The interplanetary scattering mean free path from 1 to 3 × 10³ MV

We have made a statistical study of published intensity‐time profiles of proton and electron solar particle events from 1967 to 1974 in an attempt to examine systematically the temporal and rigidity dependence of the interplanetary scattering mean free path from ∼1 to 3 × 10³ MV. The observed tmax, the time from the release at the sun to the time of maximum flux at the spacecraft, from solar particle events that complied with a stringent selection criterion were interpreted in terms of a propagation model, with Kr = K0, to obtain the average radial scattering mean free path λr. We find that (1) λr appears to be species independent when it is derived from proton and electron solar particle events, (2) λr varies less than a factor of 2 from solar maximum to solar minimum, and (3) λr is nearly rigidity independent below ∼500 MV. The values of λr that we obtain at low rigidities are inconsistent with all current theoretical models that derive λr from the interplanetary magnetic field fluctuations.