GLE and Sub-GLE Redefinition in the Light of High-Altitude Polar Neutron Monitors

TL;DR

This paper proposes a redefinition of ground-level enhancement (GLE) events to include high-altitude polar neutron monitors, distinguishing between GLEs and sub-GLEs to improve classification consistency.

Contribution

It introduces a modified GLE definition incorporating high-altitude polar neutron monitor data, addressing classification issues caused by their unique detection capabilities.

Findings

High-altitude polar neutron monitors can detect weaker SEP events.

The new definition maintains homogeneity in GLE classification.

Sub-GLEs are identified as events detected only by high-altitude monitors.

Abstract

The conventional definition of ground-level enhancement (GLE) events requires a detection of solar energetic particles (SEP) by at least two differently located neutron monitors. Some places are exceptionally well suitable for ground-based detection of SEP - high-elevation polar regions with negligible geomagnetic and reduced atmospheric energy/rigidity cutoffs. At present, there are two neutron-monitor stations in such locations on the Antarctic plateau: SOPO/SOPB (at Amundsen-Scott station, 2835 m elevation), and DOMC/DOMB (at Concordia station, 3233 m elevation). Since 2015, when the DOMC/DOMB station started continuous operation, a relatively weak SEP event that was not detected by sea-level neutron-monitor stations was registered by both SOPO/SOPB and DOMC/DOMB, and it was accordingly classified as a GLE. This would lead to a distortion of the homogeneity of the historic GLE list and the corresponding statistics. To address this issue, we propose to modify the GLE definition so that it maintains the homogeneity: A GLE event is registered when there are near-time coincident and statistically significant enhancements of the count rates of at least two differently located neutron monitors, including at least one neutron monitor near sea level and a corresponding enhancement in the proton flux measured by a space-borne instrument(s). Relatively weak SEP events registered only by high-altitude polar neutron monitors, but with no response from cosmic-ray stations at sea level, can be classified as sub-GLEs.