Variations in the polar cap area during two substorm cycles

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...

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Main Authors: S. E. Milan, M. Lester, S. W. H. Cowley, K. Oksavik, M. Brittnacher, R. A. Greenwald, G. Sofko, J.-P. Villain
Format: Article
Language:English
Published: Copernicus Publications 2003-05-01
Series:Annales Geophysicae
Online Access:https://www.ann-geophys.net/21/1121/2003/angeo-21-1121-2003.pdf
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spelling doaj-6cf67895248a45418d28dab45ecee0ae2020-11-24T23:16:14ZengCopernicus PublicationsAnnales Geophysicae0992-76891432-05762003-05-01211121114010.5194/angeo-21-1121-2003Variations in the polar cap area during two substorm cyclesS. E. Milan0M. Lester1S. W. H. Cowley2K. Oksavik3M. Brittnacher4R. A. Greenwald5G. Sofko6J.-P. Villain7Department of Physics and Astronomy, University of Leicester, Leicester, UKDepartment of Physics and Astronomy, University of Leicester, Leicester, UKDepartment of Physics and Astronomy, University of Leicester, Leicester, UKDepartment of Physics, University of Bergen, Bergen, NorwayUniversity of Washington, Geophysics Program, Seattle, Washington, USAThe Johns Hopkins University, Applied Physics Laboratory, Laurel, Maryland, USAInstitute of Space and Atmospheric Studies, Department of Physics and Engineering, University of Saskatchewan, Saskatchewan, CanadaCentre National de la Recherche Scientifique, Orléans, FranceThis 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.<br><br><b>Key words. </b>Ionosphere (plasma convection; polar ionosphere) – Magnetospheric physics (magnetospheric configuration and dynamics)https://www.ann-geophys.net/21/1121/2003/angeo-21-1121-2003.pdf
collection DOAJ
language English
format Article
sources DOAJ
author S. E. Milan
M. Lester
S. W. H. Cowley
K. Oksavik
M. Brittnacher
R. A. Greenwald
G. Sofko
J.-P. Villain
spellingShingle S. E. Milan
M. Lester
S. W. H. Cowley
K. Oksavik
M. Brittnacher
R. A. Greenwald
G. Sofko
J.-P. Villain
Variations in the polar cap area during two substorm cycles
Annales Geophysicae
author_facet S. E. Milan
M. Lester
S. W. H. Cowley
K. Oksavik
M. Brittnacher
R. A. Greenwald
G. Sofko
J.-P. Villain
author_sort S. E. Milan
title Variations in the polar cap area during two substorm cycles
title_short Variations in the polar cap area during two substorm cycles
title_full Variations in the polar cap area during two substorm cycles
title_fullStr Variations in the polar cap area during two substorm cycles
title_full_unstemmed Variations in the polar cap area during two substorm cycles
title_sort variations in the polar cap area during two substorm cycles
publisher Copernicus Publications
series Annales Geophysicae
issn 0992-7689
1432-0576
publishDate 2003-05-01
description 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.<br><br><b>Key words. </b>Ionosphere (plasma convection; polar ionosphere) – Magnetospheric physics (magnetospheric configuration and dynamics)
url https://www.ann-geophys.net/21/1121/2003/angeo-21-1121-2003.pdf
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