On some features of the solar proton event on 2021 October 28 (GLE73)

TL;DR

This study analyzes the 2021 October 28 solar proton event, revealing unique features such as low proton fluxes and a hard energy spectrum, and links these to flare magnetic reconnection processes.

Contribution

It identifies rare spectral characteristics of the SEP-GLE73 event and correlates them with flare radio burst spectra, highlighting the role of magnetic reconnection in proton acceleration.

Findings

SEP-GLE73 had very small proton fluxes and a hard energy spectrum.

Only two other GLEs shared similar spectral features.

Hard microwave radio burst spectra correlate with hard SEP spectra.

Abstract

In addition to several recent articles devoted to the rare event of ground-level enhancement of the solar relativistic proton flux observed on 2021 October 28 (GLE73), we study the 10-100 MeV solar energetic particle (SEP) component of this event. Based on the GOES satellite data for 26 GLEs recorded since 1986, we have formed a scatter plot displaying the ratio of the peak fluxes of the >10 MeV (J10) and >100 MeV (J100) protons and their energy spectra. Two extreme characteristics of the prompt component of the SEP-GLE73 event were revealed: (1) very small J10 and J100 proton fluxes and (2) a very hard energetic spectrum in the 10-100 MeV range. There are only two events with these characteristics similar to SEP-GLE73 namely, GLE40 (1989 July 25) and GLE46 (1989 November 15). A correspondence was demonstrated between the hard frequency spectrum of microwave radio bursts of initiating flares and the hard SEP energy spectrum of these two and other GLEs. These results suggest that the flare magnetic reconnection both in the impulsive and post-eruption phases plays an important role in the acceleration of the SEP-GLE protons.