Major X-Class Solar Flare from Earth-Facing Active Region AR12887 on October 28, 2021 and First Cosmic Ray GLE 73 in Solar Cycle 25

Abstract

A phenomenological study of the new Ground Level Enhancement GLE 73 of cosmic rays (CRs) on October 28, 2021 is presented in this work. The investigated GLE 73 appears as one of the first major events of the Sun in the new solar cycle 25 (2019–2030). The recent GLE 73 occurred as a result of an X-class solar flare and was measured on the surface of both the Earth (by Neutron Monitors) and PAMELA satellite-borne instruments. This GLE occurs as a result of massive acceleration of charged particles in the solar corona and interplanetary space. Usually such event provides quite a soft spectrum of energetic particles, but sometimes the spectrum is sufficiently hard so that the initial solar protons can generate secondary nucleons that can be detected as an increase in the cosmic ray flux at ground level. Such exceptional event represents the studied GLE 73. These GLEs of cosmic rays are numbered consecutively from the first events that were detected on February 28, 1942 (GLE 1) and March 7,  1942 (GLE 2) J exactly 80 years ago. During the past 80 years of GLE studies some fundamental results of space physics have been achieved. Analyzing the list of all GLEs registered until now in the period (1942–2021) some characteristics that are important parameters can be determined. The quantification of GLEs in different solar cycles is presented by calculating the frequency of GLE occurrence in solar cycles 17–25.