Strong turbulence and magnetic coherent structures in the interstellar medium

Magnetic turbulence is classified as weak or strong based on the relative amplitude of the magnetic field fluctuations compared to the mean field. These two classifications have different energy transport properties. This study analyzes interstellar turbulence based on this classification. Specifically, we examine the ISM of simulated galaxies to detect evidence of strong magnetic turbulence and provide statistics on the associated magnetic coherent structures (MCoSs), such as current sheets, that arise in this context. We analyzed MHD galaxy simulations with different initial magnetic field structures (ordered or random) and studied the magnetic field fluctuations ($\delta B/B_0$) and the MCoSs, defined here as regions where the current density surpasses a certain threshold. We also studied the MCoS sizes and kinematics. The magnetic field disturbances in both models follow a log-normal distribution, peaking at values close to unity, which turns into a power-law at large values ($\rm \delta B/B_0 > 1$). The current densities are widely distributed, with deviations from a log-normal at the largest values. These deviating values of the current density define MCoSs. We find that, in both models, MCoSs are fractally distributed in space, with a typical volume-filling factor of about 10 percent, and tend to coincide with peaks of star formation density. Their fractal dimension is close to unity below kpc scales and between 2 and 3 on larger scales. Our work challenges the prevailing paradigm of weak magnetic turbulence in the ISM by demonstrating that strong magnetic disturbances occur even when the initial magnetic field is initially ordered due to differential rotation and supernova feedback. Our findings provide a foundation for a strong magnetic turbulence description of the galactic ISM. (abridged)
Comment: Accepted for publication in A&A