Monte Carlo Model of the Highly Anisotropic Solar Proton Event of 20 April 1971
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 pitchangle 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 fieldaligned well into the decay phase of the event. A Monte Carlo technique, which gives the pitchangle 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 anefolding time of 7 hr. In this model the parallel propagation is determined by smallangle scattering in a diverging field equivalent to a uniform diffusion coefficient of 2.1 × 10^{22} cm^{2} 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.
 Publication:

Solar Physics
 Pub Date:
 February 1975
 DOI:
 10.1007/BF00162391
 Bibcode:
 1975SoPh...40..449P
 Keywords:

 Angular Distribution;
 Anisotropic Media;
 Interplanetary Medium;
 Monte Carlo Method;
 Proton Scattering;
 Solar Protons;
 Diffusion Coefficient;
 Mathematical Models;
 Pitch (Inclination);
 Radiation Distribution;
 Solar Cosmic Rays;
 Solar Physics