The first ground-level enhancement of solar cycle 25 on 28 October 2021

Abstract

Aims. The first relativistic solar proton event of solar cycle 25 was detected on 28 October 2021 by neutron monitors (NMs) on the ground and particle detectors on board spacecraft in near-Earth space. This is the first ground-level enhancement (GLE) of the current cycle. A detailed reconstruction of the NM response together with the identification of the solar eruption that generated these particles is investigated based on in situ and remote-sensing measurements. Methods. In situ proton observations from a few MeV to ∼500 MeV were combined with the detection of a solar flare in soft X-rays, a coronal mass ejection, radio bursts, and extreme ultraviolet (EUV) observations to identify the solar origin of the GLE. Timing analysis was performed, and a relation to the solar sources was outlined. Results. GLE73 reached a maximum particle rigidity of ∼2.4 GV and is associated with type III, type II, and type IV radio bursts and an EUV wave. A diversity of time profiles recorded by NMs was observed. This points to the event having an anisotropic nature. The peak flux at E > 10 MeV was only ∼30 pfu and remained at this level for several days. The release time of ≥1 GV particles was found to be ∼15:40 UT. GLE73 had a moderately hard rigidity spectrum at very high energies (γ ∼ 5.5). Comparison of GLE73 to previous GLEs with similar solar drivers is performed.

High-Resolution Spectral and Anisotropy Characteristics of Solar Protons During the GLE N∘73 on 28 October 2021 Derived with Neutron-Monitor Data Analysis

“…Here, we investigate the most recent GLE, namely, GLE N • 73 registered on 28 October 2021 (e.g. Papaioannou et al, 2022;Velinov, 2022), and present the rigidity spectra and angular distribution of SEPs, including their dynamical evolution throughout the event, employing NM data analysis.…”

The Updated Version of the A.Ne.Mo.S. GLE Alert System: The Case of the Ground-Level Enhancement GLE73 on 28 October 2021

“…Moreover, the active region AR12887 on 28 October at 15:34 UT (peak time) produced the X1.0 solar flare on the S26W02 part of the solar disk, as well as the resulting proton flux, are presented in Figure 1. This X1.0 solar flare was also detected by the GOES Solar Ultraviolet Imager (SUVI, an extreme ultraviolet telescope that detects photons that are not detectable from the Earth's surface) [2,6].…”

“…A ground-level enhancement (GLE) is defined as an increase in the flux of solar cosmic rays with sufficiently high energies (proton energies above 433 MeV) [1,2]. This sharp increase has a duration of several hours and can be recorded on Earth in the counting rate of ground-based cosmic ray detectors [3][4][5][6].…”

“…Phenomena of solar activity, such as solar flares and coronal mass ejections (CMEs) that occur on the Sun, are responsible for the acceleration processes that can accelerate particles up to a few giga-electron volts in kinetic energy. This is sufficient intensity to allow their secondary products to reach the terrestrial surface and be detected by NMs or other particle detectors on Earth [2,5,8].…”

“…The first GLEs were registered with ionization chambers in the 1940s, and neutron monitors were introduced only a few years later, covering solar cycles 17-25 [7,11]. Among all the recorded GLEs, the first event (GLE01) was observed on 28 February 1942 [12], while the last (GLE73) was recorded on 28 October 2021 [2]. During solar cycle 24 (December 2008 to December 2019), only two GLEs were recorded, GLE71 on 17 May 2012 [13,14] and GLE72 on 10 September 2017 [15]; therefore, it was characterized as a quiet solar cycle.…”