Astro FOTD: Parker Instability
M 51 in infrared. Notice how the bright clumps of star formation are strung like beads along the dusty lanes of the spiral arms and other spider web like features. It is thought that much of the clumpyness is due to Parker Instability. According to Choudhuri in Astrophysics for Physicists, magnetic pressure causes regions of stronger field to be less dense than nearby region of lesser field strength, giving rise to magnetic buoyancy. In the disc the local gravitational field has a component toward the plane of the disc, so that regions of slightly lesser field rise out of it like magnetic bubbles. The plasma in these bubbles, however, can stream down the magnetic field lines towards the disc, amplifying any small initial mass concentrations. The resulting mass concentrations can become dense enough to promote star formation.
The same buoyancy instability is involved in sunspots and solar flares.