Cosmic ray propagation in turbulent galactic magnetic fields

Abstract

Cosmic rays and magnetic fields are important non-thermal components of the interstellar medium in galaxies. This thesis explores the intermittent structure of the magnetic field generated by a fluctuation dynamo and the interaction of cosmic rays with small-scale random magnetic fields. First, the nonlinear state of fluctuation dynamo is described in terms of the statistical and structural properties of magnetic and velocity fields. Using three-dimensional fluctuation dynamo simulations, we study their properties in the kinematic and saturated stages. The alignment of the magnetic field, electric current density, and velocity field are analyzed to suggest a possible saturation mechanism for the fluctuation dynamo. Furthermore, we also study the change in the diffusion of magnetic fields by calculating local magnetic Reynolds number in the kinematic and saturated stages. We show that both the amplification and diffusion of magnetic fields are affected by nonlinearity. The dynamo-generated magnetic field is intermittent, i.e., concentrated in filaments, ribbons, and sheets. Minkowski functionals are used to characterize the shape of the magnetic structures and study its dependence on magnetic Reynolds number. We find that all three length scales of magnetic structures decreases on saturation. We also propose that observing magnetic fields in elliptical galaxies, via a grid of the Faraday rotation measures from background polarized sources, would serve as a probe of the fluctuation dynamo action in a galactic environment. Next, the effect of magnetic field intermittency on cosmic ray propagation is studied. Using test-particle simulations, it is shown that the diffusivity of low energy cosmic rays is enhanced when the magnetic field is intermittent. It is demonstrated that the cosmic ray diffusion in any random magnetic field (Gaussian or intermittent) can be better described as a correlated random walk rather the usual Brownian motion. Then, the energy equipartition between magnetic fields and cosmic rays usually assumed to infer magnetic field strength from synchrotron intensity observations is discussed. Using test-particle and magnetohydrodynamic simulations, it is shown that the cosmic ray and magnetic field energy densities are not correlated on scales less than the driving scale of the turbulence. Even when the cosmic ray and magnetic field energy densities are uncorrelated, small-scale structures are seen in the spatial distribution of cosmic rays as they are trapped between random magnetic mirrors. These results exclude the possibility of local energy equipartition between cosmic rays and magnetic fields.