Local Time and Geomagnetic Activity Dependence of the Distribution of ULF Wave Power on Azimuthal Mode Numbers: Observations and Test Particle Simulations

Abstract:

An important parameter that characterizes Ultra-Low Frequency (ULF) waves, and that is critical in correctly approximating the radial diffusion that these waves can inflict on relativistic electrons in the radiation belts, is the waves' mode number, m, which describes the azimuthal wavelength of the waves. A commonly used approximation states that all ULF wave power can be assumed to be in a single mode number, usually m=1 or m=2. We use cross-spectrogram phase-differences between multiple pairs of satellites to get an estimate of m and we find that the distribution of power in the various mode numbers can be considerably different than this approximation at times; we also find a dependence of the m-distribution of power on geomagnetic conditions and on local time, with geomagnetically active times and midnight-side magnetosphere favoring higher mode numbers. We use these results in a particle tracing simulation that includes analytic expressions for the ULF waves, and we discuss their implications for radiation belt electrons.