Shock aurora: FAST and DMSP observations

Abstract

Global signatures of the aurora caused by interplanetary shocks/pressure pulses have been studied in recent years using ultraviolet imager data from polar orbiting spacecraft. The signatures include the occurrence of the aurora first near local noon and then propagation antisunward along the auroral oval at very high speeds. To better understand the mechanisms of particle precipitation, in this paper we study shock auroras using near‐Earth observations of the FAST and DMSP satellites. We have studied the events that occurred during 1996–2000 where FAST and/or DMSP crossed the dawnside or duskside auroral zone within 10 min after shocks/pressure pulses arrived at the nose of the magnetopause. It is found that the electron precipitation increased significantly above the dawnside and duskside auroral oval zone after the shock/pressure pulse arrivals. The precipitation structure is low‐energy electrons (<∼1 keV) at higher latitudes (∼75°–83° ILAT within 0600–0900 MLT) and high‐energy electrons (∼1–10 keV) at lower latitudes (∼65°–79° ILAT) of the auroral zone. There are a few degrees (1°–4° ILAT) of overlap between these two categories of precipitated electrons. The precipitation of low‐energy electrons was along highly structured field‐aligned currents. The precipitation of the high‐energy electrons was highly isotropic filling the loss cone. Possible mechanisms of field‐aligned current generation are some dynamic processes occurring on the dayside magnetopause, such as magnetic shearing, magnetopause perturbation, magnetic reconnection, and Alfvén wave generation. Adiabatic compression might have caused the high‐energy electron precipitation. On the basis of observations of FAST and DMSP, shock auroras are speculated to be diffuse auroras at the lower latitudes of the dayside auroral oval and discrete auroras on the poleward boundary of the oval with a few latitude degree overlap of the two types of auroras.