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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 21, issue 5
Ann. Geophys., 21, 1121–1140, 2003
https://doi.org/10.5194/angeo-21-1121-2003
© Author(s) 2003. This work is distributed under
the Creative Commons Attribution 3.0 License.
Ann. Geophys., 21, 1121–1140, 2003
https://doi.org/10.5194/angeo-21-1121-2003
© Author(s) 2003. This work is distributed under
the Creative Commons Attribution 3.0 License.

  31 May 2003

31 May 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 S. E. Milan et al.
  • 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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