An extended analysis of the September 1989 cosmic ray ground level enhancement

Abstract

The September 29, 1989, cosmic ray ground level enhancement had a complex intensity‐time profile with two distinct peaks observed at some neutron monitors. The event was detected by surface and some underground muon telescopes indicating the presence of particles up to ∼30 GV. We have modeled the response of neutron monitors and surface muon telescopes at the times of both intensity peaks and during the decay phase. We have determined particle rigidity spectra and pitch angle distributions centered on apparent arrival directions. Some evidence of bidirectional flow is present during the second peak. This may be due to backscattering of particles from beyond Earth. There was a high degree of scattering from all directions resulting in a significant isotropic component. Spectra derived from ground level responses are consistent with satellite observations. The spectrum between a few MeV and a few tens of GeV is consistent with shock acceleration.