The midday, discrete auroral gap

Abstract

A gap in the midday sector of the auroral oval is examined on the basis of Defense Meteorological Satellite Program (DMSP) satellite auroral photographs. All DMSP photographs recorded in June 1975 over the southern hemisphere were examined, and 38 photographs were found suitable to specify details of auroral features in the midday sector. There is not a single example showing the discrete auroral arcs extending continuously across the noon sector, bridging the morning and afternoon sectors of the auroral oval. Hence a gap, manifested by the absence of extended, discrete auroral arcs, exists in the midday sector of the auroral oval. The gap appears typically to extend over more than 15° of longitude or, correspondingly, more than 1 hour of local time and is centered about the 1000 corrected geomagnetic local time meridian. During an early phase of substorms the gap appears to fill with rays and fragmented arcs and to empty after substorm recovery. Similarly, polar cap arcs appear and disappear at the same times that the gap fills and empties of rays and fragmented arcs. The gap therefore appears to be a semi‐permanent feature of the midday auroral distribution.