Thermal Instability

Thermal instability (TI) refers to a runaway cooling/heating process in a plasma that results in condensation of cold gas clouds forming within a hotter background medium. TI criteria were first analyzed in Field (1965) and have since been studied in detail through numerical simulations. TI is used to explain the origins of the multiphase structure in the Interstellar Medium (ISM), as well as the existence of cooler gas clouds in the Intracluster Medium (ICM) and Circumgalactic Medium (CGM).

Below are simulations of TI within the ISM from Jennings and Li (2020) arXiv:2012.05252. We use the magnetohydrodynamics (MHD) code Athena++ to simulate the formation of cold clouds from uniform background. The first movie shows TI for pure hydrodynamic simulations without thermal conduction for various spectral index alpha. The next simulations include thermal conduction and viscosity with Pr = 10-2 (low viscosity) and Pr = 2/3 (high viscosity). The final set of simulations include a magnetic field initially oriented 45 degrees above the horizontal axis. The first movie has initial plasma beta of 106 and the second has initial plasma beta of 1.

Density plots of TI runs without thermal conduction or physical viscosity for various spectral indices alpha.

Density plots of TI runs with thermal conduction and physical viscosity with overlaid velocity vectors. The first video shows the run with Prandtl number of 10-2. The second video shows the run with Prandtl number of 2/3.