A case study of relationship between substorm expansion and global plasma convection

Abstract

This letter reports observations of large‐scale ionospheric plasma flows from the SuperDARN radar network and global auroral images from the Polar ultraviolet imager during two large and rare substorm events. The substorm events were characterized by predominantly westward expansion for one and predominantly eastward expansion (or lack of westward expansion) for the other. The westward expansion case was associated with a positive y‐component of interplanetary magnetic field (IMF), whereas the eastward expansion case was associated with a negative y‐component of IMF. The concurrent plasma flow measurements from the SuperDARN radar network suggest that the westward (eastward) expansion is embedded in the large, round dusk (dawn) convection cell. A simple explanation is offered for the observed preponderance of westward expansions, namely, that the pre‐midnight onset location favors the westward plasma flow that is associated with the dusk cell. Eastward expansion occurs only when a large IMF By negative condition causes the midnight sector to be dominated by eastward plasma flows.