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1. Estimating Storm‐Time Maximum Fluxes of Outer Radiation Belt Electrons: Combining Van Allen Probes and GPS Satellite Observations

   https://doi.org/10.1029/2025JA034644  

   Hua, Man; Bortnik, Jacob; Meredith, Nigel P; Cayton, Thomas E (January 2026, Journal of Geophysical Research: Space Physics)

   Abstract By combining 6‐year Van Allen Probes data and 19‐year US Global Positioning System (GPS) satellite NS41 observations, covering the maximum and declining phases of both Solar Cycles 23 and 24, we comprehensively investigate the statistical distributions of the storm‐time maximum fluxes (j_max) of outer belt electrons and their critical controlling geomagnetic and solar wind conditions. We find that electron fluxes with energies ∼0.5–2 MeV atL = 4.2–4.5 are closely modulated by the time‐integrated AL index (Int(AL)), having strong correlation coefficients (CC > 0.7) between 6‐hr averaged electron fluxes and Int(AL) calculated with a sliding time window. The consistent observations ofj_maxfrom Radiation Belt Storm Probes and NS41 confirm the controlling influence of continuous substorms as measured by Int(AL) on producingj_max, showing strong and linear correlation between them in logarithmic space, even when applied to storms with (SYM‐H)_min < −300 nT. Meanwhile, the combination of low solar wind pressure, large and sustained substorms and solar wind electric field are important predictors for efficient electron acceleration. 

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2. Upper Limit of Outer Belt Electron Acceleration and Their Controlling Geomagnetic Conditions: A Comparison of Storm and Non‐Storm Events

   https://doi.org/10.1029/2024GL109612  

   Hua, Man; Bortnik, Jacob (July 2024, Geophysical Research Letters)

   Abstract We perform a comprehensive investigation of the statistical distribution of outer belt electron acceleration events over energies from 300 keV to ∼10 MeV regardless of storm activity using 6‐years of observations from Van Allen Probes. We find that the statistical properties of acceleration events are consistent with the characteristic energies of combined local acceleration by chorus waves and inward radial diffusion. While electron acceleration events frequently occur both at <2 MeV atL < 4.0 and at multi‐MeV atL > 4.5, significant acceleration events are confined toL > ∼4.0. By performing superposed epoch analysis of acceleration events during storm and non/weak storm events and comparing their geomagnetic conditions, we reveal the strong correlation (>0.8) between accumulated impacts of substorms as measured by time‐integrated AL (Int(AL)) and the upper flux limit of electron acceleration. While intense storms can provide favorable conditions for efficient acceleration, they are not necessarily required to produce large maximum fluxes. 

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3. Observations of Relativistic Electron Enhancement and Butterfly Pitch Angle Distributions at Low L (<3)

   https://doi.org/10.1029/2023GL106668  

   O'Brien, D.; Li, X.; Khoo, L.; Selesnick, R. S.; Hogan, B.; Zhao, H.; Mei, Y.; Hoxie, V.; Baker, D. N.; Kanekal, S. G. (January 2024, Geophysical Research Letters)

   Abstract Electrons in Earth's outer radiation belt are highly dynamic, with fluxes changing by up to orders of magnitude. The penetration of electrons from the outer belt to the inner belt is one such change observed during geomagnetic storms and was previously observed in electrons up to 1 MeV for some strong storms observed by the Van Allen Probes. We analyze pulse height analysis data from the Relativistic Electric and Proton Telescope (REPT) on the Van Allen Probes to produce electron flux measurements with lower minimum energy and significantly improved resolution compared to the standard REPT data and show that electron penetrations into the inner belt (L ≤ 2) extend to at least 1.3 MeV and penetrations into the slot region (2 < L < 2.8) extend to at least 1.5 MeV during certain geomagnetic storms. We also demonstrate that these penetrations are associated with butterfly pitch angle distributions from 1 to 1.3 MeV. 

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4. Dependence of Electron Flux Dropouts in the Earth's Outer Radiation Belt on Energy and Driving Parameters During Geomagnetic Storms

   https://doi.org/10.1029/2023JA031882  

   Hua, Man; Bortnik, Jacob; Ma, Donglai (October 2023, Journal of Geophysical Research: Space Physics)

   Abstract Using 5‐year of measurements from Van Allen Probes, we present a survey of the statistical dependence of the Earth's outer radiation belt electron flux dropouts during geomagnetic storms on electron energy and various driving parameters including interplanetary magnetic field B_z, P_SW, SYM‐H, and AE. By systematically investigating the dropouts over energies of 1 keV–10 MeV at L‐shells spanning 4.0–6.5, we find that the dropouts are naturally divided into three regions. The dropouts show much higher occurrence rates at energies below ∼100 keV and above ∼1 MeV compared to much smaller occurrence rate at intermediate energies around hundreds of keV. The flux decays more dramatically at energies above ∼1 MeV compared to the energies below ∼100 keV. The flux dropouts of electrons below ∼100 keV strongly depend on magnetic local time (MLT), which demonstrate high occurrence rates on the nightside (18–06 MLT), with the highest occurrence rate associated with northward B_z, strong P_SWand SYM‐H, and weak AE conditions. The strongest flux decay of these dropouts is found on the nightside, which strongly depends on P_SWand SYM‐H. However, there is no clear MLT dependence of the occurrence rate of relativistic electron flux dropouts above ∼1 MeV, but the flux decay of these dropouts is more significant on the dayside, with stronger decay associated with southward IMF B_z, strong P_SW, SYM‐H, and AE conditions. Our statistical results are crucial for understanding of the fundamental physical mechanisms that control the outer belt electron dynamics and developing future potential radiation belt forecasting capability. 

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5. On the Possibility of Driving the Electron Flux Probabilistic Models by the AE Index

   https://doi.org/10.1029/2025JA033880  

   Dubyagin, S; Ganushkina, N; Sicard, A; Matéo‐Vélez, J‐C; Monnin, L; Heynderickx, D; Jiggens, P; Deprez, G; Cipriani, F (February 2026, Journal of Geophysical Research: Space Physics)

   Abstract We present the new probabilistic model of the electron fluxes designed to assess the risks of the spacecraft surface charging for missions with near‐equatorial orbits in the inner magnetosphere. It is a second model developed within a frame of the European Space Agency's activity “Plasma Environment Modeling in the Earth's Magnetosphere” (PEMEM). The first model PEMEMPercentile(Dubyagin et al., 2024,https://doi.org/10.1029/2023JA032026) has a robust though somewhat simple functionality. Addressing the PEMEMPercentileweaknesses, we test a novel approach to incorporating the dependence on geomagnetic activity in probabilistic models. The model is based on Van Allen Probes particle data. The model is driven by the auroral electrojet (AE) index from a period in the past corresponding to the expected solar cycle phase during a mission lifetime. The main model inputs are the spacecraft orbit, the time interval of AE‐index to drive the model, and the confidence levels. For given confidence levels, the model outputs the worst‐case 1–100 keV integrated electron flux and corresponding differential flux spectrum. The model can output these parameters separately for the eclipse and sunlit parts of the orbit. While investigating the response of the electron flux to the AE‐index variations, we have found that lower energy electrons reveal the highest correlation with the AE‐index averaged over the substorm time scale, while higher energy electrons show a higher correlation with AE on the storm time scale. The transition between these two regimes occurs at  keV energy and has a complex dependence on radial distance and MLT. 

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