Diffusion analysis of the GLE (ground-level enhancement) of September 29, 1989

On September 29, 1989, neutron monitors and muon telescopes around the world recorded a significant increase in their count rate. This ground-level enhancement affords a unique opportunity to study the rigidity dependence of the interplanetary scattering mean free path, lambda(parallel to). By fitting the well-known diffusion model to worldwide neutron 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 particles observed by each station (which ranges from 3-16 GV) clearly indicates that lambda(parallel to) is rigidity dependent. If we assume that lambda(parallel to) proportional to P-2-q for P > 3 GV, where the power-law index q is a measure of the deviation from isotropic scattering (q = 1), then we find that q = 0.86 +/- 0.03 at the time of this ground-level enhancement.