DIFFUSION ANALYSIS OF THE GLE (GROUND-LEVEL ENHANCEMENT) OF SEPTEMBER29, 1989

On September 29, 1989, neutron monitors and muon telescopes around the world recorded a significant increase in their count rate. This groun d-level enhancement affords a unique opportunity to study the rigidity dependence of the interplanetary scattering mean free path, lambda(pa rallel to). By fitting the well-known diffusion model to worldwide neu tron monitor and muon telescope data, we calculate the value of lambda (parallel to) observed by each station. A plot of lambda(parallel to) observed by each station versus the mean rigidity of solar flare parti cles observed by each station (which ranges from 3-16 GV) clearly indi cates that lambda(parallel to) is rigidity dependent. If we assume tha t lambda(parallel to) proportional to P-2-q for P > 3 GV, where the po wer-law index q is a measure of the deviation from isotropic scatterin g (q = 1), then we find that q = 0.86 +/- 0.03 at the time of this gro und-level enhancement.