The turbulent dynamo during the formation of the first stars and galaxies

A small-scale dynamo, also called turbulent dynamo, is a magnetohydrodynamical process, in which the kinetic energy from turbulence is converted into magnetic energy. The mechanism is based on the fact that the magnetic field becomes stronger when the magnetic flux ropes are stretched. Figure 1 illustrates the stretching, twisting, and folding of the field lines schematically. The trigger of the stretching is turbulence, i.e. chaotic motion on a large range of scales, in a weakly magnetized plasma. I use an analytical description to predict the properties of the turbulent dynamo and the evolution of the magnetic field. My models take into account various astrophysical environments, for example different types of turbulence and different microphysical quantities.


Figure 1: The stretch-twist-fold model as an illustration of the turbulent dynamo.



Besides the analytical description of the small-scale dynamo, a focus of my research is its application to the formation of the first stars and galaxies. Here it might play a distinguished role as turbulence is driven efficiently by accretion and supernova explosions. These turbulent motions stretch the field lines and by this amplify the field exponentially at a typically very large growth rate. With our models we find that strong magnetic fields of the order of 10-6 Gauss can be generated by the turbulent dynamo which is comparable to the present-day Milky Way value. The amplification takes place within a few 107 years which is short compared to other dynamical timescales of the systems. A typical evolution of the magnetic field strength is shown in figure 2.

BfieldFigure 2: The typical time evolution of the magnetic field strength B(t) in the presence of a turbulent dynamo. The magnetic field increases by many orders of magnitude!





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