Turbulent dynamo in the two-phase interstellar medium

TL;DR

This study investigates how the turbulent dynamo operates differently in the multiphase interstellar medium, revealing phase-dependent behaviors in magnetic field amplification and energy ratios through non-isothermal simulations.

Contribution

It presents the first non-isothermal simulation of the turbulent dynamo in a two-phase medium, highlighting differences from isothermal models and emphasizing the importance of multiphase analysis.

Findings

Growth rate of magnetic fields is similar in both phases.

Vorticity is amplified by turbulence and baroclinic effects in warm phase.

Final magnetic to kinetic energy ratio is lower in the cold phase.

Abstract

Magnetic fields are a dynamically important component of the turbulent interstellar medium (ISM) of star-forming galaxies. These magnetic fields are due to a dynamo action, which is a process of converting turbulent kinetic energy to magnetic energy. A dynamo that acts at scales less than the turbulent driving scale is known as the turbulent dynamo. The ISM is a multiphase medium and observations suggest that the properties of magnetic fields differ with the phase. Here, we aim to study how the properties of the turbulent dynamo depend on the phase. We simulate the non-isothermal turbulent dynamo in a two-phase medium (most previous work assumes an isothermal gas). We show that the warm phase ($T\ge10^3~{\rm K}$) is transsonic and the cold phase ($T<10^3~{\rm K}$) is supersonic. We find that the growth rate of magnetic fields in the exponentially growing stage is similar in both phases. We compute the terms responsible for amplification and destruction of vorticity and show that in both phases vorticity is amplified due to turbulent motions, further amplified by the baroclinic term in the warm phase, and destroyed by the term for viscous interactions in the presence of logarithmic density gradients in the cold phase. We find that the final ratio of magnetic to turbulent kinetic energy is lower in the cold phase due to a stronger Lorentz force. We show that the non-isothermal turbulent dynamo is significantly different from its isothermal counterpart and this demonstrates the need for studying the turbulent dynamo in a multiphase medium.