Toward an observational synthesis of substorm models: Precipitation regions and high‐latitude convection reversals observed in the nightside auroral oval by DMSP satellites and HF radars

Abstract

A combination of simultaneous measurements from high, low and ground altitude instruments has been used to infer the rapid evolution of the coupled nightside magnetosphere‐ionosphere during substorms. Reversals from an eastward zonal convection to westward zonal convection become apparent inside the volume of the substorm bulge a few minutes after the intensification of the westward electrojet. These reversals persist for periods of 10–20 min and appear to have a one‐to‐one correspondence with the occurrence of dipolarizations. The changes in convection are accompanied by changes in precipitation. Flux depletion regions (FDR) are measured by the DMSP satellites inside the surge, near the equatorward portion of the westward electrojet intensification. The poleward boundary of every FDR is collocated with a convection reversal and an arc intensification that marks a poleward transition into a region initially dominated by intense discrete electron precipitation and velocity dispersed ion structures (VDIS). Convection in the FDR constitutes an eastward electrojet channel that may produce a transient recovery signature as observed by ground magnetometers. The observations of FDR's and the fast westward flows that accompany them are consistent with the scenario of a rarefaction and/or a neutral line in the near‐earth tail that produces fast earthward flows after the breakup. The arc intensification at the poleward boundary of the depletion region and the collocated transient convection reversal favor an enhancement of the magnetosphere‐ionosphere coupling and thus the continuation of the substorm expansion in a multiple cell convection system. Arguments are presented to explain how a series of pseudobreakups modify the near Earth magnetic field into an increasingly dipolar geometry until a breakup is possible.