Interplanetary diffusion model for the time behavior of intensity in a solar cosmic ray event

Abstract

The time history of the intensity of several solar cosmic ray events can be interpreted in terms of a unified interplanetary diffusion model which assumes that the diffusion coefficient D = Mrβ, where r is the heliocentric radial distance and M and β are parameters, which may be dependent on particle energy E. The model is particularly successful in accounting for the decay phases. For the September 28, 1961, event M varies as E0.33, and, assuming spherical geometry, β is equal to 1 for 55<E<500 Mev and decreases rapidly at lower energies. At r = 1 AU the mean free path λ is about 0.081 AU for E ∼200 Mev and 0.15 for E = 23 Mev. The energy dependences of β and λ suggest 0.006 AU as the order of magnitude of the scale size of irregularities in the interplanetary magnetic field. The source spectrum is inferred. The possibility of nonspherical geometry of diffusion is considered, as is the implication of the results with respect to the solar modulation of galactic cosmic radiation.