The Changing Eigenfrequency Continuum During Geomagnetic Storms: Implications for Plasma Mass Dynamics and ULF Wave Coupling. Issue 6 (13th June 2020)

• Record Type: Journal Article
• Authors: Wharton, S. J.; Rae, I. J.; Sandhu, J. K.; Walach, M.‐T.; Wright, D. M.; Yeoman, T. K.
• Is Part Of: Journal of geophysical research. Volume 125:Issue 6(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-06-13
• Subjects: ULF waves -- cross phase -- geomagnetic storms -- plasma mass density -- eigenfrequencies -- ring current; Magnetospheric physics -- Periodicals; Space environment -- Periodicals; Cosmic physics -- Periodicals; Planets -- Atmospheres -- Periodicals; Heliosphere (Astrophysics) -- Periodicals; Geophysics -- Periodicals; 523.01
• DOI: 10.1029/2019JA027648
• Languages: English
• ISSNs: 2169-9380

Abstract

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. Plain Language Summary: Periods of intense solar wind can cause geomagnetic storms, large disturbances of the magnetosphere. Previous studies have shown that this can decrease the resonant frequencies of magnetic field lines with the Earth's magnetosphere. We perform a large statistical study on 132 storms to investigate this in order to understand the physics that drives this change. We find that the resonant frequencies typically decrease on field lines where L > 4 due to a weaker magnetic field caused by an enhanced ring current and a higher plasma mass density. We suggest the higher plasma mass density could be caused by an increase in Oxygen ions. At L < 4, the eigenfrequencies increase due to a drop in plasma mass density caused by plasmaspheric erosion. Key Points: We measure the average storm time evolution of field line eigenfrequencies, accounting for diurnal variation Lower eigenfrequencies during the main phase for L > 4 are due to a weaker magnetic field and enhanced plasma mass density Higher eigenfrequencies during the main phase for L < 4 are due to plasmaspheric erosion