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The Ionospheric Leg of the Substorm Current Wedge: Combining Iridium and Ground Magnetometers
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  • Simon James Walker,
  • Karl Laundal,
  • Jone Peter Reistad,
  • Spencer Mark Hatch,
  • Anders Ohma,
  • Jesper W. Gjerloev
Simon James Walker
Univerisity of Bergen

Corresponding Author:[email protected]

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Karl Laundal
University in Bergen
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Jone Peter Reistad
Birkeland Centre for Space Science, University of Bergen
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Spencer Mark Hatch
Birkeland Centre for Space Science
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Anders Ohma
Birkeland Centre for Space Science, Department of Physics and Technology, University of Bergen
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Jesper W. Gjerloev
APL-JHU
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Abstract

Utilising magnetic field measurements made by the Iridium satellites and by ground magnetometers in North America we calculate the full ionospheric current system and investigate the substorm current wedge. The current estimates are independent of ionospheric conductance, and are based on estimates of the divergence-free (DF) ionospheric current from ground magnetometers and curl-free (CF) ionospheric currents from Iridium. The DF and CF currents are represented using spherical elementary current systems (SECS), derived using a new inversion scheme that ensures the current systems’ spatial scales are consistent. We present 18 substorm events and find a typical substorm current wedge (SCW) in 12 events. Our investigation of these substorms shows that during substorm expansion, equivalent field-aligned currents (EFACs) derived with ground magnetometers are a poor proxy of the actual FAC. We also find that the intensification of the westward electrojet can occur without an intensification of the FACs. We present theoretical investigations that show that the observed deviation between FACs estimated with satellite measurements and ground-based EFACs are consistent with the presence of a strong local enhancement of the ionospheric conductance, similar to the substorm bulge. Such enhancements of the auroral conductance can also change the ionospheric closure of pre-existing FACs such that the ground magnetic field, and in particular the westward electrojet, changes significantly. These results demonstrate that attributing intensification of the westward electrojet to SCW current closure can yield false understanding of the ionospheric and magnetospheric state.
06 Jan 2024Submitted to ESS Open Archive
16 Jan 2024Published in ESS Open Archive