  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.

Space Physics;Solar and Stellar Astrophysics