Publications

Journal articles

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Proceeding articles

Theses

PhD thesis: “On the role of the turbulent dynamo in the evolution of cosmic magnetic fields”
(Abstract)

The aim of this work is to explore the origin of magnetic fields in the Universe. We claim that the turbulent or small-scale dynamo, which amplifies weak seed fields on short timescales in the presence of turbulence, plays an important role in the evolution of cosmic magnetic fields. The theoretical model for the turbulent dynamo is generalized for various astrophysical environments, with a focus on different turbulence spectra. We derive analytical solutions for the dynamo growth rate in the kinematic phase and discuss the subsequent non-linear evolution as well as saturation. In the history of the Universe turbulence is expected to be driven efficiently at the latest during the formation of the first stars and galaxies, where gravitational energy is converted into chaotic motions as the dark matter halos accrete gas from the environment. We model these processes semi-analytically and implement magnetic field amplification by a turbulent dynamo. Our results show that unordered magnetic fields, with strengths comparable to the ones in local galaxies, were already present in the primordial Universe. A potential observational test for magnetic fields in young galaxies is suggested to probe our proposed scenario for the evolution of cosmic magnetic fields.

Diploma thesis: “The small-scale dynamo: Amplification of magnetic fields in the early Universe”
(Abstract)

In this work we explore the small-scale dynamo – a mechanism which may rapidly amplify a week magnetic seed field by converting turbulent kinetic energy into magnetic energy. %The basics for the description of this process are magnetohydrodynamics and turbulence. Both of these theories are presented in this work as far as necessary and available. Moreover, we give an introduction to the theory of magnetohydrodynamical dynamos in general. The small-scale dynamo is described by a theory of Kazantsev, which depends crucially on the nature of turbulence. We propose a model for different types of turbulence and use the Kazantsev theory to determine properties of the small-scale dynamo. With our model we find that the critical magnetic Reynolds number, which needs to be exceeded for small-scale dynamo action, lies between 110 and 2700. Furthermore, we show that the growth rate of the small-scale magnetic field depends strongly on the hydrodynamical Reynolds number Re. In the limit of infinite magnetic Prandtl numbers the growth rate Γ scales between Γ∝Re1/2 and Γ∝Re1/3 for different types of turbulence. For decreasing magnetic Prandtl number, the growth rate of the small-scale dynamo decreases. We apply our model to the magnetic fields in the formation of the first stars. For this we estimate the typical quantities of primordial gas by using a one-zone chemistry code. The resulting small-scale magnetic field reaches its saturation value almost instantly and thus we expect it to be dynamically important in primordial star formation.