A diffusive model for the initial phase of a solar proton event

The presence of an interplanetary magnetic field in the vicinity of the earth is now fairly well established, and several authors have proposed that the long duration of solar proton events is due to the storage of protons in the tubes of force of this field, which we shall assume to be of solar origin. According to one model, protons emitted by a flare diffuse through the solar atmosphere, gradually leaking off into the interplanetary field where they are stored for comparatively long periods. The early anisotropic phase of a solar proton event presumably represents the filling of the earth's local tube of force by this mechanism. An attempt is made to treat the diffusion through the solar atmosphere in a quantitative fashion, and the results are applied to measurements carried out by Explorer 12 on September 28, 1961. These data appear to fit the theory and permit a determination of the diffusion constant. Application of classical diffusion theory, though admittedly of dubious validity, yields a mean free path of about 600 km for protons of a few hundred Mev energy.