Hydromagnetic break-up of bridges and jets into aligned objects

It is shown that a cylindrical plasma column supporting a longitudinal field Bsub(z) and an azimuthal field Bsub(phi) has a fastest-growing mode in which kR0 approximately 1 (k=wave number, R0=radius of the column). If it is assumed that plasma is ejected from a galaxy to form a jet, filament or bridge of length approximately equal to 5 kpc, R0 approximately equal to 0.5 kpc, density approximately 10-24 gm cm-3 with a dragged-out field of strength Bsub(z) approximately 10-5 Gauss (from a parent field approximately 10-6 Gauss), such a column must eventually fragment by the action of a hydromagnetic instability, breaking up into a number approximately 10 of regularly-spaced condensations. It is therefore, predicted that features like the M87 jet should show incipient nucleation with 3-10 knots, and that periodically-spaced objects of the type noted by Arp may have resulted from the action of such an instability. (Auth.)