A statistical study on the peak and fluence spectra of Solar Energetic Particles observed over 4 solar cycles

TL;DR

This study analyzes over 100 solar energetic particle events across four solar cycles, revealing how their spectral properties relate to solar activity and propagation effects, advancing understanding of SEP acceleration and transport mechanisms.

Contribution

It provides a comprehensive statistical analysis of SEP spectra over four solar cycles, linking spectral features to solar activity and interplanetary conditions, and explores the origins of double-power law spectra.

Findings

SEP spectral properties are influenced by solar source and interplanetary conditions.

The double power-law spectral characteristic is linked to adiabatic cooling and diffusion processes.

SEP spectra correlate with solar flare X-ray peak intensity.

Abstract

Solar energetic particles (SEPs) are an important space radiation source, especially for the space weather environment in the inner heliosphere. The energy spectrum of SEP events is crucial both for evaluating their radiation effects and for understanding their acceleration process at the source region and their propagation mechanism. In this work, we investigate the properties of the SEP peak flux spectra and the fluence spectra and their potential formation mechanisms using statistical methods. We aim to advance our understanding of both SEPs' acceleration and propagation mechanisms. Employing the dataset of European Space Agency's Solar Energetic Particle Environment Modelling (SEPEM) program, we have obtained and fitted the peak-flux and fluence proton spectra of more than a hundred SEP events from 1974 to 2018. We analyzed the relationship among the solar activity, X-ray peak intensity of solar flares and the SEP spectral parameters. Based on the assumption that the initial spectrum of accelerated SEPs generally has a power-law distribution and also the diffusion coefficient has a power-law dependence on particle energy, we can assess both the source and propagation properties using the observed SEP event peak flux and fluence energy spectra. We confirm that SEPs' spectral properties are influenced by the solar source and the interplanetary conditions and their transportation process can be influenced by different phases of solar cycle. This study provides an observational perspective on the double power-law spectral characteristics of the SEP energy spectra, showing their correlation with the adiabatic cooling and diffusion processes during the particle propagation from the Sun to the observer. This contributes to a deeper understanding of the acceleration and propagation of SEP events, in particular the possible origins of the double-power law.