Estimation of extreme electron fluxes at geostationary orbit: a statistical and a physical approach

The outer Van Allen belt is home to relativistic electrons and experience large variations in electron fluxes during geomagnetic driving events. Many satellite orbits, especially geostationary ones, overlap with the radiation belts and experience high-energy electron radiation. This radiation causes surface and internal charging, accompanied by aging of satellite components. In order to determine the impact on satellites during strong and extreme geomagnetic storms, one needs to quantify the magnitude of the fluxes. In this communication, we compare two different methods for estimating large flux values at geostationary orbit. The first method is physical and relies on the Kennel-Petschek limit (Kennel&Petschek, 1966). The second method is purely statistical and originates in extreme value theory (Coles, 2001). Using extreme value theory (EVT), for electron energies of 130 keV, we find for the once in 150 years electron flux an expected value that is two orders of magnitude larger than electron flux during the Halloween storm of 2003. On the other hand, the Kennel-Petschek limit, for the 130 keV electrons at L=6.7, which corresponds to geostationary orbit, is 1/6 of the maximum of the Halloween storm flux. The EVT therefore provides much larger estimates than the Kennel-Petschek limit. We compare the two methodologies with their respective strengths and limitations and determine under which conditions they should be combined to estimate extreme fluxes in the radiation belts.