Toward a unified model of substorms

Numerous models of substorms have been proposed so far, and they are roughly divided into two categories, i.e., the outside-in category that is represented by the near-Earth neutral line (NENL) model and the inside-out category represented by the current disruption model or the ballooning instability model. Controversies have been raised for many years over the validity of those models. However, in recent years we have obtained important clues to solve this long-standing issue by analyzing THEMIS probe data for substorms and pseudo-substorms separately. [Fukui et al., 2017] The key is the plasma pressure in the equatorial region, and it was about 1.3 times higher in substorms, than the pseudo-substorm in the region between X -7 and -8 Re. However, no difference was found beyond X -10 Re. Therefore, the spatial gradient of the plasma pressure in the region of X -7.5 Re must be a necessary condition for the occurrence of substorm. Abrupt earthward flows originated from the catapult current sheet relaxation and subsequent magnetic reconnection at the NENL just prior to the onset is a common signature for both substorm and pseudo-substorm, which seems to be essentially a result of the tearing instability in the magnetotail. [Uchino and Machida, 2015] The subsequent earthward flows must initiate some instability, quite likely the ballooning instability around the flow braking region. Substorms do not occur only with the magnetic reconnection. If there is enough plasma pressure gradient, the system can develop into a substorm. Otherwise, it will end up with a pseudo-substorm. We emphasize that both NENL model and the ballooning instability model are partially correct but incomplete, and the true model of substorm can be constructed by synthesizing multiple models of substorm including at least these two models.