PhD Defense by Thomas Berlok – Niels Bohr Institute - University of Copenhagen

Astrophysics & Planetary Science > Calendar > 2017 > PhD Defense by Thomas ...

PhD Defense by Thomas Berlok

Weakly Collisional and Collisionless Astrophysical Plasmas


Weakly collisional and collisionless astrophysical plasmas are not well described by ideal magnetohydrodynamics (MHD) whose validity depends on a high collision frequency. This thesis aims to address this issue by moving beyond ideal MHD and the scope of the thesis is twofold. Firstly, we investigate helium mixing in the weakly collisional intracluster medium of galaxy clusters using Braginskii MHD.

Secondly, we present a newly developed Vlasov-fluid code which can be used for studying fully collisionless plasmas such as the solar wind and hot accretions flows. The equations of Braginskii MHD are used to study weakly collisional, stratified atmospheres which offer a useful model of the intracluster medium of galaxy clusters. Using linear theory and computer simulations, we study instabilities that feed off thermal and compositional gradients. We find that these instabilities lead to vigorous mixing of the composition and discuss the potential consequences for X-ray observations of galaxy clusters.

Collisionless plasmas can be subject to microscale velocity-space instabilities which are not well-described by Braginskii MHD. In contrast, Vlasov-fluid theory captures all the kinetic phenomena associated with the ions and is thus well suited for studying collisionless plasmas. We have developed a new 2D-3V Vlasov-fluid code which works by evolving the phase-space density distribution of the ions while treating the electrons as an inertialess fluid. The code uses the particle-incell (PIC) method and several options for particle interpolation (cloud-in-cell and triangular-shaped-cloud) and several methods for updating the equations in time ( the predictor-corrector and the Horowitz method) are provided. The programming language Python has been chosen for its usability but high performance is nevertheless maintained and the code is MPI-enabled. The Vlasov-fluid code has been tested and is able to convincingly reproduce results from linear theory.

The tests include one-dimensional simulations of plasma instabilities such as the firehose instability, the ion-cyclotron instability and the ion beam instability and simulations of waves, such as the ion-acoustic, ion Bernstein, ion-cyclotron and whistler waves. The thesis also contains a general introduction to the PIC method including a discussion of aliasing due to the numerical grid and the finite grid instability. We furthermore study ion-cyclotron damping and Landau damping of ion-acoustic waves and present a two-dimensional simulation of the parallel firehose and oblique firehose instability. We conclude the thesis by pointing to possible future applications of the Vlasov-fluid code.

Assessment Committee:

(Chairperson)  Associate Professor Klaus Galsgaard, Astrophysics and Planetary Science, University of Copenhagen

Associate Professor Günter Houdek, Department of Physics and Astronomy, Aarhus University

Assistant Professor Lorenzo Sironi, Department of Astronomy at Columbia University


Professor MSO Martin E. Pessah, Niels Bohr International Academy, University of Copenhagen

Associate Professor Troels Haugbølle, StarPlan, University of Copenhagen

Associate Professor Tobias Heinemann, Niels Bohr International Academy, University of Copenhagen

Copy of the thesis:

A copy of the thesis can be downloaded here.