Nightside polar rain aurora boundary gap and its applications for magnetotail reconnection

Abstract

[1] Polar rain electrons with energy around 1 keV occasionally fill the polar cap ionosphere. Their energy flux is high enough to produce detectable auroral emissions, called polar rain aurora (PRA). We report a PRA event on 2 August 2002 and the estimation of the magnetotail reconnection location and size from the PRA boundary. The distinguishing feature of this event identified from the TIMED/GUVI data is an emission depletion gap between PRA and auroral oval. The size of the gap is about 2° in latitude and extended from the dusk sector (17:00 MLT) to the premidnight sector (22:00 MLT). This gap coincides with the electron flux gap in the DMSP electron energy flux data. The electron flux data show the existence of energy dispersion at the gap. These GUVI and DMSP observations support the idea that the gap is very likely due to a magnetic reconnection in the magnetotail that blocks the solar wind electron entry into the polar ionosphere. Based on the electron energy dispersion, the nightside X line is estimated to be at X = −50 to −60 RE, which is consistent with results from other studies. The gap extends from the nightside to the duskside and presumably to the dawnside. This new information suggests that the extension of the tail X line over the entire plasma sheet in the dawn-dusk direction under a strongly southward IMF (Bz ∼ −10 nT) and the curved nature of the X line.