Characteristics of solar proton events associated with ground level enhancements

Abstract

In certain explosive events, the Sun emits large numbers of protons with energy up to tens of GeV. Particle acceleration processes on the Sun can be understood through the observation of such energetic particles. According to the definition of NOAA Space Environment Services Center, a solar proton event (SPE) is defined as an event with a peak intensity of >10 pfu (particle flux unit; 1 particle cm(-2) sr(-1) s(-1)) for >10 MeV protons. Major SPEs are not always associated with ground level enhancements (GLEs), whereas relatively minor SPEs are sometimes associated with GLEs. We examined the peak intensities of 85 SPEs after 1986 using the intensity of proton differential energy channels (P3-P10) from GOES. We identified 31 SPEs associated with GLEs having well-defined profiles with a large increase and clear peak for each proton channel. They have larger peak intensity and fluence and shorter delay time between onset and peak than SPEs without GLEs. Fluences and peak intensities of SPEs have a good correlation with percent increases of GLEs, with the best correlation coefficients obtained for the peak intensities and fluences of channels P8, P9, and P10. For these energy channels (spanning 350-700 MeV), we find that there are threshold values for GOES fluence and peak intensity such that most SPEs above the threshold are associated with GLEs, whereas almost none below the thresholds are.