Abstract
AbstractGeomagnetic 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 multipleâepoch analysis to crossâphase 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.
Highlights
Geomagnetic storms are prolonged periods of intense solar wind-magnetosphere coupling that compress the Earth's magnetosphere, energize the magnetosphere-ionosphere system, and result in strong enhancements of the ring current, among other phenomena
We investigate the changes in local eigenfrequencies by applying a superposed multiple-epoch analysis to cross-phase spectra from 132 geomagnetic storms
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
Summary
Geomagnetic storms are prolonged periods of intense solar wind-magnetosphere coupling that compress the Earth's magnetosphere, energize the magnetosphere-ionosphere system, and result in strong enhancements of the ring current, among other phenomena. These have been documented since the nineteenth century (Stewart, 1861). The most intense coupling happens if the interplanetary magnetic field (IMF) is orientated southward and results in enhanced plasma convection within the magnetosphere (Akasofu et al, 1963; Gonzalez et al, 1994). The eigenfrequencies of any given flux tube depend upon the relationship between these three parameters: Eigenfrequencies will increase if the magnetic field strength increases, the plasma mass density decreases, or the flux tube decreases in length
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