The stability of a magnetically confined radio jet

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Abstract
The linear stability against pinching modes of a model for a magnetically confined extragalactic radio jet is investigated. An analytic dispersion relation for these modes is obtained and then solved numerically. For comparison the dispersion relation for the Kelvin-Helmholtz pinching modes of a pressure confined jet is also solved numerically. The behavior of the modes is found to be fairly independent of the confinement mechanism. Subsonic and Mach 1 jets are found to be unstable to a single pinching mode at every wavenumber. In addition to this fundamental mode; supersonic jets have an infinite set of higher order (reflection) pinching modes that enter at points of marginal stability. When the jet velocity is less than hypersonic, these reflection modes have large spatial growth rates and thus may be responsible for the observed knot structure of radio jets.

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