Unusual Magnetospheric Dynamics During Intense Substorm Initiated by Strong Magnetospheric Compression

We investigate an unusual sequence and peculiar features of magnetotail changes during a storm-range substorm initiated by the interplanetary shock. Auroral observations and measurements at several favorably distributed magnetospheric spacecraft allowed the construction of an adaptive time-dependent magnetospheric model to quantitatively characterize the configurational changes and mapping variations. Several passages of low-altitude spacecraft in polar orbits near midnight help reveal the magnetic configuration of the nightside tail-dipole transition region. In this event, an intense auroral and convection activity (accompanied by an up to 1,500 nT increase in the SuperMag AL-index index) emerged in the highly compressed magnetosphere after the passage of interplanetary shock followed by strongly southward interplanetary magnetic field (IMF). This directly driven phase of the activity continued for an hour and resulted in the formation of a hybrid magnetic configuration with dipolarized midtail and stretched field lines in the transition region. Observations of energetic particle isotropy boundary latitudes near midnight are consistent with the modeled magnetic configuration. In concert with a downward turn of the solar wind (SW) flow, and weakening of the IMF driver and convection, an unusual stretching signature of the inner magnetosphere magnetic field was observed as close as at r similar to 5 to 7 Re; which resulted mostly from an increasing downward tilt of the thin azimuthal current. Classic substorm breakup signatures commenced at fairly low, similar to 60 degrees magnetic latitude, deep in the closed field line region, in association with the current sheet upward motion. It was followed by strong stepwise poleward auroral expansion. We discuss how these signatures deviate from standard substorm scenarios and may be potentially imparted by the aforementioned changes in SW flow direction and pressure.