Dayside magnetosphere‐ionosphere coupling during IMF clock angle ∼90°: Longitudinal cusp bifurcation, quasi‐periodic cusp‐like auroras, and traveling convection vortices

Abstract

In spite of the great progress achieved in the understanding the Earth's magnetosphere‐ionosphere system, its configuration and dynamics during periods with large horizontal interplanetary magnetic field (IMF ∣By∣ ≫ 0) is still poorly investigated. In such time intervals, the cusp/low‐latitude boundary layer (LLBL) entry regions are characterized by a more complex magnetic reconnection topology, than during IMF Bz‐dominated periods, which in turn leads to peculiar magnetosheath plasma injections into the magnetosphere and associated ionospheric signatures. In this context, we discuss the 19 December 2002 dayside aurora activity, observed by the ITACA2 twin all‐sky camera system, along the Greenland‐Svalbard sector, during a nearly horizontal IMF (By ∼ 15 nT and Bz ∼ 0). The event originates from a longitudinally bifurcated cusp configuration and then develops into quasi‐periodic red aurora activations, between about 1000 and 1140 UT. By analyzing the Greenlandic magnetometer data, we identify a series of westward traveling convection vortices (TCVs), moving away from magnetic noon at 2–3 km/s and synchronized with the transit of red‐dominated auroral forms. We find that the auroral emission takes place in coincidence with the ionospheric footprint of the downward field‐aligned currents (counterclockwise TCVs), instead of the upward ones (clockwise TCVs), as it would be expected for precipitating electrons. This fact and several analogies with previously reported quasi‐periodic TCV activity, observed during IMF By‐dominated periods, suggest that the geomagnetic activity could be associated with FACs that are triggered by the magnetic merging on the dayside, either by inducing azimuthal pressure gradients at the magnetopause or by driving magnetopause/LLBL wave activity.