Models of magnetic field evolution and effective viscosity in weakly collisional extragalactic plasmas

Abstract

In weakly collisional plasmas such as the intracluster medium (ICM), the viscous stress and the rate of change of the magnetic energy are proportional to the local pressure anisotropy, so subject to constraints imposed by the pressure-anisotropydriven microinstabilities (mirror and firehose) and controlled by the local instantaneous plasma β. The dynamics of such plasmas can be dramatically different from a conventional MHD fluid. The plasma is expected to stay locally marginal with respect to the instabilities, but how it does this remains an open question. Two models of magnetic-field evolution are investigated. In the first, marginality is achieved via suppression of the rate of change of the field. In the second, the instabilities give rise to anomalous collisionality, reducing pressure anisotropy to marginal -at the same time decreasing viscosity and so increasing the turbulent rate of strain. Implications of these two models are studied in a simplified zero-dimensional setting. In the first model, the field grows explosively but on a time scale that scales with the initial β, while in the second, dynamical field strength can be reached in one large-scale turbulence turn-over time regardless of the initial seed. Both models produce very intermittent fields. Both also suffer from fairly strong constraints on their applicability: for typical cluster-core conditions, scale separation between the fluid motions (with account of suppressed viscous stress) and the miscoscale fluctuations breaks down at β ∼ 10 4 − 10 5 . At larger β (weaker fields), a fully collisionless plasma dynamo theory is needed to justify field growth from a tiny primordial seed. However, the models discussed here are appropriate for studying the structure of the currently observed field as well as large-scale dynamics and thermodynamics of the magnetized ICM or similarly dilute astrophysical plasmas.

Fluctuation Dynamo in a Collisionless, Weakly Magnetized Plasma

“…Since the viscous-scale rate of strain increases as Re 1/2 eff , the dynamo in this intermediate second regime should, at some point, be self-accelerating, with the field-stretching eddies becoming smaller and faster as the magnetic field is amplified. Similar scenarios have previously formed the basis for theories of explosive dynamo in collisionless plasmas (Schekochihin & Cowley 2006a,b;Melville et al 2016;Mogavero & Schekochihin 2014), a topic that will be the subject of a separate publication.…”

Forecasts for the detection of the magnetised cosmic web from cosmological simulations

“…Kronberg et al 1999;Donnert et al 2009;Xu et al 2009;Schober et al 2013;Beck et al 2013). Fast-growing plasma instabilities, such as firehose and mirror instabilities in large β plasmas (where β is the ratio between the thermal and the magnetic energy), might be able to amplify magnetic fields up to values we observe in clusters (Schekochihin et al 2005;Kunz et al 2011;Mogavero & Schekochihin 2014), or at least provide additional seeding for the dynamo. Indications of substantial magnetisation along the filamentary region in the SW sector of the Coma cluster have been reported in Bonafede et al (2013).…”

Pressure-anisotropy-driven microturbulence and magnetic-field evolution in shearing, collisionless plasma

“…In order to make that happen, the firehose and mirror fluctuations must impose an upper bound on the effective rate of change of the magnetic field (which gives rise to the pressure anisotropies) and/or increase the effective collisionality of the plasma (which relaxes the anisotropies)-either scenario can lead to macroscopic dynamics dramatically different from the conventional MHD evolution. This topic was discussed at length (if speculatively) by Mogavero & Schekochihin (2014), with particular focus on the question of how, under pressure-anisotropic conditions, the magnetic field might be able to grow from a weak primordial seed (believed to be anywhere between 10 −21 and 10 −9 G, corresponding to β ∼ 10 32 − 10 8 ; see review by Durrer & Neronov 2013) to its observed value (B ∼ 10 6 G, β ∼ 10 2 ), even as the rate at which it is allowed to change locally is constrained by the condition…”