Adiabatic Deceleration of Solar Energetic Particles as Deduced from Monte Carlo Simulations of Interplanetary Transport |
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Authors: | Kocharov L Vainio R Kovaltsov G A Torsti J |
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Institution: | (1) Space Research Laboratory, Turku University, FIN-20014, Finland;(2) Ioffe Physical-Technical Institute, St. Petersburg, 194021, Russia |
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Abstract: | Monte Carlo simulations of interplanetary transport are employed to study adiabatic energy losses of solar protons during
propagation in the interplanetary medium. We consider four models. The first model is based on the diffusion-convection equation.
Three other models employ the focused transport approach. In the focused transport models, we simulate elastic scattering
in the local solar wind frame and magnetic focusing. We adopt three methods to treat scattering. In two models, we simulate
a pitch-angle diffusion as successive isotropic or anisotropic small-angle scatterings. The third model treats large-angle
scatterings as numerous small-chance isotropizations. The deduced intensity–time profiles are compared with each other, with
Monte Carlo solutions to the diffusion-convection equation, and with results of the finite-difference scheme by Ruffolo (1995).
A numerical agreement of our Monte Carlo simulations with results of the finite-difference scheme is good. For the period
shortly after the maximum intensity time, including deceleration can increase the decay rate of the near-Earth intensity essentially
more than would be expected based on advection from higher momenta. We, however, find that the excess in the exponential-decay
rate is time dependent. Being averaged over a reasonably long period, the decay rate of the near-Earth intensity turns out
to be close to that expected based on diffusion, convection, and advection from higher momenta. We highlight a variance of
the near-Earth energy which is not small in comparison with the energy lost. It leads to blurring of any fine details in the
accelerated particle spectra. We study the impact of realistic spatial dependencies of the mean free path on adiabatic deceleration
and on the near-Earth intensity magnitude. We find that this impact 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 intensities observed at 1 AU. Computations invoked during the study are described in detail. |
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