GLE and Sub-GLE Redefinition in the Light of High-Altitude Polar Neutron Monitors
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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.
The work was supported by the projects of the Academy of Finland Centre of Excellence ReSoLVE (No. 272157), CRIPA and CRIPA-X (No. 304435), and by the Finnish Antarctic Research Program (FINNARP). We acknowledge Askar Ibragimov for the support of the International GLE database ( gle.oulu.fi ) and are grateful to the worldwide neutron-monitor database ( nmdb.eu ), which is a product of an EU Project. We thank Marc Duldig, Erwin Flückiger, John Humble, and Roger Pyle for valuable discussions.
Disclosure of Potential Conflicts of Interest
The authors declare that they have no conflicts of interest.
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