Cosmic ray propagation in interstellar space.

摘要

The astrophysics of cosmic rays is a very broad field of research, concerned with questions like where and how cosmic rays are accelerated, how they propagate and interact with the interstellar medium. In recent years a huge number of studies have been devoted to cosmic rays and to questions related to them as new and accurate observational data become available. Many models have been developed and have succeeded in reproducing observational data of many kinds related to cosmic rays using realistic astrophysical parameters; however many problems can't be addressed using these models such as inhomogeneities and small scale structure in the interstellar space. With this incentive we developed a new numerical model to describe the cosmic rays interactions and propagation in the galaxy and to handle many limitations of previously developed models. Our model depends on using the time backward Markov Stochastic solution of the general diffusion transport equation developed by Zhang [1999]; however we extend the solution to solve a group of diffusion transport equations, each representing a particular element or isotope in the cosmic ray nuclei. The new technique fits the observational data for several elemental and isotopic abundance ratios over a wide energy range. The new method is not only helpful in addressing the effects of the low and high density regions in the interstellar space on the elemental and isotopic ratio but also on the determination of the contribution of individual nuclei and galactic locations to the production of certain cosmic ray nuclei. The Markov Stochastic method was also used to calculate cosmic rays propagation within a halo diffusion model. The energy dependent path-length probability distribution (PLD) is combined with a weighted slab model to determine the production of secondary cosmic ray nuclei. This combination allows the seamless incorporation of the low density Local Bubble surrounding the solar system. While the effect of the Local Bubble on the primary to secondary abundance ratios of stable nuclei is minimal, the effect on unstable short lived isotopes is significant. This study also pointed out that many modulation models describe the modulation level using a single number called the modulation parameter; however using the same modulation parameter with different selected models doesn't usually result in the same level modulation. This means that the modulation parameter is a model.