An axisymmetric bubble in a magnetic field

The Rayleigh–Taylor instability (RTI) plays a key role in the evolution and collapse of astrophysical bubbles, such as those formed by stellar winds or supernova explosions. Magnetic fields are thought to stabilize these bubbles. To investigate this, we model a bubble of light gas surrounded by a denser gas with a radially inward gravity and compare results with and without a background vertical magnetic field. Magnetohydrodynamic equations describe the electrically conducting fluid and magnetic field in and around the bubble. To simplify, we apply a Boussinesq approximation, which ignores density variations except in terms multiplied by the acceleration due to gravity. We use a vorticity-streamfunction approach and formulate a series solution using spectral methods. In each simulation, an initially spherical bubble in a fluid at rest is subjected to a small perturbation to the velocity of the fluid or to the vertical magnetic field. Without the magnetic field, the bubble can exhibit a wide range of large-scale deformations and often develops bipolar jets or disk-like structures that can fragment into rings. These disks and jets form prominent mushroom-cap shapes that are typical of the RTI for small density ratios. The addition of the magnetic field suppresses the growth of the RTI and Kelvin–Helmholtz instability, especially for small wavelength disturbances. It can also be a source of asymmetry.