DETECTION OF 0.5-15-GEV SOLAR PROTONS ON 29 SEPTEMBER 1989 AT AUSTRALIAN STATIONS

A major cosmic ray ground-level enhancement, the largest in 33 years, occurred on 29 September 1989 during which intensity enhancements at Australian observatories ranged up to a maximum of 344% at Mt. Wellington. The Darwin neutron monitor (cutoff rigidity 14.1 GV) recorded an approximately 13% increase in the five-minute counting rate indicating that solar particles up to at least 15 GeV must have been present. Surface muon detectors at Hobart and Mawson recorded increased fluxes, but the event was not recorded by underground muon detectors at either station. Preliminary analysis of the solar particle flux during the initial phase of the event shows a hard spectrum approximated equally well by an exponential spectra with a P(o) of 2.0 GV or by a modified power law spectra of exponent approximately -2.9. Particles arriving at the detectors from non-vertical directions make a significant contribution to the total increase recorded at mid and low latitude stations.