionospheric electric field at the same period. Each positive or negative half-cycle of the oscillating electric field gave rise to one Joule heating enhancement which, in turn, generated a pair of gravity waves (two wave pulses). Six successive gravity wave pairs were observed. We suggest that the reason for two gravity wave pulses was that the Joule heating events in the present case were of longer duration than those in the previous events. We also show that the periodic velocities seen in the far-range ionospheric echoes are due to the direct auroral electric field modulation and not to indirect modulation caused by the equatorward-moving gravity waves.