Abstract. In this work, we analyze the ballooning stability of the near-Earth plasma sheet prior to the initial expansion onset of the 23 March 2007 THEMIS substorm event. Using 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 expansion 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 ballooning instability criterion is evaluated using eigenvalue analyses for the near-Earth plasma sheet region when the configuration attains a quasi-static equilibrium condition prior to the onset. Our analysis of the evolution of the near-Earth magnetotail region during the substorm growth phase (10:10 UT to 10:43 UT) reveals a correlation between the breaching of the ballooning stability condition and the initial expansion onset in the temporal domain. The analysis indicates that the ballooning instability is at first initiated by the stretching of closed field lines near the region with a local nonzero minimum in normal component of magnetic field (Bz>0) associated with the formation of a bipolar-flow pattern in the near-Earth plasma sheet around 10 RE. The subsequent initiation of ballooning instability with enhanced growth rate occurs (about 28 min later) on those most stretched closed field lines Earthward of the neutral line following the formation and the tailward recession of a plasmoid.
