The Dynamics of Internal Working Surfaces in Magnetohydrodynamic Jets

The dynamical effects of magnetic fields in models of radiative Herbig-Haro jets have been studied in a number of papers. For example, magnetized radiative jets from variable sources have been studied with axisymmetric and three-dimensional numerical simulations. In this paper, we present an analytical model describing the effect of a toroidal magnetic field on the internal working surfaces that result from the presence of variability in the ejection velocity. We find that for parameters appropriate to Herbig-Haro jets, the forces associated with the magnetic field dominate over the gas pressure force within the working surfaces. Depending on the ram pressure radial cross section of the jet, the magnetic field can produce a strong axial pinch or, alternatively, a broadening of the internal working surfaces. We check the validity of our analytical model with axisymmetric numerical simulations of variable magnetized jets.