The pulse shape of cosmic-ray ground-level enhancements

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.