Upper Limit of Electron Fluxes Observed in the Radiation Belts

Radiation belt electrons have a complicated relationship with geomagnetic activity. We select electron measurements from 7 years of DEMETER and 6 years of Van Allen Probes data during geomagnetic storms to conduct statistical analysis focusing on the correlation between electron flux and Dst index. We report, for the first time, an upper limit of electron fluxes observed by both satellites throughout the inner and outer belts across a wide energy range from ∼100s keV to multi‐MeV. The upper flux limit is determined at different L's and energies, for example, 1.9 × 107/cm2‐s‐sr‐MeV at 470 keV at L = 1.5 and 3.6 × 105/cm2‐s‐sr‐MeV at 3.4 MeV at L = 4 (Van Allen Probes). We present the energy spectra of the electron flux upper limit at different L shells and find the measured upper flux limit to be at least three times higher than the predicted flux from the AE8/AE9 models, although the spectral shape is remarkably similar. We show that the average flux with an applied time lag is better correlated with the Dst index and that the time lag optimizing the correlation coefficient is larger at lower L and at higher energies. These findings present the underlying challenges to model the dynamic variation of relativistic electrons in the inner magnetosphere and are important information for space weather considerations. Key Points Upper limit of electron fluxes during storm times is observed by both DEMETER and the Van Allen Probes The upper limit of electron flux is higher than the AE8/AE9 model predictions, though the spectral shape is remarkably similar Electrons at lower L and higher energy take longer time to fully respond to geomagnetic storms