Magnetohydrodynamic flow patterns in high-pressure jet exhausts containing toroidal fields

ABSTRACT A magnetic field is generally present within astrophysical jets. Constraining the field properties just as the jet emerges may help to understand the principles behind jet propagation and jet feedback into their surroundings. Taking an adiabatic, supermagnetosonic flow in which the field is frozen-in provides a set-up in which the flow characteristics depend mainly on the jet pressure, Mach number and relative magnetic field strength. To perform numerical simulations we also take a uniform density and implement a toroidal field. We distinguish conditions which generate Mach shock discs rather than a diamond pattern of oblique regular reflections. Some diagnostics that were explored included the shock pattern, the flow pattern, shock stand-off distance from the nozzle, velocity sheath, and oscillations in shock positions. Rapid oscillations occur in the divergent-convergent pattern through a feedback/hysteresis effect promoted by the ambient medium only for a passive field. Slow, high-amplitude variations are promoted by back-flow in a variable cocoon flow. We also study the energy transferred into the environment. Overpressured jets may contribute to noise and sound wave generation through screeching and droning. However, these oscillations that are due to the near-field are shown to be sufficiently unlikely to regulate star and galaxy formation. Overall, the major consequence reported here is that a toroidal field alters the shape of jet knots from diamonds to elongated prolate lenses.