How the Shortest and Longest HILDCAAs Shaped Earth Outer Radiation Belt During the Van Allen Probes Era?

TL;DR

This study compares two HILDCAA events of contrasting durations using Van Allen Probes data, revealing how event length influences electron flux enhancements and magnetospheric wave activity, with implications for space weather modeling.

Contribution

It provides the first detailed spectral and temporal analysis of contrasting HILDCAA events, highlighting the role of event duration in radiation belt dynamics.

Findings

Short events cause rapid, transient electron energy increases.

Long events lead to higher electron flux variations and higher energy acceleration.

Elevated ULF wave power correlates with longer HILDCAA events.

Abstract

High-intensity long-duration continuous auroral electrojet (AE) activity (HILDCAA) events are associated with the enhancement of relativistic electron fluxes in the inner magnetosphere. The physical mechanisms underlying this enhancement are not well established yet. In this study, we analyze two contrasting HILDCAA events, one representing the shortest and the other the longest duration, using NASA Van Allen Probes observations, which have provided unprecedented, unique in-situ observations of the harsh radiation environment around the Earth. Detailed spectral and temporal analyses reveal that while both events trigger enhancements in electron flux across multiple energy channels, the shortest event is characterized by rapid, transient increases in energy levels. In contrast, the longest event produced sudden and markedly higher flux variation. The long duration event showed an acceleration of electrons to higher energy as compared to the shorter one. Moreover, a clear correlation between elevated ULF wave power for the longest event compared to the shortest is observed, apart from chorus waves responsible for relativistic electron acceleration. These findings underscore the importance of considering the duration of events in space weather models and assessment and provide valuable insights into the magnetospheric processes that modulate the variability of the radiation belt during HILDCAA conditions.