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Ann. Geophys., 20, 461-470, 2002
www.ann-geophys.net/20/461/2002/
© European Geosciences Union 2002
He+ dominance in the plasmasphere during geomagnetically disturbed periods: 1. Observational results
M. H. Denton1, G. J. Bailey2, C. R. Wilford2, A. S. Rodger3, and S. Venkatraman4
1Radio and Space Physics Group, University of Wales, Aberystwyth, UK (formerly of Space and Atmosphere Research Group, University of Sheffield, UK)
2Space and Atmosphere Research Group, University of Sheffield, UK
3Physical Sciences Division, British Antarctic Survey, Cambridge, UK
4Center for Space Sciences, University of Texas at Dallas, USA
Correspondence to: M. H. Denton
(mick.denton@aber.ac.uk)
Abstract. Observations made by the
DMSP F10 satellite during the recovery phase from geomagnetic disturbances in
June 1991 show regions of He+ dominance around 830 km altitude at
09:00 MLT. These regions are co-located with a trough in ionisation observed
around 55° in the winter hemisphere. Plasma temperature and concentration
observations made during the severe geomagnetic storm of 24 March 1991 are used
as a case study to determine the effects of geomagnetic disturbances along the
orbit of the F10 satellite. Previous explanations for He+ dominance
in this trough region relate to the part of the respective flux tubes that is
in darkness. Such conditions are not relevant for this study, since the whole
of the respective flux tubes are sunlit. A new mechanism is proposed to explain
the He+ dominance in the trough region. This mechanism is based on
plasma transport and chemical reaction effects in the F-region and topside
ionosphere, and on the time scales for such chemical reactions. Flux tubes
previously depleted by geomagnetic storm effects refill during the recovery
phase from the ionosphere as a result of pressure differences along the flux
tubes. Following a geomagnetic disturbance, the He+ ion recovers
quickly via the rapid photoionisation of neutral helium, in the F-region and
the topside. The recovery of the O+ and H+ ions is less
rapid. This is proposed as a result of the respective charge exchange reactions
with neutral atomic hydrogen and oxygen. Preliminary model calculations support
the proposed mechanism.
Key words. Magnetospheric physics
(storms and sub-storms, plasmasphere)
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