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In weakly collisional extragalactic plasmas such as the intracluster medium, viscous stress and the rate of change of the magnetic-field strength are proportional to the local pressure anisotropy, so subject to constraints imposed by the pressure-anisotropy-driven mirror and firehose instabilities and controlled by the local instantaneous plasma beta. The dynamics of such plasmas is dramatically different from a conventional MHD fluid. The plasma is expected to stay locally in a marginal state with respect to the instabilities, but how it does this is 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 models are studied in a simplified 0D setting. In the first model, the field grows explosively but on a time scale that scales with initial beta, 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 strong constraints on their applicability: for typical cluster-core conditions, scale separation between the fluid motions and the microscale fluctuations breaks down at beta~10^5-10^4. At larger beta (weaker fields), a fully collisionless plasma dynamo theory is needed in order to justify the growth of the field 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.
The physical foundations of the dissipation of energy and the associated heating in weakly collisional plasmas are poorly understood. Here, we compare and contrast several measures that have been used to characterize energy dissipation and kinetic-sc
This white paper, submitted for the Plasma 2020 Decadal Survey, concerns the physics of weakly collisional, high-beta plasmas -- plasmas in which the thermal pressure dominates over the magnetic pressure and in which the inter-particle collision time
We perform fully kinetic simulations of flows known to produce dynamo in magnetohydrodynamics (MHD), considering scenarios with low Reynolds number and high magnetic Prandtl number, relevant for galaxy cluster scale fluctuation dynamos. We find that
The nonlinear state of a high-beta collisionless plasma is investigated when an imposed linear shear amplifies or diminishes a uniform magnetic field, driving pressure anisotropies and hence firehose/mirror instabilities. The evolution of the resulti
We introduce the idea of weakly coherent collisional models, where the elements of an environment interacting with a system of interest are prepared in states that are approximately thermal, but have an amount of coherence proportional to a short sys