Global magnetospheric response to IMF driving: ISTP observations, empirical modeling, and MHD simulations

The energy flow through the magnetosphere-ionosphere system is examined during an isolated substorm event (May 15, 1996) using multi-spacecraft ISTP observations, empirical modeling techniques, and global MHD simulations. Detailed timings show that the growth phase signatures were evident in the ionosphere ~5 min after the IMF southward turning - ~7 min before the energy loading on open tail lobe field lines was initiated. After this initial period, the energy input and storage derived using existing empirical formulas are shown to be in balance. Based on both empirical model results and MHD simulations for this interval, it is argued that the magnetotail has two active regions that develop during the growth phase: The one in the inner magnetotail is necessary for the substorm pre-conditioning, but it is the one in the midtail region which initiates the global instability growth. The differences between pseudobreakups and full substorm onsets are discussed; it is suggested that although the pseudobreakup signatures in the inner tail appear localized, the midtail may already be undergoing large-scale reconfiguration.