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Seasonal and hemispheric asymmetries in the cold ion outflow source region: Swarm and CHAMP observations of F-region polar cap plasma density
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  • Spencer Mark Hatch,
  • Stein Haaland,
  • Karl M. Laundal,
  • Therese Moretto Jorgensen,
  • Andrew Yau,
  • Lindis Merete Bjoland,
  • Jone Peter Reistad,
  • Anders Ohma,
  • Kjellmar Oksavik
Spencer Mark Hatch
Birkeland Centre for Space Science, Birkeland Centre for Space Science

Corresponding Author:[email protected]

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Stein Haaland
Birkeland Centre for Space Science, Birkeland Centre for Space Science
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Karl M. Laundal
University of Bergen, University of Bergen
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Therese Moretto Jorgensen
UNIVERSITY OF BERGEN, UNIVERSITY OF BERGEN
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Andrew Yau
University of Calgary, University of Calgary
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Lindis Merete Bjoland
The University Centre in Svalbard, The University Centre in Svalbard
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Jone Peter Reistad
Birkeland Centre for Space Science, University of Bergen, Birkeland Centre for Space Science, University of Bergen
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Anders Ohma
Birkeland Centre for Space Science, University of Bergen, Birkeland Centre for Space Science, University of Bergen
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Kjellmar Oksavik
University of Bergen, University of Bergen
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Abstract

One of the primary mechanisms of loss of Earth’s atmosphere is the persistent “cold” (T ≲ 20 eV) ion outflow that has been observed in the magnetospheric lobes over large volumes with dimensions of order several Earth radii. As the main source of this cold ion outflow, the polar cap F-region ionosphere and conditions within it have a disproportionate influence on these magnetospheric regions. Using 15 years of measurements of plasma density Ne made by the Swarm spacecraft constellation and the CHAMP spacecraft within the F region of the polar cap above 80° Apex magnetic latitude, we report evidence of several types of seasonal asymmetries in polar cap Ne. Among these, the transition between “winter-like” and “summer-like” median polar cap Ne occurs one week prior to local spring equinox in the Northern Hemisphere (NH), and one week after local spring equinox in the Southern Hemisphere (SH). Thus the median SH polar cap Ne lags the median NH polar cap Ne by approximately two weeks with respect to hemispherically local spring and fall equinox. From interhemispheric comparison of statistical distributions of polar cap plasma density around each equinox and solstice, we find that distributions in the SH are often flatter (i.e., less skewed and kurtotic) than in the NH. Perhaps of most significance to cold ion outflow, we find no evidence of an F-region plasma density counterpart to a previously reported hemispheric asymmetry whereby cold plasma density is higher in the NH magnetospheric lobe than in the SH lobe.