Research Byte

Published in the RSAA Lunations
Vol1 Issue31 1–31 August 2022

Unravelling magnetised turbulence in galaxies

The interstellar medium (ISM) of galaxies is magnetised and turbulent. Therefore, any theory of the ISM, including star formation, cosmic ray transport, the origin of magnetic fields, metal mixing, the structure of the different ISM phases, etc., must include turbulence. This means, for any astrophysical turbulence and plasma theorist, the ISM provides a plethora of opportunities to stretch ones’ legs, tackle the problem of magnetised turbulence, and apply it to our understanding of the ISM. Alas, the problem of magnetised turbulence is a notoriously challenging one. The philosophy of many theorists is therefore to start as simply as possible — understand the basics and then add complexity to our models as nature requires us, aided by in silico observations of numerical experiments. After all, how can we understand the complexity of the magnetised ISM without first understanding the basic building blocks?

Over the past three years in my PhD I have been doing exactly that, working with many different theorists here on the mountain and overseas, simulating and using pen and paper to try to understand plasma models that capture some important aspects of interstellar medium turbulence. One interesting problem is: given a model of the balance between the turbulent energy and the magnetic energy, can we derive relations between the plane-of-sky dispersion of dust polarisation angles and the magnetic field strength? The underlying idea being, if a magnetic field is strong, the dispersion of the plane-of-sky dust polarisation angles should be small (magnetic field strength is inversely proportional to curvature), and vice versa. This links an underlying turbulence phenomenology (turbulent energy balance) with an observable (dust polarisation angles) and helps us unravel the nature and structure of magnetic fields in galaxies using polarisation data.

The original solutions to this problem were proposed by Davis (1951) and Chandrasekhar & Fermi (1953). They hypothesised that the kinetic energy balances with incompressible Alfvénic magnetic field fluctuations in the plasma. But the ISM is highly-compressible, and we are understanding in more and more detail, that incompressible Alfvénic fluctuations are simply not in energy balance with the kinetic energy in plasma regimes relevant to both the molecular and atomic ISM. In Beattie et al. (2022) (https://arxiv.org/abs/2202.13020), which is currently under-review, we provide a new analytical, statistical (involving moments of distribution functions) energy balance model that matches our numerical experiments exactly, with no fit parameters, involving compressible plasma modes. My collaborators have also already derived dust polarisation angle relations (https://arxiv.org/abs/2109.10925).

Statistical models are good, and used with fervour in the turbulence community, however, we think we can do better than even that, and are currently in the process of finishing a fluid dynamical model, derived directly from the equations of magnetised gas dynamics. These models involve emergent supersonic vortices in the plasma and I hope one day we might actually see signatures of these vortices in ISM observations. We will be continuing this project, and many others, while I continue my research journey from the sunny skies of California on my Fulbright exchange this year.

James Beattie

_{Figure 1: Our latest numerical experiments resolve over 10,000^3 grid elements (more grid resolution than the IllustrisTNG simulations, probing from parsecs to AU everywhere on the grid), which helps us understand the characteristic scales and energy transfer in the magnetised turbulence in galaxies.}