TY  - JOUR
A1  - Thomas, Timon
A1  - Pfrommer, Christoph
A1  - Pakmor, Rüdiger
T1  - A finite volume method for two-moment cosmic ray hydrodynamics on a moving mesh
JF  - Monthly notices of the Royal Astronomical Society
N2  - We present a new numerical algorithm to solve the recently derived equations of two-moment cosmic ray hydrodynamics (CRHD). The algorithm is implemented as a module in the moving mesh AREPO code. Therein, the anisotropic transport of cosmic rays (CRs) along magnetic field lines is discretized using a path-conservative finite volume method on the unstructured time-dependent Voronoi mesh of AREPO. The interaction of CRs and gyroresonant Alfven waves is described by short time-scale source terms in the CRHD equations. We employ a custom-made semi-implicit adaptive time stepping source term integrator to accurately integrate this interaction on the small light-crossing time of the anisotropic transport step. Both the transport and the source term integration step are separated from the evolution of the magnetohydrodynamical equations using an operator split approach. The new algorithm is tested with a variety of test problems, including shock tubes, a perpendicular magnetized discontinuity, the hydrodynamic response to a CR overpressure, CR acceleration of a warm cloud, and a CR blast wave, which demonstrate that the coupling between CR and magnetohydrodynamics is robust and accurate. We demonstrate the numerical convergence of the presented scheme using new linear and non-linear analytic solutions.
KW  - hydrodynamics
KW  - MHD
KW  - methods: numerical
KW  - cosmic rays
Y1  - 2021
U6  - https://doi.org/10.1093/mnras/stab397
SN  - 0035-8711
SN  - 1365-2966
VL  - 503
IS  - 2
SP  - 2242
EP  - 2264
PB  - Oxford University Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Thomas, T.
A1  - Pfrommer, Christoph
T1  - Cosmic-ray hydrodynamics
BT  - alfvén-wave regulated transport of cosmic rays
JF  - Monthly notices of the Royal Astronomical Society
N2  - Star formation in galaxies appears to be self-regulated by energetic feedback processes. Among the most promising agents of feedback are cosmic rays (CRs), the relativistic ion population of interstellar and intergalactic plasmas. In these environments, energetic CRs are virtually collisionless and interact via collective phenomena mediated by kinetic-scale plasma waves and large-scale magnetic fields. The enormous separation of kinetic and global astrophysical scales requires a hydrodynamic description. Here, we develop a new macroscopic theory for CR transport in the self-confinement picture, which includes CR diffusion and streaming. The interaction between CRs and electromagnetic fields of Alfvenic turbulence provides the main source of CR scattering, and causes CRs to stream along the magnetic field with the Alfven velocity if resonant waves are sufficiently energetic. However, numerical simulations struggle to capture this effect with current transport formalisms and adopt regularization schemes to ensure numerical stability. We extent the theory by deriving an equation for the CRmomentum density along the mean magnetic field and include a transport equation for the Alfven-wave energy. We account for energy exchange of CRs and Alfven waves via the gyroresonant instability and include other wave damping mechanisms. Using numerical simulations, we demonstrate that our new theory enables stable, self-regulated CR transport. The theory is coupled to magnetohydrodynamics, conserves the total energy and momentum, and correctly recovers previous macroscopic CR transport formalisms in the steady-state flux limit. Because it is free of tunable parameters, it holds the promise to provide predictable simulations of CR feedback in galaxy formation.
KW  - hydrodynamics
KW  - radiative transfer
KW  - methods: analytical
KW  - methods: numerical
KW  - cosmic rays
Y1  - 2019
U6  - https://doi.org/10.1093/mnras/stz263
SN  - 0035-8711
SN  - 1365-2966
VL  - 485
IS  - 3
SP  - 2977
EP  - 3008
PB  - Oxford Univ. Press
CY  - Oxford
ER  -