Small‐Scale Aurora Associated With Magnetospheric Flow Vortices After a Solar Wind Dynamic Pressure Decrease.

Sudden changes of the solar wind dynamic pressure have significant impacts on the dynamics of the magnetosphere‐ionosphere system. On 18 February 2011, a sudden decrease in solar wind dynamic pressure was observed by the Wind satellite, which drove the entire magnetosphere‐ionosphere system as recorded in many ground‐based and space‐based measurements. In the magnetosphere, Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft observed a counterclockwise plasma flow vortex propagating tailward. Near the magnetic footprints of the THEMIS spacecraft, the ground magnetometers observed magnetic field variations that corresponded to a counterclockwise vortex in the equivalent ionospheric currents, which in turn indicated the presence of upward field‐aligned currents. The all‐sky imager at RANK station near the THEMIS footprints also observed a simultaneous enhancement of the auroral brightness. Therefore, this comprehensive case study demonstrates a causal chain that links the solar wind dynamic pressure variations to magnetospheric, ionospheric, and auroral activities. Plain Language Summary: In the near‐Earth space, the shape and dynamics of the magnetosphere and the ionosphere are strongly affected by solar wind variations. There have been numerous studies demonstrating the effects of magnetospheric compression caused by a sudden enhancement in the solar wind dynamic pressure. However, the opposite case with sudden decrease in the solar wind dynamic pressure has been rarely investigated. Here we perform a comprehensive, observational study to demonstrate the consequences of sudden decrease of solar wind dynamic pressure in the entire magnetosphere‐ionosphere system, which include the formation of magnetospheric flow vortices, generation of field‐aligned currents, perturbations of the geomagnetic field, and enhancement of auroral brightness near the magnetic footprints of the magnetospheric flow vortices. In other words, this case study provides a causal chain that shows the magnetospheric, ionospheric, and auroral responses to a sudden decrease of the solar wind dynamic pressure. Key Points: Small‐scale auroral brightening is observed after a sudden decrease in solar wind dynamic pressureSolar wind discontinuity produces a tailward‐moving magnetospheric flow vortex associated with field‐aligned currentsClear relationship between upward field‐aligned currents, ionospheric currents, and auroral activities are presented [ABSTRACT FROM AUTHOR]