Observation of a unipolar field‐aligned current system associated with IMF By‐triggered theta auroras

Abstract

AbstractWe investigate the existence of a specific field‐aligned current (FAC) system predicted by numerical magnetohydrodynamic simulations in a past study. The FAC system is expected to occur when a drifting θ aurora is formed in response to a stepwise transition of interplanetary magnetic field (IMF) By during strongly northward IMF periods. When the IMF By changes from positive to negative, a crossbar forms in the Northern Hemisphere that moves dawnward, while in the Southern Hemisphere the crossbar moves in the opposite direction. The crossbar motion reverses when the IMF By changes from negative to positive. The FAC system appears on the trailing side of the drifting crossbar of the θ aurora as it moves either dawnward or duskward. When the θ aurora is drifting dawnward, the FACs flow into the ionosphere. The FAC polarity reverses when the θ aurora is drifting duskward. Using low‐altitude satellite data, we confirmed the real existence of the above model‐predicted FAC system.