Magnetic Reconnection Modifies Cosmic-Ray Transport in the Interstellar Medium
We present numerical magnetohydrodynamic (MHD) simulations of the interstellar medium in gravity-stratified boxes with the Piernik MHD code. The system contains thermal gas, spectrally resolved cosmic-ray (CR) nuclei within the CRESP algorithm, and spectrally unresolved CR protons. CR protons drive gas outflows and undergo Coulomb and hadronic losses. CR nuclei undergo Coulomb losses. We also include the spallation of primary Carbon by thermal gas atoms, thereby decaying Car- bon into secondary stable Boron isotopes. We use the resistive-MHD approximation as a subgrid model for unresolved turbulent magnetic dissipation in current sheets. We perform a parameter study of the magnetic diffusivity η. The Boron-to-Carbon ratio B/C and hadronic γ-ray luminosity density are computed as diagnostics. We find that non-zero magnetic diffusivity modifies the magnetic topology, leading to a more horizontal field in the halo, thereby accumulating CRs in the disk region and signifi- cantly increasing the CR escape time. As a consequence, the local turbulent conditions and turbulent reconnection in the interstellar medium should be considered when interpreting B/C and hadronic γ-ray luminosity observed at Earth. Our results suggest that the escape time and diffusion coefficient of CRs deduced from observational data in the vicinity of the Solar System cannot be extrapolated to the whole Galaxy.