Does the aurora provide evidence for the occurrence of antiparallel magnetopause reconnection?


Antiparallel merging at the magnetopause predicts a longitudinal bifurcation of the cusp during intervals of southeast or southwest interplanetary magnetic field (IMF) orientation. In the aurora this should manifest itself as a longitudinal gap in the midday sector, bracketed by bright auroral forms on either side. With the purpose of testing this prediction, we searched for an auroral signature in our database from the arctic islands of Svalbard, focusing on strong, isolated auroral brightenings belonging to the category of events which has been previously established as a signature of transient magnetopause reconnection (flux transfer events (FTEs)). We found some very interesting events, showing clear brightenings in the eastern and western parts of the field of view (FOV), with a gap in between. In this paper we present a 1 hour long interval of auroral observations in the 1100–1400 MLT sector, showing two strong, isolated auroral events, which were separated by 20 min. The IMF was steady and directed southeast (By = 3 nT; Bz = −3 nT; clock angle = 135°), in a solar wind of low speed (370 km s−1) and low dynamic pressure (0.2 nPa). Each auroral event is characterized by an initial, strong brightening of the cusp equatorward boundary, previously referred to as equatorward boundary intensification (EBI), which is followed by a westward and poleward moving auroral form (PMAF). The main point here is that the initial brightening is longitudinally bifurcated, with near-simultaneous auroral brightenings in the eastern (1300–1400 MLT) and western (1100–1130 MLT) parts of the all-sky camera FOV being separated by a ∼500 km wide “gap.” The brightening event is discussed as a possible signature of transient, antiparallel reconnection, taking place at high magnetopause latitudes, while the “gap” aurora is attributed to quasi steady component reconnection in the subsolar region.

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JGR: Space Physics



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