Magnetohydrodynamic turbulence and the associated spatial diffusion tensor of cosmic rays in dynamical galactic halos

TL;DR

This paper develops a numerical framework to compute the evolution of magnetohydrodynamic turbulence in galactic halos and derives the cosmic-ray diffusion tensor from first principles, revealing spatial variations in turbulence and diffusion properties.

Contribution

It introduces a novel numerical approach to model MHD turbulence evolution in galactic halos and calculates the cosmic-ray diffusion tensor ab initio, incorporating realistic galactic parameters.

Findings

Turbulence quantities vary significantly within the halo.

The cosmic-ray diffusion tensor shows substantial spatial variation.

The model aligns with observed properties of NGC4631.

Abstract

A detailed understanding of cosmic-ray transport in galactic halos is essential for explaining various observations, such as the radio continuum measurements of synchrotron radiation from energetic electrons. Of central importance is the spatial diffusion tensor of cosmic rays, which can be computed in an ab~initio manner if the turbulence in the background medium is known. The study aimed to establish a suitable framework to compute the evolution of magnetohydrodynamic (MHD) turbulence and, hence, the diffusion tensor in the dynamical halos of galaxies. The Reynolds-averaged single-fluid MHD equations were solved numerically on a cylindrical grid, assuming axial symmetry and fixed boundary conditions on a central ellipsoid representing the galaxy. The physical properties of both large-scale MHD and small-scale turbulent quantities, including the coefficients of parallel and perpendicular diffusion, were extracted from the resulting steady state. Hydrodynamic validation revealed a persistent instability of the near-axis flow, which could be traced to the galaxy's gravitating mass. The results for the evolution of MHD turbulence in a galactic halo -- using parameters approximating those of NGC4631 -- enabled an ab initio computation of the spatial diffusion tensor of cosmic rays through the application of a state-of-the-art nonlinear theory. The simulation results reveal variation in turbulence quantities, (i.e., fluctuation energy, correlation scale, and cross helicity) in a dynamical halo. Furthermore, the corresponding diffusion tensor of cosmic rays exhibits significant variation throughout such a halo.