Discrete polar cap aurora observed from Spitsbergen

Abstract

ABSTRACTDiscrete auroral arcs frequently bridge the polar cap connecting the morning and evening sectors of the auroral oval along the line of the transpolar (electron) current. Very high-latitude stations that lie wholly within the oval during the earth's diurnal rotation pass under this bridge twice a day, giving morning and evening maxima. Stations at slightly lower latitudes on Spitsbergen lie within the oval in the evening but under, or even south of, the oval in the morning. From such stations the evening, but not the morning, maximum is readily observed. This study is primarily concerned with the orientation of discrete polar cap arcs in the evening skies over Spitsbergen. It shows that the geomagnetic alignment of these arcs is latitude-dependent between geomagnetic colatitudes 6 to 20°N. At the highest latitudes within this range, the arcs are in transpolar alignment, whereas at the lowest latitudes within the polar cap, they are oval-aligned. At intermediate latitudes, the arcs are observed in transitional phases between transpolar and ovalalignment. The solar alignment of discrete polar cap arcs is a function of corrected geomagnetic local time. In the early afternoon, solar alignment is poor but this gradually improves throughout the late afternoon until there is excellent alignment at the time of the evening maximum. Recent satellite studies of plasma convection in the polar ionosphere have helped to explain some of the visual characteristics of discrete polar cap aurora observed from Spitsbergen, particularly the irregular alignment of arcs in the region of the Harang discontinuity.