Strong turbulence and magnetic coherent structures in the interstellar medium

¹ Scuola Normale Superiore, Piazza dei Cavalieri, 7 56126 Pisa, Italy
² Institute of Astrophysics, Foundation for Research and Technology (FORTH), Nikolaou Plastira 100, Vassilika Vouton 711 10, Heraklion, Crete, Greece
³ Department of Physics, Aristotle University of Thessaloniki, 541 24 Thessaloniki, Greece
⁴ Department of Physics and ITCP, University of Crete, 71003 Heraklion, Greece

^★ Corresponding author; evangelia.ntormousi@sns.it

Received: 14 May 2024
Accepted: 23 September 2024

Abstract

Context. 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 classes have different energy transport properties.

Aims. The purpose of this study is to analyze turbulence in the interstellar medium (ISM) based on this classification. Specifically, we examined 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.

Methods. We analyzed magnetohydrodynamic galaxy simulations with different initial magnetic field structures (either completely ordered or completely random) and recorded statistics on the magnetic field fluctuations (δB/B₀) and the MCoSs, which are defined here as regions where the current density surpasses a certain threshold. We also studied the MCoS sizes and kinematics.

Results. The magnetic field disturbances in both models follow a log-normal distribution, peaking at values close to unity; this distribution turns into a power law at large values (δB/B₀ > 1), which is consistent with strong magnetic turbulence The current densities are widely distributed, with non-power-law 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%, and tend to coincide with peaks of star formation density. Their fractal dimension is close to unity on sub-kiloparsec scales, and between 2 and 3 on larger scales. These values are consistent with MCoSs having a sheet-like or filament-like morphology.

Conclusions. Our work challenges the prevailing paradigm of weak magnetic turbulence in the ISM by demonstrating that strong magnetic disturbances can occur even when the initial magnetic field is completely ordered. This strong magnetic turbulence arises self-consistently from differential rotation and supernova feedback. Our findings provide a foundation for a magnetic turbulence description of the galactic ISM that includes strong fluctuations of the magnetic field.