The September 29, 1989 ground‐level event observed at high rigidity

Abstract

Ground‐level solar cosmic ray events are usually observed by high latitude neutron monitors at relatively low rigidities (> 1 GV) and are not usually observed by underground cosmic ray detectors because of their much higher threshold rigidity. However, the September 29, 1989 ground‐level event was sufficiently large and unusual that it was recorded by the Embudo, New Mexico underground muon telescope which has a threshold rigidity of 19 GV. The observed increase was 2.2 ± 0.2% in the hour interval from 1100 to 1200 UT. There was no statistically significant increase in the data from the Socorro underground muon telescope which has a threshold rigidity of 45 GV. This is the first detection of a ground‐level solar cosmic ray event by any underground cosmic ray muon telescope where the event has been clearly distinguishable above the background cosmic ray intensity.