On the formation of near‐Earth X‐line at substorm expansion onset

Abstract

Convection in the plasma sheet can be enhanced by the MDXL (middle‐to‐distant X‐line) during the growth phase and by the NEXL (near‐Earth X‐line) during the expansion phase. Cowling current intensifies by the blockage of the northward Hall current due to lack of the radial currents or azimuthal pressure gradient in the midnight sector of the near‐Earth plasma sheet. Interaction of intensified Cowling current loop with the near‐Earth plasma sheet causes dipolarization. Dipolarization‐induced thinning tailward of the dipolarizing region can cause the NEXL formation to drive the substorm expansion phase. Further intensifications of the Cowling current in the substorm current wedge, the tailward expansion of dipolarization region, and the expansion of the auroral bulge are all driven by the NEXL during the expansion phase.