Magnetic Fields in Simulation Data: Spatial Structure
Leonard Kaiser
Magnetic fields play an important role in the interstellar medium (ISM) of our Galaxy. According to the magneto-hydrodynamic (MHD) equations that we use to describe astrophysical plasmas, the magnetic field lines move with the gaseous matter. And since the energy density of magnetic fields is comparable to that of other components and processes within the ISM, such as cosmic rays and turbulent motions, they influence the dynamics of the gas. Because of this, galactic magnetic fields play a crucial role in our understanding of many astrophysical processes.
We investigate the magnetic field in the SILCC simulations – a set of three-dimensional MHD simulations of the ISM in slabs of Galaxies. Here, we present a first insight into the spatial structure of the field by examining how its value at one point is correlated with that at another point, separated from the first by a certain distance along a line of sight through the simulation domain.
The second row of Fig. 1 shows two measures of the spatial correlation of the magnetic field along the z coordinate. The patch length is the average line-of-sight distance between sign flips of the parallel magnetic field. The lower right panel shows the correlation length, which is the distance at which the normalised correlation between the parallel magnetic field at two points along a line of sight drops below 0.5. Both measures are well correlated and represent a typical length scale at which the magnetic field loses spatial coherence.
The first row of Fig. 1 shows two measures of turbulence in the simulation. The right panel plots the velocity dispersion, a measure of kinetic energy. It is influenced by the star formation rate (SFR) – the stellar mass formed per unit time and area. The higher the SFR, the higher the velocity dispersion: The medium is more turbulent.
Our analysis suggests a weak anticorrelation between the measures of turbulence and those of magnetic field correlation: For instance, at 140 Myr, the SFR decreases sharply, leading to a slight increase in patch length and correlation length. This indicates that a more perturbed, turbulent medium results in a less coherent magnetic field. This effect modulates the spatial coherence length of approx. 20 pc. This is important information for astronomical observations and modelling: Whenever the magnetic field is assumed to be constant, the assumption will likely not hold on scales larger than this.