Stability of Magnetic Jet Equilibria

Abstract

Magnetically confined plasma is notoriously unstable. Jets self-confined by magnetic fields can have advantages, however, if their streaming velocities exceed the Alfven speed. Traditional MHD modes (kink, pinch) have reduced growth rates in flowing jets. If the confining field threads a surrounding dense cocoon, this lowers the growth rates for wavelengths exceeding the jet radius, since motions must drag this cocoon mass along with the perturbation. The very straight jets of the well-aligned classical double radio sources imply remarkable sideways stability over distances of 1000 r_j. This implies either Mach numbers exceeding 50, quite dense jets, or stabilization by magnetic linking of jet and cocoon, which increases growth lengths by M_cocoon/M_j• Wriggling produces sideways shocks, which can also slow growth. B_z builds up from a seed field by velocity shear amplification, and thus may not influence stability until it becomes correlated over distances > r_j. Reflection modes riceochet around the interior, probably driving shocks which in turn may accelerate particles. Detection of a magnetically ordered cocoon would imply a major role for magnetic fields in the origin and propagation of jets.