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Ann. Geophys., 21, 1121-1140, 2003
www.ann-geophys.net/21/1121/2003/
© European Geosciences Union 2003
Variations in the polar cap area during two substorm cycles
S. E. Milan1, M. Lester1, S. W. H. Cowley1, K. Oksavik2, M. Brittnacher3, R. A. Greenwald4, G. Sofko5, and J.-P. Villain6
1Department of Physics and Astronomy, University of Leicester, Leicester, UK
2Department of Physics, University of Bergen, Bergen, Norway
3University of Washington, Geophysics Program, Seattle, Washington, USA
4The Johns Hopkins University, Applied Physics Laboratory, Laurel, Maryland, USA
5Institute of Space and Atmospheric Studies, Department of Physics and Engineering, University of Saskatchewan, Saskatchewan, Canada
6Centre National de la Recherche Scientifique, Orléans, France
Abstract. This study employs
observations from several sources to determine the location of the polar cap
boundary, or open/closed field line boundary, at all local times, allowing the
amount of open flux in the magnetosphere to be quantified. These data sources
include global auroral images from the Ultraviolet Imager (UVI) instrument on
board the Polar spacecraft, SuperDARN HF radar measurements of the convection
flow, and low altitude particle measurements from Defense Meteorological
Satellite Program (DMSP) and National Oceanographic and Atmospheric
Administration (NOAA) satellites, and the Fast Auroral SnapshoT (FAST)
spacecraft. Changes in the open flux content of the magnetosphere are related
to the rate of magnetic reconnection occurring at the magnetopause and in the
magnetotail, allowing us to estimate the day- and nightside reconnection
voltages during two substorm cycles. Specifically, increases in the polar cap
area are found to be consistent with open flux being created when the IMF is
oriented southwards and low-latitude magnetopause reconnection is ongoing, and
decreases in area correspond to open flux being destroyed at substorm breakup.
The polar cap area can continue to decrease for 100 min following the onset of
substorm breakup, continuing even after substorm-associated auroral features
have died away. An estimate of the dayside reconnection voltage, determined
from plasma drift measurements in the ionosphere, indicates that reconnection
can take place at all local times along the dayside portion of the polar cap
boundary, and hence presumably across the majority of the dayside magnetopause.
The observation of ionospheric signatures of bursty reconnection over a wide
extent of local times supports this finding.
Key words. Ionosphere (plasma
convection; polar ionosphere) – Magnetospheric physics (magnetospheric
configuration and dynamics)
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