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

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.

Average Ionospheric Electric Field Morphologies During Geomagnetic Storm Phases

“…Further evidence for the plasmaspheric plume being responsible for this moving of the dayside convection throat is available from comparing our results to those of Wharton et al. (2020): In their study, Wharton et al. (2020) looked at the eigenfrequencies in‐ground magnetometer variations on the dayside during the same storm phases as ours.…”

“…This approximately corresponds to a geomagnetic latitude of 60° or less (see Table 1 in Wharton et al [2020]), which corresponds to the dayside throat location we see during the main phase of the storm. Wharton et al (2020) find that at L > 4 (which maps to higher latitudes and thus inside the convection pattern on the dayside), the eigenfrequencies decrease by ∼50% during the main phase, due to a weaker magnetic field and an enhanced plasma mass density. This may be further evidence of the plasmaspheric plume.…”

“…Wharton et al. (2020) find that at L > 4 (which maps to higher latitudes and thus inside the convection pattern on the dayside), the eigenfrequencies decrease by ∼50% during the main phase, due to a weaker magnetic field and an enhanced plasma mass density. This may be further evidence of the plasmaspheric plume.…”

“…To study the characteristic ionospheric convection morphologies of the storms in detail, we make a superposed epoch analysis. Similarly to Hutchinson et al (2011) and Wharton et al (2020), we make a superposed epoch analysis of the storms which treats each storm phase independently. The duration of each storm phase is scaled to the median duration, such that the initial, main, and recovery phases are 19.5-, 9.1-, and 55.8-h long, respectively.…”

“…(2011) and Wharton et al. (2020), we make a superposed epoch analysis of the storms which treats each storm phase independently. The duration of each storm phase is scaled to the median duration, such that the initial, main, and recovery phases are 19.5‐, 9.1‐, and 55.8‐h long, respectively.…”