Small Size Ground Level Enhancements During Solar Cycle 24

Abstract

We continue the systematical empirical search for small size ground level enhancements (GLEs) (also called “hidden” or sub-GLEs) using data from ground-based instruments for Solar Cycle 24. The starting point of this research is the hypothesis that small size GLEs may be indicative of the acceleration of solar energetic particles (SEPs) by shocks driven by coronal mass ejections (CMEs). A crucial parameter for solving the problem seems to be the SEP energy spectrum at the Earth’s orbit measured by spacecraft detectors and ground-based neutron monitors (NMs). We try to recover the SEP spectrum in a wide range of energies – from GOES non-relativistic energy channels to the relativistic range from NM data, as well as from relevant measurements of some ground-based non-standard (mainly muon) cosmic ray detectors. The main factors that determine the SEP intensity and spectrum shape near the Earth are the source power, location, and/or shock strength. Every “suspected” small GLE is analyzed separately. Finally, we compile the list of statistically confirmed small GLEs and give our interpretation within the frame of the above hypothesis. The three considered models of shock wave acceleration are not suitable to physically and unambiguously explain some features of the observed solar cosmic ray (SCR) spectra. The results emphasize the importance of studying the GLEs of low intensity (hidden GLEs) for better understanding the SEP spectrum formation, especially in the range of relativistic energies. The GLE events from behind-the-limb sources are of special interest.