Cluster magnetotail observations of a tailward-travelling plasmoid at substorm expansion phase onset and field aligned currents in the plasma sheet boundary layer

Draper, N. C. and Lester, M. and Cowley, S. W. H. and Wild, J. A. and Milan, S. E. and Provan, G. and Grocott, A. and Fazakerley, A. N. and Lahiff, A. and Davies, J. A. and Bosqued, J. M. and Dewhurst, J. P. and Nakamura, R. and Owen, C. J. and Watermann, J. F. and Henderson, M. G. and Singer, H. J. and Donovan, E. (2005) Cluster magnetotail observations of a tailward-travelling plasmoid at substorm expansion phase onset and field aligned currents in the plasma sheet boundary layer. Annales Geophysicae, 23 (12). pp. 3667-3683. ISSN 0992-7689

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Abstract

We present data from both ground- and space- based instruments for a substorm event which occurred on 5 October 2002, with an expansion phase onset time of 02:50 UT determined from the ground magnetometer data. During this substorm, the Cluster spacecraft were located around 15 RE downtail, 8 RE from midnight in the pre- midnight sector and just 2 RE above the equatorial plane (in GSM coordinates). At expansion phase onset the Cluster spacecraft were located in the plasma sheet, tailward of a near-Earth neutral line and detected a significant time delay of 6 min between the tail field Bz component becoming negative and the subsequent detection of Earthward flows. This is explained by the formation of a tailward-directed travel- ling compression region initially Earthward of the spacecraft; 7 min later the Cluster spacecraft entered the plasma sheet boundary layer; they remained in and close to the plasma sheet boundary layer for around 15 min before exiting to the lobe. The spacecraft then re-entered the plasma sheet 30 min after onset. Earthward then tailward directed currents detected in the plasma sheet boundary layer after onset indicate that the Cluster spacecraft encountered the dawnward and duskward portions of the reconnection flow associated current system with Region 1 sense, respectively. The reconnection site and current system were initially skewed towards the pre-midnight sector, consistent with previous observations that found the majority of substorm onsets located in this sector. At later times the reconnection site and current system had moved towards dawn, to be located more centrally in the midnight sector.

Item Type: Journal Article
Journal or Publication Title: Annales Geophysicae
Additional Information: © European Geosciences Union 2005
Uncontrolled Keywords: /dk/atira/pure/researchoutput/libraryofcongress/t1
Subjects:
Departments: Faculty of Science and Technology > School of Computing & Communications
Faculty of Science and Technology > Physics
ID Code: 6676
Deposited By: ep_importer_dcs
Deposited On: 21 Apr 2008 10:56
Refereed?: Yes
Published?: Published
Last Modified: 22 Aug 2019 00:40
URI: https://eprints.lancs.ac.uk/id/eprint/6676

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