Observation of a unipolar field-aligned current system associated with IMF B_y-triggered theta auroras

We 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) B_y during strongly northward IMF periods. When the IMF B_y 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 B_y 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.