On Diffusive and Non-Diffusive Fast ULF Wave Radiation Belt Losses during Intense Geomagnetic Storms

Abstract:

It is well-known that outwards ULF wave radial diffusion can enhance radiation belt losses due to transport to a compressed magnetopause during magnetic storms. Here we use data from intense magnetic storms to assess the expected rates of diffusion based on in-situ measurements from the THEMIS, Van Allen Probes and GOES satellites, and from ground-based magnetometers from around the world. Specifically we compare the rates of electric and magnetic diffusion and their relative importance, our observations revealing that at times the magnetic diffusion rates during the main phase may exceed the electric rates, in contrast to the usual paradigm. Moreover, at times the values of the diffusion coefficients when constrained by observational data can reach small fractions of a day, suggesting that the diffusive paradigm might not be valid. We specifically investigate conditions during the March 2015 storm, comparing the observed diffusion rates and revealing conditions where the compressional magnetic field fluctuations can have amplitudes which reach dB/B≈0.3 around geosynchronous orbit. We also consider the effects of intense convection during periods of southward IMF, and find evidence that such processes may be responsible for large field fluctuations in the equatorial plane. Combining the multi-satellite and ground data we examine the impacts of these processes on radiation belt particle dynamics, including an assessment of the possibility that non-diffusive effects might be significant and might be able to explain the fast radiation belt losses observed in the main phase of this intense magnetic storm.