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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Research output: Contribution to Journal/Magazine › Journal article › peer-review
Research output: Contribution to Journal/Magazine › Journal article › peer-review
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TY - JOUR
T1 - The Changing Eigenfrequency Continuum during Geomagnetic Storms
T2 - Implications for Plasma Mass Dynamics and ULF Wave Coupling
AU - Wharton, S. J.
AU - Rae, I.J.
AU - Sandhu, J. K.
AU - Walach, Maria
AU - Wright, D. M.
AU - Yeoman, T. K.
N1 - Accepted for publication in Journal of Geophysical Research: Space Physics. Copyright 2020 American Geophysical Union. Further reproduction or electronic distribution is not permitted.
PY - 2020/6/13
Y1 - 2020/6/13
N2 - 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.
AB - 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.
KW - ULF waves
KW - cross phase
KW - geomagnetic storms
KW - plasma mass density
KW - eigenfrequencies
KW - ring current
U2 - 10.1029/2019JA027648
DO - 10.1029/2019JA027648
M3 - Journal article
VL - 125
JO - Journal of Geophysical Research: Space Physics
JF - Journal of Geophysical Research: Space Physics
SN - 2169-9402
IS - 6
M1 - e2019JA027648
ER -