Ground Level Enhancement (GLE) events are a particular class of solar energetic particle (SEP) events in which the particle spectrum is hard enough to produce effects on the Earth’s surface. Historically a GLE had to be observed by two separate ground-based monitors to be added to the informal ‘GLE list’ (http://neutronm.bartol.udel.edu/~pyle/GLE_List.txt). As these events are among the most energetic of SEP events, they are a clear space weather concern and understanding the conditions under which they occur and the processes that create them is critical for improving our predictive capabilities. By their very definition, GLEs cannot be identified with only space-based instrumentation or at locations far from Earth, yet as we increasingly send instrumentation to distant locations, our need for predicting space weather throughout the heliosphere becomes more urgent.
Using the 13-100 MeV proton intensities observed during the 13 Dec 2006 GLE by the STEREO-B spacecraft (when it was still close to Earth) as a guide, we have identified several SEP events observed by STEREO of similar or larger intensity. None of these events were strong enough at Earth to register as a GLE, but had Earth been in the STEREO location, they might have been. We have calculated the event-integrated proton spectra for these events and found they fell into two categories: those with hard spectra and those with soft spectra above a few tens of MeV. Extrapolation of the hard spectra to higher energies indicates that these events had >500 MeV fluences that exceeded those of the 13 Dec 2006 and 17 May 2012 GLE events by factors of ~4 to 500, suggesting that they would have caused GLEs had the Earth been suitably positioned. The spectral indices of these events are consistent with those found in previous studies of GLE events. Simulations using the WSA-ENLIL+Cone model indicate that the observing spacecraft was magnetically connected to both the event coronal mass ejection (CME) and a preceding CME during the hard spectrum events. This may suggest a favorable configuration for the generation of extreme SEP events and/or GLEs.