Nonlinear Simulations of the Heat Flux Driven Buoyancy Instability and its Implications for Galaxy Clusters

TL;DR

This paper investigates the heat flux driven buoyancy instability (HBI) in galaxy cluster plasmas, showing it reorients magnetic fields, reduces heat conduction, and impacts thermal dynamics in cluster cores.

Contribution

The study presents novel 2D and 3D simulations of the HBI, revealing its role in magnetic field reorientation and suppression of heat flux in galaxy clusters.

Findings

HBI amplifies magnetic fields by a factor of ~20.

Magnetic fields become nearly perpendicular to temperature gradients after saturation.

Heat flux is reduced to less than 1% of the Spitzer value in the saturated state.

Abstract

In low collisionality plasmas heat flows almost exclusively along magnetic field lines, and the condition for stability to convection is modified from the standard Schwarzschild criterion. We present local two and three-dimensional simulations of a new heat flux driven buoyancy instability (the HBI) that occurs when the temperature in a plasma decreases in the direction of gravity. We find that the HBI drives a convective dynamo that amplifies an initially weak magnetic field by a factor of ~20. In simulations that begin with the magnetic field aligned with the temperature gradient, the HBI saturates by rearranging the magnetic field lines to be almost purely perpendicular to the initial temperature gradient. This magnetic field reorientation results in a net heat flux through the plasma that is less than 1% of the field-free (Spitzer) value. We show that the HBI is likely to be present in the cool cores of clusters of galaxies between ~0.1-100 kpc, where the temperature increases outwards. The saturated state of the HBI suggests that inward thermal conduction from large radii in clusters is unlikely to solve the cooling flow problem. Finally, we also suggest that the HBI may contribute to suppressing conduction across cold fronts in galaxy clusters.