Abstract
We review numerical methods for simulations of cosmic ray (CR) propagation on galactic and larger scales. We present the development of algorithms designed for phenomenological and self-consistent models of CR propagation in kinetic description based on numerical solutions of the Fokker–Planck equation. The phenomenological models assume a stationary structure of the galactic interstellar medium and incorporate diffusion of particles in physical and momentum space together with advection, spallation, production of secondaries and various radiation mechanisms. The self-consistent propagation models of CRs include the dynamical coupling of the CR population to the thermal plasma. The CR transport equation is discretized and solved numerically together with the set of MHD equations in various approaches treating the CR population as a separate relativistic fluid within the two-fluid approach or as a spectrally resolved population of particles evolving in physical and momentum space. The relevant processes incorporated in self-consistent models include advection, diffusion and streaming propagation as well as adiabatic compression and several radiative loss mechanisms. We discuss, applications of the numerical models for the interpretation of CR data collected by various instruments. We present example models of astrophysical processes influencing galactic evolution such as galactic winds, the amplification of large-scale magnetic fields and instabilities of the interstellar medium.
Highlights
1.1 IntroductionCosmic rays (CRs) are charged particles with non-thermal energy distributions (Strong et al 2007; Grenier et al 2015; Gabici et al 2019)
If the coupling of CRs is reduced, due to wave damping mechanisms, the effective propagation speed of the CR population might be higher than the Alfven speed, it is argued that the heating rate is still given by the expression (8), because all momentum and energy transfer between the CRs and gas is mediated by hydromagnetic waves which propagate at the speed vA (e.g. Zweibel 2013)
We have reviewed the numerical treatment of CRs assuming a fluid description of this high-energy component
Summary
Cosmic rays (CRs) are charged particles with non-thermal energy distributions (Strong et al 2007; Grenier et al 2015; Gabici et al 2019). There are both hadronic and leptonic cosmic rays among which protons and electrons are the most abundant particles. Naab and Ostriker 2017; Zweibel 2017) In addition they might have an impact on the distribution of gas in the galactic disc and alter the star formation process on molecular cloud scales, even though their dynamical impact on molecular clouds is expected to be much weaker compared to other drivers in the ISM like radiation or supernovae. If CRs are efficiently coupled to the gas or penetrate into regions dense enough such that direct particle-particle collisions become relevent, they provide a temperature floor, which directly impacts the fragmentation scale and the seeds of star formation
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