Ultralow-frequency modulation of whistler-mode wave growth

Abstract

Measurements from ground-based magnetometers and riometers at auroral latitudes have demonstrated that energetic (∼30-300 keV) electron precipitation can be modulated in the presence of magnetic field oscillations at ultralow frequencies. It has previously been proposed that an ultralow-frequency (ULF) wave would modulate field and plasma properties near the equatorial plane, thus modifying the growth rates of whistler-mode waves. In turn, the resulting whistler-mode waves would mediate the pitch angle scattering of electrons resulting in ionospheric precipitation. In this paper, we investigate this hypothesis by quantifying the changes to the linear growth rate expected due to a slow change in the local magnetic field strength for parameters typical of the equatorial region around 6.6RE radial distance. To constrain our study, we determine the largest possible ULF wave amplitudes from measurements of the magnetic field at geosynchronous orbit. Using nearly ten years of observations from two satellites, we demonstrate that the variation in magnetic field strength due to oscillations at 2 mHz does not exceed 10% of the background field. Modifications to the plasma density and temperature anisotropy are estimated using idealized models. For low temperature anisotropy, there is little change in the whistler-mode growth rates even for the largest ULF wave amplitude. Only for large temperature anisotropies can whistler-mode growth rates be modulated sufficiently to account for the changes in electron precipitation measured by riometers at auroral latitudes. Copyright 2011 by the American Geophysical Union.