Electron precipitation in the midday auroral oval

Abstract

Simultaneous observations of auroral displays and electron precipitations by the DMSP 33 satellite provide an excellent and unique opportunity to study precipitation characteristics of the midday auroral oval. Attention is given to two topics: (1) the nature of the ‘gap’ of the midday discrete auroras which is a permanent feature of the dayside auroral oval observed by both Isis 2 and DMSP satellites and (2) the relationship of this gap with the polar cusp region. Based on 2‐month (June, July 1975) observations of the midday auroras over the southern hemisphere, it is found that inside the ‘gap’ of the discrete auroras along the dayside auroral oval, soft electron precipitations with a magnetosheathlike spectrum were invariably detected. The spatial extent of this region was about few degrees in latitude and about 2–3 hours in local time near 1130 magnetic local time meridian. No significant electron precipitation was detected poleward of the instantaneous midday auroral oval. Typical plasma sheet and discrete auroral types of electron precipitations were detected in the other parts of the midday auroral oval. Therefore it is proposed that the ionospheric projection of the polar cusp is a small region of the instantaneous dayside auroral oval near the noon meridian, coinciding with the ‘gap’ of the midday discrete auroras.