Detailed analysis of a substorm event on 6 and 7 June 1989. 2. Stepwise auroral bulge evolution during expansion phase
 We have analyzed in detail the auroral bulge evolution during the expansion phase of an isolated substorm, which was observed by the UV imager aboard the Akebono satellite. It was found that there were three distinct stages in the evolution. Stage 1 was characterized by rapid poleward and azimuthal ( predominantly westward) expansions in a short time (about 2 min). Stage 2 was characterized by a very slow poleward and slower and continuous azimuthal expansions. There was a certain period for transition between stage 1 and stage 2, and it was characterized by a very slow poleward and rapid eastward expansions. Stage 3 started about 11 min after the onset and was characterized by a sudden reactivation of the rapid poleward and azimuthal expansions. The reactivation started around the initial onset meridian and then spread both eastward and westward. At the azimuthal front, the expansion first occurred at the lowest latitudes, spread poleward to around the highest latitudes of stage 1, and then spread further poleward after a brief interval. Hence, the local expansion also had three distinct stages similar to the global one. The ground-based observations showed that the highest latitude of the local first stage was very close to the latitude of auroral activity that appeared near the ionospheric plasma sheet boundary layer (PSBL) region a few minutes before the onset. The further poleward expansion during the local third stage started with a significant intensification of the poleward-most auroral activity. During the local third stage, the bright electron auroral region was bifurcated into a poleward expanding part and an equatorward moving part. The proton auroral emission coexisted in the bulge during the local first and second stages and almost disappeared soon after the bifurcation during the local third stage. Based on these observations, we discuss the evolution in the magnetosphere during the expansion phase.
Authors: Kadokura, Akira, Yukimatu, Akira-Sessai, Ejiri, Masaki, Oguti, Takasi, Pinnock, Michael, Sutcliffe, Peter R.