Shifting Gears: How Changes in Cosmic-Ray Transport Drive Buoyancy in the Galactic Halo

The plasma properties of the interstellar medium (ISM), galactic halo, and circumgalactic medium (CGM) differ significantly. These properties determine how GeV cosmic rays are transported and how the total cosmic-ray pressure reacts to compressive bulk plasma motions as the cosmic rays move between different temperature, density, and plasma beta regimes. A simple buoyancy instability can develop which drives cold, neutral plasma to be buoyantly unstable but hot, ionized plasma to be buoyantly stable. The instability results from a change in the cosmic-ray pressure polytropic index. The instability will also occur on a larger scale at the interfaces of the ISM, galactic halo, and CGM. We present both analytical prescriptions describing this theory and cosmic-ray-magnetohydrodynamic simulations illustrating the development of the instability. This instability could help explain cold clouds at high latitude in our own galaxy, as well as increase the mass loading of galactic winds.