The interactions of magnetic fields and cosmic rays, and their role in galaxies

Abstract

I study the physics of charged relativistic cosmic ray (CR) particles and the dynamics of the interstellar medium (ISM) including the CR particles along with other physical effects such as gravity and differential rotation. I use two complementary approaches to model cosmic rays in astrophysical systems. The first approach uses test particle simulations of high energy cosmic rays in various magnetic field configurations to understand their ensemble distribution with respect to the field. Unlike most other studies of this type, I consider realistic, non-Gaussian magnetic fields produced by the fluctuation dynamo rather than ad-hoc Gaussian random fields. Particle simulations in an idealised axisymmetric mirror trap defined by an analytical function, and a dynamo-generated random field which was interpolated from a three-dimensional grid, demonstrate that the spatial distribution of the cosmic-ray protons and electrons in a random magnetic field is not only uncorrelated with the magnetic field strength but is even statistically independent of it. The spatial distributions of the protons and electrons are different from each other, although they are correlated. The spatial distribution of cosmic rays at the turbulent scales is sensitive to the discrete nature of the cosmic ray sources in the sense that it is important that the particles are injected into the interstellar medium non-uniformly. These high-energy particles are accelerated into the interstellar medium from the shock fronts of supernovae remnants which are inhomogeneous, and to study their realistic evolution in various magnetic field geometries, it is vital to include the inherent non-uniformity in the spatial distribution of cosmic rays. The second part of my research is aimed at forming a better understanding of the dynamics of galaxies in the presence of magnetic fields and cosmic rays through magnetohydrodynamic (MHD) simulations. Here, I study the evolution and saturation of the fully nonlinear Parker and magnetic buoyancy instabilities. I use the sixth order differential equation solver Pencil code (Pencil Code Collaboration et al., 2021) developed for non-ideal astrophysical MHD simulations. I simulate the system in external gravity typical of the solar neighbourhood in a local rectangular box of 4×4 kpc2 horizontally and 3.5 kpc vertically (with the mid-plane at the box middle). In order to simulate the nonlinear, statistically steady-state of the system I impose background horizontal magnetic field, gas density and cosmic ray distributions as functions of height symmetric about the midplane. The simulations show that the nonlinear development of the Parker instability leads to a decrease in the gas scale height, accompanied by the evolution of the magnetic field towards a nearly uniform vertical distribution and vigorous spread of the cosmic rays through the system guided by the localized vertical magnetic fields. I also consider the effect of rotation on the evolution and saturation of the Parker instability. Rotation introduces dynamo effects in the system, which affect the dynamics of magnetic fields and cosmic rays. In particular, rotation results in a reversal of the imposed horizontal magnetic field and reduces the loss of cosmic rays from the system. The qualitative evolution of cosmic rays and thermal gas is similar in both rotating and non-rotating cases, but the loss and saturation levels of magnetic field and cosmic rays differ quantitatively.