A classification of polar cap auroral arcs

Abstract

Global auroral imagery obtained by DMSP satellites during the years 1972–1979 over both the northern and southern high latitude polar regions were examined to study the morphology of the discrete arcs known as polar cap arcs. Based upon their morphology, the polar cap arcs can be generally classified into three types viz. (1) the distinctly sun-aligned polar cap arcs—Type 1 arcs, (2) the morning/evening polar cap arcs expanded from the auroral oval—Type 2 arcs and (3) the hook shaped arcs connecting the polar cap arc with the oval arc (including the hitherto unreported oppositely oriented hook shaped arcs)—Type 3 arcs. Concurrent auroral electrojet indices (AE) and interplanetary magnetic field (IMF) data were used to study the occurrence of the polar cap arcs. It was found that Type 1 arcs were observed mostly during low geomagnetic activity conditions, bright Type 2 arcs during the recovery phase of the substorms and Type 3 arcs do not occur during the recovery phase of the substorm. Over both hemispheres, the polar cap arcs were observed mostly during northward IMF. Furthermore, Type 1 arcs were obeserved over the northern polar cap during mostly negative B x periods and over the southern polar cap during mostly positive B x periods. The latter observation suggests that these types of arcs may be non-conjugate.