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Abstract

Ultra-low frequency (ULF) waves are known to radially diffuse hundreds-keV to few-MeV electrons in the magnetosphere, as the range of drift frequencies of such electrons overlaps with the frequencies of the waves, leading to resonant interactions. The theoretical framework for this process is described by analytic expressions of the resonant interactions between electrons and toroidal and poloidal ULF wave modes in a background magnetic field. However, most expressions of the radial diffusion rates are derived only for equatorially mirroring electrons, and are based on estimates of the power of ULF waves that are obtained either from spacecraft close to the equatorial plane or from the ground. We present recent statistical observations based on THEMIS and Arase multi-year in-situ observations, showing that the wave power in magnetic fluctuations is significantly enhanced away from the magnetic equator, consistent with models simulating the natural modes of oscillation of magnetospheric field lines. We present 3D particle simulation results, which show different effects of these waves on electrons of different pitch angles, as electrons mirroring at higher magnetic latitudes will experience considerably higher ULF wave fluctuations than equatorial electrons. We discuss implications of these observations for the estimation of radial diffusion rates and highlight the need for incorporating pitch-angle-dependent radial diffusion coefficients in global models of the radiation belts.