Three-dimensional Magnetohydrodynamic Simulations of Relativistic Jets Injected along a Magnetic Field

We present the first numerical simulations of moderately hot, supersonic jets propagating initially along the field lines of a denser magnetized background medium with Lorentz factor W = 4.56 and evolving in a four-dimensional spacetime. Compared with previous simulations in two spatial dimensions, the resulting structure and kinematics differ noticeably: the density of the Mach disk is lower, and the head speed is smaller. This is because the impacted ambient fluid and its embedded magnetic field make efficient use of the third spatial dimension as they are deflected circularly off of the head of the jet. As a result, a significant magnetic field component normal to the jet is created near the head. If the field is strong, backflow and field reversals are strongly suppressed; upstream, the field closes back on the surface of the beam and assists the collimation of the jet. If the field is weak, backflow and field reversals are more pronounced, although still not as extended as in the corresponding plane-parallel case. In all studied cases, the high-pressure region is localized near the jet head irrespective of the presence/strength of the magnetic field, and the head decelerates efficiently by transferring momentum to the background fluid that recedes along a thin bow shock in all directions. Furthermore, two oppositely directed currents circle near the surface of the cylindrical beam, and a third current circles on the bow shock. These preliminary results underline the importance of performing fully three-dimensional simulations to investigate the morphology and propagation of relativistic extragalactic jets.