The Acceleration Mechanism of Resistive Magnetohydrodynamic Jets Launched from Accretion Disks

We analyzed the results of non-linear resistive magnetohydrodynamical (MHD)simulations of jet formation to study the acceleration mechanism ofaxisymmetric, resistive MHD jets. The initial state is a constant angularmomentum, polytropic torus threaded by weak uniform vertical magnetic fields.The time evolution of the torus is simulated by applying the CIP-MOCCT schemeextended for resistive MHD equations. We carried out simulations up to 50rotation period at the innermost radius of the disk created by accretion fromthe torus. The acceleration forces and the characteristics of resistive jetswere studied by computing forces acting on Lagrangian test particles. Since theangle between the rotation axis of the disk and magnetic field lines is smallerin resistive models than in ideal MHD models, magnetocentrifugal accelerationis smaller. The effective potential along a magnetic field line has maximumaround $z \sim 0.5r_0$ in resistive models, where $r_0$ is the radius where thedensity of the initial torus is maximum. Jets are launched after the diskmaterial is lifted to this height by pressure gradient force. Even in thiscase, the main acceleration force around the slow magnetosonic point is themagnetocentrifugal force. The power of the resistive MHD jet is comparable tothe mechanical energy liberated in the disk by mass accretion. Joule heating isnot essential for the formation of jets.Comment: 15 pages, 15 figures, 1 table, accepted for publication in Ap