UW-CPTC 08-11 Initiation of ballooning instability by reconnection in the near-Earth plasma sheet

In this work, an alternative scenario for the substorm onset process is proposed, based on ideal ballooning stability analysis of the near-Earth plasma sheet during the March 23, 2007 THEMIS substorm event. In this scenario, the ballooning instability is initiated by the magnetic reconnection in the near-Earth plasma sheet, which in turn directly contributes to the trigger of a full onset. Using the solar wind data from WIND satellite observation for the substorm event as an input at dayside, we reconstructed a sequence of global magnetospheric configurations around the substorm onset by means of OpenGGCM simulation. These simulations have reproduced most of the salient features, including the onset timing, observed in the THEMIS substorm event (Raeder et al., 2008). The local ballooning instability criterion is evaluated for the near-Earth plasma sheet region when the configuration attains a quasi-static equilibrium condition. Our analysis of the evolution of the near-Earth magnetotail region during this substorm event reveals a correlation between the breaching of the ballooning stability condition and the substorm onset in both temporal and spatial domains. The analysis suggests that the Earthward bulk plasma flow induced by the reconnection event in the near-Earth plasma sheet, leads to the pressure build-up and creates a favorable condition for the initiation of the ballooning instability in that same region.