Unraveling the Critical Geomagnetic Conditions Controlling the Upper Limit of Electron Fluxes in the Earth's Outer Radiation Belt

We investigate the statistical distribution of the upper limit of outer radiation belt electron fluxes during active times using 5-year measurements from Van Allen Probes. The upper flux limit at different energies peaks around L similar to 4.7, with larger flux variation at higher energies >1 MeV compared to energies at hundreds of keV. Based on the correlation analysis between maximum fluxes and various time-integrated geomagnetic indices, including SYM-H, AE, and AL, we reveal that maximum fluxes strongly correlate to cumulative effects of substorms instead of storms, with the strongest dependence on the time-integrated AL (Int(AL)). We investigate the correlation coefficients (CC) between maximum fluxes and time-integrated substorm indices at different time lags and identify the time lag corresponding to the best CC. Finally, we develop a prediction model of maximum fluxes using only Int(AL) as input, which shows CC of similar to 0.80 and allows future forecasts of extreme cases of outer belt electrons.