Timing and localization of ionospheric signatures associated with substorm expansion phase onset
Article first published online: 22 JAN 2009
Copyright 2009 by the American Geophysical Union.
Journal of Geophysical Research: Space Physics (1978–2012)
Volume 114, Issue A1, January 2009
How to Cite
2009), Timing and localization of ionospheric signatures associated with substorm expansion phase onset, J. Geophys. Res., 114, A00C09, doi:10.1029/2008JA013559., et al. (
- Issue published online: 22 JAN 2009
- Article first published online: 22 JAN 2009
- Manuscript Accepted: 14 OCT 2008
- Manuscript Revised: 28 AUG 2008
- Manuscript Received: 2 JUL 2008
- ULF waves;
 In this paper, we present case studies of the optical and magnetic signatures of the characteristics of the first minute of substorm expansion phase onset observed in the ionosphere. We find that for two isolated substorms, the onset of magnetic pulsations in the 24–96 s period wavelet band are colocated in time and space with the formation and development of small-scale optical undulations along the most equatorward preexisting auroral arc prior to auroral breakup. These undulations undergo an inverse spatial cascade into vortices prior to the release of the westward traveling surge. We also present a case study of a multiple activation substorm, whereby discrete onsets of ULF wave power above a predetermined quiet time threshold are shown to be associated with specific optical intensifications and brightenings. Moreover, in the multiple activation substorm event, we show that neither the formation of the small-scale undulations nor the formation of similar structures along a north–south aligned arc is sufficient to produce auroral breakup associated with expansion phase onset. It is only ∼10 min after these two disparate activation regions initiate that auroral breakup and the subsequent formation of a westward traveling surge occur. We discuss the implications of these results in terms of the triggering mechanisms likely to be occurring during these specific events.