A Climatology of Long‐Duration High 2‐MeV Electron Flux Periods in the Outer Radiation Belt

Abstract

Since the advent of the Space Age, the importance of understanding and forecasting relativistic electron fluxes in the Earth’s radiation belts has been steadily growing due to the threat that such particles pose to satellite electronics. Here, we provide a model of long‐duration periods of high time‐integrated 2‐MeV electron flux deep inside the outer radiation belt, based on the significant correlation obtained in 2001–2017 between time‐integrated electron flux measured by satellites and a measure of the preceding time‐integrated homogenized aaH geomagnetic index. We show that this correlation is likely due to a stronger cumulative chorus wave‐driven acceleration of relativistic electrons and a stronger cumulative inward radial diffusion of such electrons during periods of higher time‐integrated geomagnetic activity. Return levels of 2‐MeV electron flux are provided based on Extreme Value analysis of time‐integrated geomagnetic activity over 1868–2017, in rough agreement with estimates based on 20‐year data sets of measured flux. A high correlation is also found between our measure of time‐integrated geomagnetic activity averaged over each solar cycle and averaged sunspot numbers, potentially paving the way for forecasts of time‐integrated relativistic electron flux during future solar cycles based on predictions of solar activity.

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

“…Since the relativistic electron fluxes are the cumulative result of acceleration processes that take hours to days, statistical studies by Mourenas et al. (2019, 2022) demonstrated that time‐integrated geomagnetic indices were more informative in predicting outer belt electron dynamics than instantaneous values. Specifically, Mourenas et al.…”

“…Moreover, Mourenas et al. (2022) constructed a predictive model of high time‐integrated 2‐MeV electron fluxes using a time‐integrated aa H index. Nevertheless, the studies of Mourenas et al.…”

“…A high correlation between 2-MeV flux peaks and Int(AE) and Int(ap) in the heart of the outer belt was found. Moreover, Mourenas et al (2022) constructed a predictive model of high time-integrated 2-MeV electron fluxes using a time-integrated aa H index. Nevertheless, the studies of Mourenas et al (2019Mourenas et al ( , 2022 only focused on the time-integrated 2-MeV electron fluxes, and did not separate the dependence of the fluxes on substorms from storms, nor investigate a range of energies.…”

“…It has been well acknowledged that local electron heating by chorus waves and inward radial diffusion due to ULF waves are the two major mechanisms to energize outer belt energetic electrons at hundreds of keV to relativistic and ultra-relativistic energies (Drozdov et al, 2022;Hudson et al, 2008;Jaynes et al, 2018;Li, Ma, et al, 2016;Ma et al, 2018;Reeves et al, 2013;Thorne et al, 2013;Tu et al, 2013Tu et al, , 2014Zhao et al, 2018). Since the relativistic electron fluxes are the cumulative result of acceleration processes that take hours to days, statistical studies by Mourenas et al (2019Mourenas et al ( , 2022 demonstrated that time-integrated geomagnetic indices were more informative in predicting outer belt electron dynamics than instantaneous values. Specifically, Mourenas et al (2019) analyzed the correlation between the 10-day-averaged 2-MeV electron fluxes and time-integrated (denoted below as Int(*)) geomagnetic indices, including Int(Dst), Int(ap), Int(AE), and Int(AL), and preceding geomagnetic indices including (Dst) min , (ap) max , (AE) max .…”

“…Moreover, Mourenas et al (2022) constructed a predictive model of high time-integrated 2-MeV electron fluxes using a time-integrated aa H index. Nevertheless, the studies of Mourenas et al (2019Mourenas et al ( , 2022 only focused on the time-integrated 2-MeV electron fluxes, and did not separate the dependence of the fluxes on substorms from storms, nor investigate a range of energies. Furthermore, the time series of AL rather than AE were found to be the most critical parameter in the outer belt electron flux model using a neural network approach (Chu et al, 2021;Ma et al, 2022), possibly because the AL index is more relevant to injections.…”