THE INFLUENCE OF THE LARGE-SCALE INTERPLANETARY SHOCK STRUCTURE ON A LOW-ENERGY PARTICLE EVENT

We have developed a numerical model to study the influence of the large-scale shock topology on the associated low-energy (less than 2 MeV) particle event upstream of the shock. It includes particle injection at the solar corona, two-dimensional MHD simulation of the propagation of the shock, modeling of particle propagation through the interplanetary medium, and particle injection at the shock. The model does not attempt to simulate the physical processes of shock particle acceleration, therefore the injection at the shock is represented by a numerical source function in the equation that describes particle propagation. The MHD simulation provides the time when the shock intersects the interplanetary magnetic field line which connects with the spacecraft, the evolution of this connection point over the shock front, and its plasma characteristics as it propagates out into the heliosphere. Considering these results, and by fitting the observed particle flux and anisotropy profiles, the model allows us to determine the parameters which characterize particle propagation and the rate of particle injection at the shock front. Therefore, it is possible to infer the history of the development of the particle event and to analyze the possible relations with the parameters at the point of the shock that is, at each moment, magnetically connected with the spacecraft. The model can be applied to those low-energy particle events that are associated with strong interplanetary shocks and that are characterized by a steady upstream magnetic field and solar wind velocity. Particularly, we have studied the particle event associated with the interplanetary shock observed on 1979 April 24 by ISEE-3 in the energy range 35-1600 keV. The magnetic connection between the shock and the spacecraft is established about 18 hr after the filament disappearance which generated the event. We then infer from the model that the shock is quasi-perpendicular at the intersection point and becomes quasi-parallel when it approaches the spacecraft. The fit to the observations requires that the injection rate at the shock increases with time. It is also necessary to assume the existence of a region of enhanced scattering upstream of the shock, 0.15 AU wide (the minimum allowed by the numerical model). The mean free path derived in that region is one order of magnitude smaller than farther upstream.