Proton activity of the Sun in current solar cycle 24

TL;DR

This study analyzes seven major solar proton events in solar cycle 24, examining their profiles, timing, and spectra to understand proton acceleration mechanisms and their relation to solar activity.

Contribution

It provides detailed analysis of proton flux profiles, timing of solar particle release, and spectral characteristics, supporting CME-driven shock acceleration as a key process.

Findings

Proton flux profiles depend on source locations.

SPR times occur during flare decay and CME travel.

The 2012 GLE event had the hardest spectrum.

Abstract

We present a study of 7 large solar proton events (SPEs) of current solar cycle 24 (from 2009 January up to date). They were recorded by GOES spacecraft with highest proton fluxes over 200 pfu for energies $>$10 MeV. In situ particle measurements show that: (1) The profiles of the proton fluxes are highly dependent of the locations of their solar sources, namely flares or coronal mass ejections (CMEs); (2) The solar particle release (SPR) times fall in the decay phase of the flare emission, and are in accordance with the times when the CMEs travel to an average height of 7.9 solar radii; (3) The time differences between the SPR and the flare peak are also dependent of the locations of the solar active regions (ARs). The results tend to support the concept of proton acceleration by the CME-driven shock, even though there exists a possibility of particle acceleration at flare site with subsequent perpendicular diffusion of accelerated particles in the interplanetary magnetic field (IMF). We derive the integral time-of-maximum (TOM) spectra of solar protons in two forms: a single power-law distribution and a power law broken with an exponential tail. It is found that the unique Ground Level Enhancement (GLE) event on 2012 May 17 displays a hardest spectrum and a largest broken energy that may explain why the this event could extend to relativistic energy.