Vortex Generation And Auroral Response To A Solar Wind Dynamic Pressure Increase: Event Analyses

In this study, we investigate ionospheric responses, including currents and aurorae, to solar wind dynamic pressure (SW P-dyn) sudden increases, which are critical for understanding solar wind-magnetosphere-ionosphere coupling. We focus on two similar SW P-dyn pulse events that occurred on January 24, 2012 and November 12, 2010. In both cases, equivalent ionospheric currents (EIC) vortices were generated within about 10 min after the pressure pulse arrival, with a counter-clockwise rotating vortex (viewed from above) observed on the dusk side in the former case, and a clockwise vortex observed on the dawn side in the latter. Simultaneous ground-based All-Sky Imager (ASI) observations in the vicinity of the observed EIC vortex in each case showed that aurorae intensified on the dusk side and diminished on the dawn side. These observations provide direct evidence of the scenario proposed by Shi et al. (2014) that magnetospheric flow vortices generated by a solar wind pressure pulse carry field-aligned currents into the ionosphere and thereby modulate auroral activity. The dawn/dusk asymmetry in the auroral intensification is a direct result of the opposite sense of vortex rotation on the dawn and dusk sides, which generate oppositely directed field-aligned currents into/out of the ionosphere.Plain Language Summary Earth's magnetosphere and ionosphere are strongly affected by variations of the solar wind, especially the variation of solar wind dynamic pressure (SW P-dyn). A SW P-dyn increase/decrease leads to the compression/expansion of the Earth's magnetosphere. In response, field-aligned currents, which are carried by precipitating or escaping plasma particles, are generated in the magnetosphere, and in turn lead to variations in the auroral intensity. In previous studies of events with similar variations of SW P-dyn, cases of both auroral brightening and dimming have been observed. Therefore, it has not been clear whether the aurorae become darker or brighter after SW P-dyn increase/ decrease events. In this article, we clarify the relationship between the auroral intensity variation and positive SW P-dyn pulses. We show that auroral brightening on the dusk side and dimming on dawn side during SW P-dyn increase events are due to oppositely directed field-aligned currents in these regions. These are generated by oppositely rotating flow vortices on the dusk and dawn flanks of the magnetosphere, which are both produced by the passage of the SW P-dyn pulse.