Impact of high-intensity long-duration continuous auroral electrojet activity (HILDCAAs) on relativistic electrons in Earth's outer radiation belt during van allen probes era

Abstract

This study investigates the impact of High-Intensity Long-Duration Continuous Auroral Electrojet Activity (HILDCAA) on the relativistic electrons in the outer radiation belt of Earth. Utilizing in situ observations from the Van Allen Probe mission of NASA, we conducted a comprehensive analysis to understand the impact of HILDCAA events on the outer radiation belt electron fluxes. A superposed epoch analysis was performed to determine how relativistic electron fluxes respond to HILDCAA events as a function of L-shell, pitch angle, and energy. Results show a significant flux enhancement in the relativistic electron fluxes, predominantly occurring with a delay of 0 to 2 days following the onset of HILDCAA events. The general response indicates that the maximum energy of accelerated electrons reaches up to 6 MeV. Notably, electrons with perpendicular pitch angles exhibit greater enhancement than field-aligned populations, implying a critical role for pitch angle-dependent acceleration mechanisms. While the very-low frequency (VLF) waves, specifically chorus waves, also showed enhanced power during the HILDCAA period, the time-delayed and pitch angle-dependent response related to the onset of HILDCAAs highlights the significant influence of wave-particle interactions, particularly driven by ultra-low frequency (ULF) waves in this context. Rather than classical radial diffusion, typically requiring coherent ULF wave structures, the observed acceleration is likely facilitated by persistent, enhanced ULF wave activity, even in the presence of partial incoherence. This is further supported by ground-based magnetometers and in situ magnetic field observations from the RBSP probe, which demonstrated enhanced power of ULF waves during HILDCAA events. These findings reinforce the pivotal role of ULF waves in particle acceleration processes of the outer radiation belt, with implications for satellite operations and other space-based technologies, both on Earth and in the magnetospheres of other planets. © 2025 COSPAR