# Highly relativistic electron flux enhancement during the weak geomagnetic storm of April-May 2017

## TL;DR

This study shows that a weak geomagnetic storm can cause significant relativistic electron flux enhancements lasting over 24 days, driven by chorus waves and ULF wave diffusion, which are not detected by typical geosynchronous measurements.

## Contribution

It reveals that even weak geomagnetic storms can produce substantial relativistic electron enhancements through specific wave-particle interactions, expanding understanding of space weather effects.

## Key findings

- Relativistic electron flux increased by two orders of magnitude during a weak storm.
- Enhancements lasted for at least 24 days, exceeding typical storm durations.
- Different mechanisms, chorus waves and ULF waves, drive electron acceleration at different energies.

## Abstract

We report observations of energetic electron flux and Phase Space Density (PSD) to show that a relatively weak magnetic storm with $Sym-H_{min} \approx -50 $nT, resulted in a relativistic and ultra--relativistic electron enhancement of two orders of magnitude similar to the St. Patrick's event of 2015, an extreme storm with $Sym-H_{min} \approx -235 nT$. This enhancement appeared at energies up to $\approx 10$ MeV, lasted for at least 24 days and was not recorded in geosynchronous orbit where most space weather alert data are collected. By combined analysis of PSD radial profiles and Fokker--Planck simulation, we show that the enhancement of relativistic and ultra--relativistic electrons is caused by different mechanisms: first, chorus waves during the intense substorm injections of April 21--25 accelerate the seed electron population to relativistic energies and redistribute them while inward diffusion driven by Pc5 ULF waves further accelerates them to ultra--relativistic energies.