A 12-hour case study of auroral phenomena in the midnight sector: Oval, polar cap, and continuous auroras

Abstract

This paper and two companion papers describe a case study of multiple auroral phenomena in the midnight sector. The duration of the study was 12 hours on December 9, 1971. For 9 of the 12 hours the Air Force Geophysics Laboratory airborne ionospheric observatory flew near local midnight across North America so as to intersect many successive passes of Defense Meteorological Satellite Program (DMSP) and Isis 2 satellites which were also in the midnight sector. The large-scale DMSP auroral photographs have been combined with the temporally continuous aircraft- and ground-based measurements to provide extensive observations of long duration compared to the lifetimes of the two substorms observed. All the types of observations (satellite pictures, aircraft ionospheric soundings, all-sky camera photographs, photometer recordings, as well as particle and other satellite- and ground-based measurements) have been combined to describe the temporal histories of a large number of phenomena and of their interrelations. The phenomena are the discrete auroras in the oval and polar cap, the continuous (E layer) aurora, auroral absorption (nearly absent on this day), and (in the companion papers) the F layer irregularity zone and 6300-Å emission, particle precipitation, the electrojet, and interplanetary magnetic field and magnetospheric phenomena. DMSP photographs are found to detect both discrete and continuous auroras; uses and limitations of the data are described. In terms of the overall perspective of this study the phenomena are found to fall into six periods of 1–3 hours each. Two of these periods contain substorms of differing characteristics; the remaining four periods are relatively quiet but also differ from one another. Throughout each of the periods the polar cap, oval, and continuous auroras as a whole retain the same configuration as described, in terms of whether each of the auroras is present or absent and whether adjacent auroras are joined or separated. These configurations are described as phenomenological states of the aurora, and the periods are regarded as the lifetimes of these states. The large-scale, long-term features of the auroral states suggest that they reflect individual states of the magnetosphere.