부산대학교 도서관

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 {plus minus} 0.2% in the hour interval from 1,100 to 1,200 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.