Stability of galactic radio jets
Gregory Benford · The Astrophysical Journal · 1981
A simple cold flowing beam undergoing sidewise perturbations is investigated, ignoring internal dynamics and using Newton's law. Effects of large scale firehose-type instability are avoided or diminished by sizable internal magnetic fields, a dense cocoon increasing in pressure with distance from the beam, or linking a large, dense cocoon to the beam. Exponential growth is stopped when radial displacement is greater than one, shifting growth to longer wavelengths, and when the beam boundary sidewise velocity exceeds the sound speed. Bow shocks can yield hot spots where beam sidewise amplitude is largest, providing particle reacceleration, and sinusoidal nodes may account for regular hot spot formation by sidewise moving shocks. Curved beams are also investigated and are less stable due to the difficulty of retaining a dense cocoon downwind from the intracluster pressure. Explosive expansion of the beam may be due to an onset of local heating, and a nonlinear stage of instability may be linked to particle reacceleration due to the range of phenomena that the simple sidewise instability produces.