Buoyancy of Cosmic Ray Loaded Magnetic Flux Tubes in the Galactic Disk*

Interstellar gas in disk galaxies is vertically supported against gravity by the pressure of thermal gas, magnetic fields, and cosmic rays. When nonthermal pressure support exceeds a threshold, the Parker instability can appear. Like the Rayleigh-Taylor instability, over-dense regions sink, and under-dense regions rise. This produces peaks and valleys in the magnetic field. Gravitational energy provides the free energy necessary to compress the interstellar gas into the valleys.

Since cosmic rays are unaffected by the galaxy’s gravity, they increase the buoyancy of the ISM. However, the cosmic ray fluid has a finite compressibility, increasing the energy required to form valleys. Linear theory suggests this compressibility dominates buoyancy, suppressing the instability. 

To address this counterintuitive result, we run local simulations of injections of cosmic ray pressure in the galactic disk. This assumes a supernova as the source. If this physically motivated perturbation creates buoyant magnetic flux tubes, then it is likely the Parker instability can develop in the ISM even if instability criteria from linear theory are not met. 

Simulation results will be presented, along with implications for vertical stability in the ISM.