[Paper Review] Cosmic Magnetic Fields: from Stars and Galaxies to the Primordial Universe

E. M. de Gouveia Dal Pino|arXiv (Cornell University)|Mar 19, 2010

Solar and Space Plasma Dynamics2 references3 citations

TL;DR

This paper reviews the role of cosmic magnetic fields across astrophysical scales—from stars and galaxies to the primordial universe—highlighting their critical influence on star formation, plasma dynamics, and cosmic structure. Using MHD and KMHD simulations, it demonstrates that kinetic effects such as fire-hose instabilities significantly enhance magnetic field wrinkling and amplification in collisionless plasmas, particularly in the intergalactic medium.

ABSTRACT

Most of the baryonic matter in the Universe is permeated by magnetic fields which affect many, if not most, of astrophysical phenomena both, in compact sources and in diffuse gas. Recent years have been marked by a worldwide surge of interest in the astrophysical magnetic fields, their origin, and their influence on the formation and evolution of astrophysical objects (stars, galaxies, cooling flows). This growing interest is in part due to the fact that it has become possible to trace magnetic fields in molecular clouds, over vast extensions of the Milky Way and to study extragalactic magnetic fields, including fields in clusters of galaxies. With the combination of various techniques, such as Zeeman and Faraday rotation measurements with synchrotron and aligned grain polarimetry, it is now possible to undertake quantitative observational studies of magnetic fields, the results of which can be compared with high resolution dynamo and MHD turbulence simulations. This brings the field to a new stage. In this paper, I will briefly review the importance of the cosmic magnetic fields both from a theoretical and from an observational perspective, focusing on their role in stellar and compact objects, in the interstellar medium and star formation regions, and in galaxies, clusters of galaxies, and the primordial Universe.

Motivation & Objective

• To synthesize current theoretical and observational understanding of cosmic magnetic fields across astrophysical scales.
• To address the unresolved origin and evolution of primordial magnetic fields in the early universe.
• To evaluate the role of magnetic fields in star formation, jet stability, and interstellar medium dynamics.
• To compare collisional MHD with kinetic MHD (KMHD) simulations in modeling magnetic field amplification in collisionless plasmas.
• To identify key observational and simulation challenges for advancing the field, especially with upcoming instruments like the SKA.

Proposed method

• Utilizes the magnetic induction equation ∂B/∂t = ∇×(u×B) + η_Ohm∇²B to model magnetic field evolution in conducting plasmas.
• Applies high-resolution 3D MHD and KMHD simulations using a Godunov-based numerical code to study turbulent magnetic field amplification.
• Compares non-helical, forced turbulence simulations with and without collisional effects, focusing on the role of anisotropic pressure and fire-hose instabilities.
• Incorporates observational techniques such as Zeeman splitting, Faraday rotation, synchrotron polarization, and dust grain alignment to infer magnetic field structure.
• Evaluates the impact of kinetic effects (e.g., pressure anisotropy, heat conduction) on magnetic field morphology and energy distribution.
• Relies on numerical benchmarks from Santos-Lima et al. (2010b) to contrast magnetic field folding and energy growth in MHD vs. KMHD regimes.

Experimental results

Research questions

• RQ1 How do kinetic effects such as fire-hose instabilities influence magnetic field amplification and morphology in collisionless plasmas?
• RQ2 To what extent do KMHD simulations differ from collisional MHD in predicting magnetic field structure and energy distribution?
• RQ3 What observational techniques provide the most reliable constraints on magnetic fields in the interstellar and intergalactic media?
• RQ4 What is the role of magnetic fields in the formation and stability of jets, accretion disks, and cosmic rays?
• RQ5 How can upcoming instruments like the Square Kilometer Array (SKA) revolutionize the mapping of cosmic magnetic fields?

Key findings

• Magnetic fields are conserved in most astrophysical plasmas due to high conductivity, with diffusion timescales far exceeding dynamical timescales.
• KMHD simulations show that fire-hose instabilities lead to enhanced small-scale magnetic fluctuations, producing a more wrinkled field structure compared to collisional MHD.
• The most efficient magnetic field amplification occurs in systems unstable to the fire-hose instability, as confirmed by energy evolution in Figure 5.
• Magnetic field energy growth is significantly modulated by pressure anisotropy and kinetic effects, which alter the power spectrum inclination in turbulent media.
• The SKA will enable detection of over 10,000 pulsars in the Milky Way, allowing high-resolution mapping of the galactic magnetic field structure.
• Next-generation instruments like ALMA, EVLA, MeerKAT, and ASKAP will provide 10× better angular resolution for magnetic field studies in galaxies and the IGM.