Radiation belt dynamics during the shortest and longest HILDCAA events: Van Allen probes observations

Abstract

High-intensity long-duration continuous auroral electrojet activity (HILDCAA) is a characteristic phenomenon of long-lasting geomagnetic disturbance in the Earth�s magnetosphere. They have been related to increased relativistic electron fluxes in the Earth�s magnetosphere, but the exact mechanisms are not fully established. By scanning Van Allen Probes data from 2014 to 2019, this study investigates two distinct HILDCAA events: one short and one long duration. Both events result in increases in electron flux across several energy channels, according to spectral and temporal investigations. The longer event, however, produces a sharper, stronger, and longer-lasting enhancement in flux, whereas the shorter event exhibits a weaker and more fleeting reaction. Sustained solar wind energy input that enables effective particle acceleration is maintained during extended HILDCAA intervals, accompanied by prolonged geomagnetic activity and repeated substorm injections. In comparison to the shorter event, this one likewise shows a stronger energization of electrons to higher energies. Analysis of plasma waves reveals that the observed flux enhancements are due to increased chorus wave activity and increased ultra-low frequency (ULF) wave activity during the long-duration event. The phase space density (PSD) profiles show local acceleration at L* = 4.5 to 5.0, while PSD shifts outward and decreases at higher energies. Spectral steepening and greater PSD peaks suggest that the longer HILDCAA event generally enables more efficient and sustained electron acceleration. These findings highlight, that event duration is a significant controlling parameter of outer radiation belt electron dynamics, modulating the efficiency of local acceleration, spectral evolution, and wave-particle interaction. While the two events not exactly identical and differ in solar wind driving conditions, the results are consistent with past statistical studies and indicate need of modeling studies to rigorously separate these effects. The necessity of including long-term geomagnetic disturbances and their duration in magnetospheric specifically radiation belt simulations is strongly supported by these observations. � The Author(s), under exclusive licence to Springer Nature B.V. 2026.