The pulse shape of cosmic-ray ground-level enhancements

TL;DR

This study analyzes the pulse shapes of cosmic-ray ground-level enhancements to understand their origins, revealing a continuum from gradual to impulsive events and linking pulse shape to propagation conditions from Sun to Earth.

Contribution

It provides a comprehensive analysis of GLE pulse shapes, highlighting their variability and connection to solar and interplanetary conditions, challenging previous models of acceleration.

Findings

Range from gradual to impulsive GLEs identified

Fast risers linked to well-connected heliolongitudes

Pulse shape indicates propagation conditions between Sun and Earth

Abstract

Enhancements of the comic-ray intensity as observed by detectors on the ground have been observed 71 times since 1942. They are due to solar energetic particles accelerated in the regions of solar flares deep in the corona, or in the shock front of coronal mass ejections (CMEs) in the solar wind. The latter is the favoured model for the classical gradual ground-level enhancement (GLE). In several papers since the one of McCracken et al. (2008), we pointed out, however, that some GLEs are too impulsive to be accelerated in the CME shocks. With this hypothesis in mind we study the time profiles of all the available GLEs. The main results are that there is a continuous range from gradual to impulsive, that the fastest risers are concentrated at heliolongitudes that are magnetically well-connected to Earth, and that the shape of the pulse is a powerful indicator of propagation conditions between Sun and Earth. This ranges from relatively quiet to highly disturbed.