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    Rights statement: Accepted for publication in Journal of Geophysical Research: Space Physics. Copyright 2020 American Geophysical Union. Further reproduction or electronic distribution is not permitted.

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The Changing Eigenfrequency Continuum during Geomagnetic Storms: Implications for Plasma Mass Dynamics and ULF Wave Coupling

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  • S. J. Wharton
  • I.J. Rae
  • J. K. Sandhu
  • Maria Walach
  • D. M. Wright
  • T. K. Yeoman
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Article numbere2019JA027648
<mark>Journal publication date</mark>13/06/2020
<mark>Journal</mark>Journal of Geophysical Research: Space Physics
Issue number6
Volume125
Number of pages20
Publication StatusPublished
<mark>Original language</mark>English

Abstract

Geomagnetic storms are one of the most energetic space weather phenomena. Previous studies have shown that the eigenfrequencies of ultralow frequency (ULF) waves on closed magnetic field lines in the inner magnetosphere decrease during storm times. This change suggests either a reduction in the magnetic field strength and/or an increase in its plasma mass density distribution. We investigate the changes in local eigenfrequencies by applying a superposed multiple‐epoch analysis to cross‐phase spectra from 132 geomagnetic storms. Six ground magnetometer pairs are used to investigate variations from approximately 3 < L < 7 and across the whole dayside sector. We find that at L > 4, the eigenfrequencies decrease by as much as 50% relative to their quiet time values. Both a decrease in magnetic field strength and an increase in plasma mass density, in some locations by more than a factor of 2, are responsible for this reduction. The enhancement of the ring current and an increase in oxygen ion density could explain these observations. At L < 4, the eigenfrequencies increase due to the decrease in plasma mass density caused by plasmaspheric erosion.

Bibliographic note

Accepted for publication in Journal of Geophysical Research: Space Physics. Copyright 2020 American Geophysical Union. Further reproduction or electronic distribution is not permitted.