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Ann. Geophys., 20, 1577-1601, 2002
www.ann-geophys.net/20/1577/2002/
© European Geosciences Union 2002
Excitation of twin-vortex flow in the nightside high-latitude ionosphere during an isolated substorm
A. Grocott1, S. W. H. Cowley1, J. B. Sigwarth2, J. F. Watermann3, and T. K. Yeoman1
1Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK
2Department of Physics and Astronomy, The University of Iowa, Iowa City, Iowa 52242-1479, USA
3Danish Meteorological Institute, Lyngbyvej 100, 2100 Copenhagen, Denmark
Correspondence to: A. Grocott (ag27@ion.le.ac.uk)
Abstract. We present SuperDARN radar observations of
the ionospheric flow during a well-observed high-latitude substorm which
occurred during steady northward IMF conditions on 2 December 1999. These data
clearly demonstrate the excitation of large-scale flow associated with the
substorm expansion phase, with enhanced equatorward flows being observed in the
pre-midnight local time sector of the expansion phase auroral bulge and
westward electrojet, and enhanced return sunward flows being present at local
times on either side, extending into the dayside sector. The flow pattern
excited was thus of twin-vortex form, with foci located at either end of the
substorm auroral bulge, as imaged by the Polar VIS UV imager. Estimated total
transpolar voltages were ~40 kV prior to expansion phase onset, grew to ~80 kV
over a ~15 min interval during the expansion phase, and then decayed to ~35 kV
over ~10 min during recovery. The excitation of the large-scale flow pattern
resulted in the development of magnetic disturbances which extended well
outside of the region directly disturbed by the substorm, depending upon the
change in the flow and the local ionospheric conductivity. It is estimated that
the nightside reconnection rate averaged over the 24-min interval of the
substorm was ~65– 75 kV, compared with continuing dayside reconnection rates
of ~30–45 kV. The net closure of
open flux during the sub-storm was thus ~0.4–0.6
× 108 Wb, representing ~15–20%
of the open flux present at onset, and corresponding to an overall contraction
of the open-closed field line boundary by ~1°
latitude.
Key words. Ionosphere (auroral ionosphere;
ionosphere-magnetosphere interactions; plasma convection)
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