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

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Acknowledgements

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.

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Poluianov, S.V., Usoskin, I.G., Mishev, A.L. et al. GLE and Sub-GLE Redefinition in the Light of High-Altitude Polar Neutron Monitors. Sol Phys 292, 176 (2017). https://doi.org/10.1007/s11207-017-1202-4

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Keywords

  • Energetic particles