$\textit{Eppur Si Muove}$: Self-Sustained Streaming Motions in Multi-Phase MHD

TL;DR

This paper uses MHD simulations to show that magnetic fields cause cooling gas to form long-lived, field-aligned streaming flows with counter-streaming motions, contrasting with chaotic shattering.

Contribution

It reveals how magnetic fields influence multi-phase gas dynamics, leading to coherent, self-sustained streaming flows and counter-streaming, a novel insight into cooling gas behavior.

Findings

Magnetic fields cause gas to stream along field lines at high speeds.

Counter-streaming flows arise from a cooling-induced MHD thin shell instability.

Magnetic tension influences the non-linear evolution of these flows.

Abstract

Radiative cooling can drive dynamics in multi-phase gas. A dramatic example is hydrodynamic `shattering', the violent, pressure-driven fragmentation of a cooling cloud which falls drastically out of pressure balance with its surroundings. We run MHD simulations to understand how shattering is influenced by magnetic fields. In MHD, clouds do not `shatter' chaotically. Instead, after initial fragmentation, both hot and cold phases coherently `stream' in long-lived, field-aligned, self-sustaining gas flows, at high speed ($\sim 100 \, {\rm km \, s^{-1}}$). MHD thermal instability also produces such flows. They are due to the anisotropic nature of MHD pressure support, which only operates perpendicular to B-fields. Thus, even when $P_{\rm B} + P_{\rm gas} \approx$const, pressure balance only holds perpendicular to B-fields. Field-aligned gas pressure variations are unopposed, and results in gas velocities $v \sim (2 \Delta P/\rho)^{1/2}$ from Bernoulli's principle. Strikingly, gas in adjacent flux tubes $\textit{counter-stream}$ in opposite directions. We show this arises from a cooling-induced, MHD version of the thin shell instability. Magnetic tension is important both in enabling corrugational instability and modifying its non-linear evolution. Even in high $\beta$ hot gas, streaming can arise, since magnetic pressure support grows as gas cools and compresses. Thermal conduction increases the sizes and velocities of streaming cloudlets, but does not qualitatively modify dynamics. These results are relevant to the counter-streaming gas flows observed in solar coronal rain, as well as multi-phase gas cooling and condensation in the ISM, CGM and ICM.