ADIABATIC DECELERATION OF SOLAR ENERGETIC PARTICLES AS DEDUCED FROM MONTE-CARLO SIMULATIONS OF INTERPLANETARY TRANSPORT

Monte Carlo simulations of interplanetary transport are employed to st udy adiabatic energy losses of solar protons during propagation in the interplanetary medium. We consider four models. The first model is ba sed on the diffusion-convection equation. Three other models employ th e focused transport approach. In the focused transport models, we simu late elastic scattering in the local solar wind frame and magnetic foc using. We adopt three methods to treat scattering. In two models, we s imulate a pitch-angle diffusion as successive isotropic or anisotropic small-angle scatterings. The third model treats large-angle scatterin gs as numerous small-chance isotropizations. The deduced intensity-tim e profiles are compared with each other, with Monte Carlo solutions to the diffusion-convection equation, and with results of the finite-dif ference scheme by Ruffolo (195). A numerical agreement of our Monte Ca rlo simulations with results of the finite-difference scheme is good. For the period shortly after the maximum intensity time, including dec eleration can increase the decay rate of the near-Earth intensity esse ntially more than would be expected based on advection from higher mom enta. We, however, find that the excess in the exponential-decay rate is time dependent. Being averaged over a reasonably long period, the d ecay rate of the near-Earth intensity turns out to be close to that ex pected based on diffusion, convection, and advection from higher momen ta. We highlight a variance of the near-Earth energy which is not smal l in comparison with the energy lost. It leads to blurring of any fine details in the accelerated particle spectra. We study the impact of r ealistic spatial dependencies of the mean free path on adiabatic decel eration and on the near-Earth intensity magnitude. We find that this i mpact is essential whenever adiabatic deceleration itself is important . It is also found that the initial angular distribution of particles near the Sun can markedly affect MeV-proton energy losses and intensit ies observed at 1 AU. Computations invoked during the study are descri bed in detail.