Abstract
The anisotropy of the particle distribution and its variation with time at 1 AU early in a solar cosmic ray event can provide information on the pitch-angle scattering of the particles in the interplanetary medium. The proton event of 20 April 1971 is described in which the anisotropy of the 7.6–55 MeV energy channel remained large (≳ 100%) and field-aligned well into the decay phase of the event. A Monte Carlo technique, which gives the pitch-angle distribution, is employed to investigate two models put forward to explain this sustained anisotropy. It is shown that the observed event is consistent with one model in which the injection of particles at the Sun decayed with ane-folding time of 7 hr. In this model the parallel propagation is determined by small-angle scattering in a diverging field equivalent to a uniform diffusion coefficient of 2.1 × 1022 cm2 s−1 (the corresponding classical mean free path is 0.90 AU). A model with impulsive injection and in whichκ(r) increases strongly with distance from the Sun cannot satisfactorily explain the observations.
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Work performed in part at University of California, LASL, Los Alamos, New Mexico.
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Palmer, I.D., Palmeira, R.A.R. & Allum, F.R. Monte Carlo model of the highly anisotropic solar proton event of 20 April 1971. Sol Phys 40, 449–460 (1975). https://doi.org/10.1007/BF00162391
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DOI: https://doi.org/10.1007/BF00162391