Lower midlatitude ionospheric disturbances and the perkins instability

Abstract

This paper presents a preliminary study of ionospheric disturbances at dip latitudes less than 40° as seen by the ion drift meter and the retarding potential analyzer on board Atmosphere Explorer E during a period of low solar activity. The altitude of observation was relatively low, mostly below 300 km. The emphasis is on the midlatitude region, where some features resemble equatorial bubbles; no clear demarcation in latitude could be recognized between the bubbles and other midlatitude disturbances. Excellent evidence was found that the Perkins instability is responsible for very structured disturbances which were frequently observed. In most cases where regions of inward and outward E × B drift were encountered, diffusive motion up or down the field line partially cancelled the vertical component of the cross-field drift. In some cases cancellation was almost perfect, but in others it appeared not to occur at all (probably cases involving rapid changes). Within the larger structures caused by the Perkins instability there were places where the secondary gradient drift instability was also active.