In-situ spatio-temporal measurements of the detailed azimuthal substructure of the substorm current wedge

C. Forsyth, A. N. Fazakerley, I. J. Rae, C. E. J. Watt, K. Murphy, J. A. Wild, T. Karlsson, R. Mutel, C .J. Owen, R. Ergun, A. Masson, M. Berthomier, E. Donovan, H. U. Frey, J. Matzka, C. Stolle, Y. Zhang

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The substorm current wedge (SCW) is a fundamental component of geomagnetic substorms. Models tend to describe the SCW as a simple line current flowing into the ionosphere towards dawn and out of the ionosphere towards dusk, linked by a westward electrojet. We use multi-spacecraft observations from perigee passes of the Cluster 1 and 4 spacecraft during a substorm on 15 Jan 2010, in conjunction with ground-based observations, to examine the spatial structuring and temporal variability of the SCW. At this time, the spacecraft travelled east-west azimuthally above the auroral region. We show that the SCW has significant azimuthal sub-structure on scales of 10026km at altitudes of 4,000-7,00026km. We identify 26 individual current sheets in the Cluster 4 data and 34 individual current sheets in the Cluster 1 data, with Cluster 1 passing through the SCW 120-24026s after Cluster 4 at 1,300-2,00026km higher altitude. Both spacecraft observed large-scale regions of net upward and downward field-aligned current, consistent with the large-scale characteristics of the SCW, although sheets of oppositely directed currents were observed within both regions. We show that the majority of these current sheets were closely aligned to a north-south direction, in contrast to the expected east-west orientation of the pre-onset aurora. Comparing our results with observations of the field-aligned current associated with bursty bulk flows (BBFs) we conclude that significant questions remain for the explanation of SCW structuring by BBF driven ``wedgelets". Our results therefore represent constraints on future modelling and theoretical frameworks on the generation of the SCW.
Original languageEnglish
Pages (from-to)927-946
Number of pages20
JournalJournal of Geophysical Research: Space Physics
Issue number2
Publication statusPublished - 12 Feb 2014


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