GLE and sub-GLE redefinition in the light of high-altitude polar neutron monitors

• Main Authors: Poluianov, S. V. (S. V.), Usoskin, I. G. (I. G.), Mishev, A. L. (A. L.), Shea, M. A. (M. A.), Smart, D. F. (D. F.)
• Format: Article in Journal/Newspaper
• Language: English
• Published: Springer Nature 2017
• Subjects: Energetic particles; Amundsen-Scott; Antarctic; Concordia Station; The Antarctic; Antarc*
• Online Access: http://urn.fi/urn:nbn:fi-fe201801081130

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.