2019MNRAS.484.2364R

We present 2.5D global, ideal magnetohydrodynamic (MHD) simulations of magnetically and rotationally driven protostellar jets from Keplerian accretion discs, wherein only the initial magnetic field strength at the inner radius of the disc, B_i, is varied. Using the AMR-MHD code AZEuS, we self-consistently follow the jet evolution into the observational regime (> 10³au^) with a spatial dynamic range of ∼6.5 x 10⁵. The simulations reveal a three-component outflow: (1) A hot, dense, super-fast, and highly magnetized 'jet core'; (2) a cold, rarefied, trans-fast, and highly magnetized 'sheath' surrounding the jet core and extending to a tangential discontinuity; and (3) a warm, dense, trans-slow, and weakly magnetized shocked ambient medium entrained by the advancing bow shock. The simulations reveal power-law relationships between B_i and the jet advance speed, v_jet, the average jet rotation speed, <v_φ> , as well as fluxes of mass, momentum, and kinetic energy. Quantities that do not depend on B_i include the plasma-β of the transported material that, in all cases, seems to asymptote to order unity. Jets are launched by a combination of the 'magnetic tower' and 'bead-on-a-wire' mechanisms, with the former accounting for most of the jet acceleration - even for strong fields - and continuing well beyond the fast magnetosonic point. At no time does the leading bow shock leave the domain and, as such, these simulations generate large-scale jets that reproduce many of the observed properties of protostellar jets including their characteristic speeds and transported fluxes.