The nature of diffusive particle transport in turbulent magnetic fields
MetadataShow full item record
The transport of charged particles in turbulent plasma is a crucially important area of research in astrophysics, since it directly impacts our ability to interpret observations done with a major group of messenger particles. In order to understand these measurements of highly energetic charged particles - or cosmic rays - a comprehension of their interactions with the turbulent magnetic fields, which permeate the heliosphere, the interstellar and the intergalactic medium, is equally important as the understanding of their generation and acceleration at the sources, if not more so. In this work, a numerical approach is taken to derive transport parameters for charged energetic particles in the heliosphere. A spectral incompressible MHD code is used to generate realistic turbulence in a self-consistent way. The properties of the turbulence are then probed by injecting test particles and analysing their propagation. New numerical analysis methods, that were developed to work especially well in strong turbulence scenarios, where classical methods and analytical solutions fail, are presented, together with their validation and transport parameter results obtained for various simulation setups. In most astrophysical scenarios the magnetic field fluctuations can be assumed to be much larger than the fluctuating electric fields δB >>δE, consequently the predominant transport process is the change of the direction of the particle momentum relative to the magnetic field - or pitch angle - as opposed to the change of the absolute value of the momentum p, which is suppressed in comparison. Hence, the focus of the analysis is on the pitch angle diffusion coefficient Dµµ. To demonstrate that the concept can also be applied to other quantities, results for the perpendicular spacial diffusion coefficient D┴ are derived and presented as well. Additionally, an alternative method to generate turbulence in magnetised plasmas using Perlin gradient noise is described and its characteristics concerning particle transport are analysed and compared with the self-consistent MHD-turbulence in order to test its validity. Although the properties of the Perlin noise turbulence are not in complete agreement with MHD, the deviations can be neglected in specific cases (especially in strong turbulence) and are offset somewhat by the much lower computational effort.