Dayside auroral activities and their implications for impulsive entry processes in the dayside magnetosphere

Abstract

Dayside aurorae often exhibit transient features with latitudinal widths of about 100–200 km, longitudinal extents of < 500 km, and durations of about 1–10 min. Many researchers regard such auroral activity as evidence for direct, impulsive entry of magnetosheath plasma into the magnetosphere via magnetopause penetration across closed field lines or flux transfer events (FTEs) on open field lines. Recent work suggests that a number of previously reported low-altitude transient events in the auroral zone could be directly associated with solar wind dynamic pressure variations (either increases or decreases) in which the dynamic pressure is seen to change by up to a factor of 3 for a duration of 1–10 min. In this paper, we first investigate the dayside auroral bright spots as seen by the Viking satellite and discuss evidence which argues against the notion that these auroral bright spots are the optical signatures of FTEs in the ionosphere. We then re-examine past reports of transient dayside auroral phenomena which were interpreted as the optical signatures of FTEs to determine the solar wind conditions at the time of the events. We find that many of the previously reported transient occurrences of poleward drifting auroral arcs in the dayside auroral region can be associated with sudden increases in the solar wind dynamic pressure. An explanation of these events in terms of solar wind dynamic pressure enhancement is proposed