Antiparallel magnetic merging signatures during IMF B_Y>>0: longitudinal and latitudinal cusp aurora bifurcations

Abstract. A prominent dayside auroral event, occurred during an IMF B_Y-dominated time interval, and characterized by the contemporaneous longitudinal and latitudinal cusp bifurcations, is reported. The event was recorded the 19 December 2002, between about 09:30–10:45 UT, by the ITACA² twin auroral monitors system, in the Greenland-Svalbard zone. The splitting of the ionospheric footprint of the geomagnetic cusp, traced by the dayside auroral activity, was recently identified with the signatures of antiparallel reconnection episodes ongoing at different magnetopause locations, during large IMF B_Y periods. The first part of the event showed a broad longitudinal bifurcation of the red-dominated cusp aurora, displaced in the prenoon and postnoon, with a separation up to ~1800 km, during northeast directed IMF (clock-angle rotating from 45° to 90°). This observation widens the range of IMF regimes that are known to drive a longitudinal bifurcation of the cusp, since previous case-studies reported these events to occur during southeast/southwest oriented IMF (clock-angle ≈135°). The second part of the event, developed when the IMF turned to a nearly horizontal orientation (B_Y>>0, B_Z~0, clock-angle ~90°), and exhibited the simultaneous activation of the cusp auroras in three distinct areas: i) two of them associated to the above-mentioned longitudinally bifurcated cusp (~73°–75° CGM latitude, type 1 cusp aurora), and linked to (near)antiparallel magnetic reconnection patches equatorward the northern and the southern cusp, ii) the other one characterized by isolated high-latitude (~76°–77° CGM latitude, type 2 cusp aurora) rayed arc(s) with intense green emission, and triggered by (near)antiparallel merging at the northern lobe (usually observed during positive IMF B_Z), poleward the local cusp. During this phase, the longitudinal separation of the low-latitude type~1 cusp aurora was about 1000 km wide, with a 500 km gap, while the latitudinal separation between low- (type 1) and high-latitude (type 2) cusp auroras, in the postnoon, was about 270–280 km at its maximum. The longitudinal gap, corresponding to a zone with weak auroral emission, was found to likely map to the component reconnection region at the subsolar magnetopause. The magnetic merging topology that can be drawn on the basis of the reported cusp auroras support the idea of a "mixed" merging scheme, with (near)antiparallel reconnection at high-latitudes, and component reconnection in the subsolar region, as recently proposed by other authors.