Cosmic ray transport and production in the Galaxy: A stochastic propagation simulation approach

Galactic cosmic-ray composition reflects the effects of nuclear interactions during propagation through the interstellar medium. In order to use measurements to determine the source of cosmic rays, one needs a model to deconvolute the propagation effect. This paper presents a new numerical method to solve cosmic-ray diffusive transport equations with a complete network of nuclear interactions. The new method uses the time backward Markov stochastic processes to solve the transport equation by tracing the particles' trajectories starting from the solar system back to their sources in the Galaxy. To make the calculation efficient for large arrays of equations for many cosmic-ray species, a matrix representing the composition of all cosmic-ray heavy nuclei and location is used. The results for abundance ratios of key elements such as B/C and sub-Fe/Fe compared with observations from HEAO-3, ACE, and Ulysses and for isotopic ratios such as Cl-36/Cl-37 and Mn-54/Mn-55 are shown to be consistent with other numerical approaches. The ability to track particles through the Galaxy, which is inherent in this technique, offers new opportunities for investigations. For example, one can look at the spatial and propagation time distributions and element composition of source particles that arrive in the solar system.