The cosmic-ray ground-level enhancements of 29 September 1989 and 20 January 2005

TL;DR

This paper analyzes two significant ground-level enhancements of cosmic rays from 1989 and 2005, comparing their profiles to understand different acceleration mechanisms of solar energetic particles.

Contribution

It demonstrates that some GLEs are too impulsive to be caused by CME shocks, challenging the traditional gradual GLE model.

Findings

GLE 29 September 1989 was a gradual event

GLE 20 January 2005 was a prompt event

Impulsive GLEs may not be caused by CME shocks

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. This hypothesis, together with other properties of GLEs, is demonstrated graphically in this paper by plotting and comparing the time profiles of GLEs 42 of 29 September 1989 and GLE 69 of 20 January. These two events are respectively the largest examples of gradual and prompt events.