Start‐to‐end global imaging of a sunward propagating, SAPS‐associated giant undulation event

Abstract

We present high time resolution global imaging of a sunward propagating giant undulation event from start to finish. The event occurred on 24 November 2001 during a very disturbed storm interval. The giant undulations began to develop at around 1345 UTC and persisted for approximately 2 h. The sunward propagation speed was on the order of 0.6 km/s (relative to magnetic latitude–magnetic local time coordinate system). The undulations had a wavelength of ∼718 km and amplitudes of ∼890 km and produced ULF pulsations on the ground with a period of ∼1108 s. We show (1) that the undulations were associated with subauroral polarization stream (SAPS) flows that were caused by the proton plasma sheet penetrating substantially farther earthward than the electron plasma sheet on the duskside and (2) that they may have been related to the arrival on the duskside of a substorm‐associated westward traveling surge‐like structure. The observations appear to be consistent with the development of a shear flow and/or ballooning type of instability at the plasmapause driven by intense SAPS‐associated shear flows.