Multiple injections of energetic electrons associated with the flare/CME event on 9 October 2021

TL;DR

This study analyzes a complex solar energetic particle event on 9 October 2021, revealing multiple electron acceleration episodes linked to flare emissions and shock wave activity, with detailed timing and anisotropy insights.

Contribution

It identifies multiple sources and phases of energetic electron acceleration during a solar event, combining multi-instrument observations for comprehensive analysis.

Findings

Electrons were accelerated over at least 20 minutes during five HXR pulses.

Shock wave contributed to electron acceleration during and after the flare.

High-energy electrons were released with a delay compared to low-energy electrons.

Abstract

We study the solar energetic particle (SEP) event observed on 9 October 2021, by multiple spacecraft including Solar Orbiter (SolO). The event was associated with an M1.6 flare, a coronal mass ejection (CME) and a shock wave. During the event, high-energy protons and electrons were recorded by multiple instruments located within a narrow longitudinal cone. An interesting aspect of the event was the multi-stage particle energization during the flare impulsive phase and also what appears to be a separate phase of electron acceleration detected at SolO after the flare maximum. We aim to investigate and identify the multiple sources of energetic electron acceleration. We utilize SEP electron observations from the Energetic Particle Detector (EPD) and hard X-ray (HXR) observations from the Spectrometer/Telescope for Imaging X-rays (STIX) on-board SolO, in combination with radio observations at a broad frequency range. We focus on establishing an association between the energetic electrons and the different HXR and radio emissions associated with the multiple acceleration episodes. We have found that the flare was able to accelerate electrons for at least 20 minutes during the nonthermal phase observed in the form of five discrete HXR pulses. We also show evidence that the shock wave has contributed to the electron acceleration during and after the impulsive flare phase. The detailed analysis of EPD electron data shows that there was a time difference in the release of low- and high-energy electrons, with the high-energy release delayed. Also, the observed electron anisotropy characteristics suggest different connectivity during the two phases of acceleration.