Amplification of magnetic fields by supernova‐driven turbulence

Abstract

Observations of µG magnetic fields in radio galaxies at cosmological epochs as early as around z = 2 have shortened the available time for dynamo action. This fact suggests that the mean-field dynamo mechanism in a global galactic scale either is too slow to amplify a seed field generated by the Biermann battery effect to the level of the observed field strength at z ∼ 2 or needs much stronger seed fields of an order of 10 −10 G. A "contamination" picture that amplified magnetic fields in smaller objects, such as stars or AGNs, within a relatively shorter timescale spread out through supernova ejecta, stellar winds, and AGN jets to nearby environments is gaining momentum. In line with this picture, we demonstrate, through three-dimensional numerical experiments, that magnetic fields can be amplified by supernova-driven turbulence with two orders of magnitude smaller e-folding timescale than that of the mean-field dynamo mechanism. Therefore, supernova-driven turbulence may play an important role in amplifying small-scale B-fields in any astrophysical systems that have harbored massive stars.

Magnetic-Field Amplification by Turbulence in a Relativistic Shock Propagating Through an Inhomogeneous Medium

“…Thus the turbulent velocity is subsonic and super-Alfvénic in most of the postshock region in both cases A and B. This result is consistent with previous non-relativistic studies (e.g., Balsara et al 2001;Kim et al 2001;Balsara et al 2004;Kim & Balsara 2006;Haugan et al 2004, Giacalone & Jokipii 2007Inoue et al 2009), although the turbulent velocity in the postshock region is locally supersonic in these previous non-relativistic simulations.…”

“…Therefore, the electromagneticfield spectrum is flatter than Kolmogorov. A flat magnetic-energy spectrum is generally seen in turbulent-dynamo simulations for incompressible MHD (e.g., Schekochihin et al 2004) and compressible MHD (e.g., Balsara et al 2004;Balsara & Kim 2005;Kim & Balsara 2006). The same properties are also observed in relativistic MHD turbulence induced by the Kelvin-Helmholtz instability .…”

“…Here the amplification of the magnetic field arises from sequences of a "stretch, twist, and fold" nature. While the limit (Re m = ∞) is unrealistic in astrophysical simulations, published results of incompressible MHD simulations (e.g., Schekochihin et al 2004) and compressible MHD simulations (e.g., Balsara et al 2004;Balsara & Kim 2005;Kim & Balsara 2006) are nevertheless in agreement with the existence of a small-scale turbulent driven dynamo.…”

“…Kim et al (2001) studied the energetics, structure, and spectra of a supernova (SN) driven turbulent medium with higher resolution simulations. Balsara et al (2004) have investigated in detail the role of magnetic field amplification in SN-driven turbulence and found that the helicity generation mechanism by SN-driven, turbulent, multiphase ISM processes was adequate to sustain the magnetic field growth (also Balsara & Kim 2005;Kim & Balsara 2006). Haugen et al (2004) have performed simulations and found that magnetic field amplification can be sustained by shock-driven turbulence in an isothermal, compressible, shockdriven medium.…”

IONIZATION-DRIVEN FRAGMENTATION OF GAS OUTFLOWS RESPONSIBLE FOR FeLoBALs IN QUASARS

“…Their models for acceleration of BAL clouds yield magnetic field strengths of the order of 10 mG. Further, Furlanetto and Loeb (2001) argue that BAL quasar outflows could carry a large fraction of the magnetic energy of the intergalactic medium in clusters of galaxies. The source of such magnetic fields could be either from the accretion disc of the AGN (de Kool and Begelman 1995), or from dynamo-like effects in the plasmas such as in the solar winds and supernova remnants (Giacalone and Jokipii 2007;Kim and Balsara 2006;Balsara and Kim 2005).…”