Abstract

The worldwide network of neutron monitors (NMs) includes two stations, South Pole (SOPO/B) and Dome C (DOMC/B), which are exceptionally sensitive to solar energetic particles. Their locations on the high Antarctic plateau make them favorable for detection of low-energy particles because of the low geomagnetic rigidity cutoff and the thin atmosphere above. This pair of cosmic ray stations is able to register relatively weak solar energetic particle events, which would not have been detected by the NM network otherwise. Since DOMC/B station is in operation only since 2015, now the NM network is more sensitive to a SEP event than ever before. Considering such weak events as the "official" ground-level enhancements (GLEs) may break the homogeneity of the GLE definition and cause an observational bias in studies based on the GLE occurrence rate over decades.

In order to keep the "official" GLE list homogeneous, we propose to slightly modify the conventional definition of a GLE, which refers to a statistically significant enhancement of the count rate of at least two differently located neutron monitors over the background. The proposed change is as follows: "... at least two neutron monitors, one of which is located near the sea level". We also propose an introduction of a new class of SEP events called sub-GLE to identify the events registered by high-elevation polar NMs but not satisfying the revised definition of a GLE.