Optical signatures and sources of prenoon auroral precipitation

Abstract

Optical observations from Ny Ålesund, Svalbard, of prenoon auroral structures are compared with simultaneous measurements of energetic particle precipitation and plasma drifts taken during a nearby overpass of the DMSP‐F9 satellite near the 0830 MLT meridian. Close to the time of this pass, a stable band of red‐line emissions was detected from south of Ny Ålesund. Very weak, transient auroral activity was observed from the sky above and north of the station. An automatic classification algorithm applied to the DMSP‐F9 particle‐precipitation data, identified source regions in the central plasma sheet, the boundary plasma sheet, the plasma mantle, and the polar rain. We interpreted the poleward and equatorward parts of the red auroral band as being collocated with regions of mantle and boundary plasma sheet precipitation on tailward and sunward convecting field lines, respectively. However, the mantle part of the red auroral band, as interpreted by the automated algorithm, is difficult to reconcile with simultaneously measured, high‐latitude convection distributions. Another interpretation of the data, which includes magnetic mapping considerations and constraints imposed by the global electric field pattern, suggests that the entire band of red‐line emissions from south of Ny Ålesund is excited by electrons originating in the nightside, boundary plasma sheet. Although neither interpretation is definitive, resolving dilemmas that they pose is critical for understanding this common and simple form of dayside aurora.