Detailed analysis of a substorm event on 6 and 7 June 1989 1. Growth phase evolution of nightside auroral activities and ionospheric convection toward expansion phase onset

Abstract

We have analyzed in detail the evolution of nightside auroral activities and ionospheric convection during the growth phase of an isolated substorm. The following three characteristic auroral activities were identified by meridian scanning photometers (MSP) at ground stations: (1) proton main oval, (2) fast equatorward moving (FEM) arc, and (3) near plasma sheet boundary layer (NPSBL) aurora. The proton main oval corresponds to the main part of the proton auroral region and was located at the equatorward‐most region of the electron auroral emissions and gradually moved equatorward during the growth phase. The FEM arc was a discrete auroral arc having a clear longitudinally elongated form. It appeared about 20 min before the onset on the high‐latitude side of the auroral region and moved equatorward toward the proton main oval. Its equatorward motion gradually slowed as it approached the proton main oval. As the FEM arc moved equatorward, the location of the peak velocity in the nightside return flow of the two‐cell convection also moved equatorward. The auroral breakup occurred in a localized area around the demarcation region between the two return flows, close to the latitude of the FEM arc. This was observed by the UV imager aboard the Akebono satellite. The NPSBL aurora appeared a few minutes before the onset around the poleward‐most region of the aurora and continued even after the onset. Our detailed analysis of these phenomena can give us important clues as to how the growth phase proceeds toward the expansion phase onset.