A Unified Scenario of Near-Earth Substorm Onset: Analysis of THEMIS Events

Physics

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2723 Magnetic Reconnection (7526, 7835), 2752 Mhd Waves And Instabilities (2149, 6050, 7836), 2753 Numerical Modeling, 2764 Plasma Sheet, 2790 Substorms

Scientific paper

We propose an alternative scenario for the substorm onset process, based on ideal ballooning stability analysis of the near-Earth plasma sheet during recent THEMIS substorm events. 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 events [Raeder et al, 2008]. The ballooning instability criterion and growth rate are evaluated for the near-Earth plasma sheet region where the configuration satisfies a quasi-static equilibrium condition. Our analysis of the evolution of the near-Earth magnetotail region during the substorm events 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. This new alternative scenario further elaborates earlier conjectures on the roles of reconnection and ballooning instability [Bhattacharjee et al, 1998], and has the potential to integrate both the near-Earth neutral-line model [McPherron et al, 1973] and the near-Earth current-sheet- disruption model [Lui et al, 1988] into a unified model of the near-Earth substorm onset. Research supported by U.S. NSF Grant No. ATM-0542954.

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