Pressure Change Associated with Dipolarization in the Near-Earth Plasma Sheet

We have studied pressure changes that occur associated with dipolarization in the plasma sheet of the near- Earth magnetotail around substorm onsets. Using Geotail data, we performed a superposed epoch analysis and examined in detail a few individual cases that were observed in the vicinity of the magnetic equator. Here the contribution of high-energy particles to the ion pressure was considered. It is found that, unlike the previously reported results, the ion pressure increases in association with dipolarization within X ~ -10 RE, where the initial dipolarization takes place. This pressure change is largely contributed by high- energy particles. Tailward of this region, the ion pressure does not significantly increase even after the beginning of dipolarization, while it decreases in some cases; the contribution of high-energy particles is not very large in both cases. Furthermore, the ion beta enhances around substorm expansion onset in the close vicinity of the magnetic equator of the initial dipolarization region. These observations suggest that the characteristics of the dipolarization differ between the initial dipolarization and tailward regions. We discuss implications for the current disruption model (the rarefaction wave), pressure balance, and relationship with auroras.