<i>Eppur Si Muove</i> : self-sustained streaming motions in multiphase MHD

ABSTRACT Radiative cooling can drive dynamics in multiphase gas. A dramatic example is hydrodynamic ‘shattering’, the violent pressure-driven fragmentation of a cooling cloud that falls drastically out of pressure balance with its surroundings. We run magnetohydrodynamic (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 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 multiphase gas cooling and condensation in the interstellar medium, circumgalactic medium, and intracluster medium.