Journal
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
Volume 491, Issue 1, Pages 993-1007Publisher
OXFORD UNIV PRESS
DOI: 10.1093/mnras/stz2961
Keywords
astroparticle physics; MHD; plasmas; methods: numerical; cosmic rays
Categories
Funding
- DFG Priority Program 1573 Physics of the Interstellar Medium
- European Research Council under ERC-CoG grant [CRAGSMAN-646955]
- (Polish) National Science Centre [2015/19/ST9/02959]
- DFG cluster of excellence ORIGINS
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Cosmic ray (CR) protons are an important component in many astrophysical systems. Processes like CR injection, cooling, adiabatic changes as well as active CR transport through the medium strongly modify the CR momentum distribution and have to be taken into account in hydrodynamical simulations. We present an efficient novel numerical scheme to accurately compute the evolution of the particle distribution function by solving the Fokker-Planck equation with a low number of spectral bins (10-20), which is required to include a full spectrum for every computational fluid element. The distribution function is represented by piecewise power laws and is not forced to be continuous, which enables an optimal representation of the spectrum. The Fokker-Planck equation is solved with a two-moment approach evolving the CR number and energy density. The low numerical diffusion of the scheme reduces the numerical errors by orders of magnitude in comparison to classical schemes with piecewise constant spectral representations. With this method not only the spectral evolution of CRs can be computed accurately in magnetohydrodynamic simulations but also their dynamical impact as well as CR ionization. This allows for more accurate models for astrophysical plasmas, like the interstellar medium, and direct comparisons with observations.
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