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Magnetically Driven Jets from Accretion Disks. III. 2.5-dimensional Nonsteady Simulations for Thick Disk Case

We present the results of 2.5-dimensional MHD simulations of jet formation by magnetic accretion disks in which both ejection and accretion of disk plasma are included self-consistently. Although the jets in nonsteady MHD simulations have often been described as transient phenomena resulting from a particular choice of initial conditions, we found that the characteristics of the nonsteady jets are very similar to those of steady jets: (1) The ejection point of the jet, which corresponds to the slow magnetosonic point in steady MHD jet theory, is determined by the effective potential resulting from gravitational and centrifugal forces along a field line. (2) The dependences of the velocity (vz) and mass outflow rate (Ṁw) on the initial magnetic field strength are approximately where B0 is the initial poloidal magnetic field strength and ΩF is the "angular velocity of the field line" (essentially the Keplerian angular velocity where the jet is ejected). These are consistent with the results of one-dimensional steady solutions, although their explanation is a little more complicated in the 2.5-dimensional case, because of an avalanche-like accretion flow that is present. The dependence of the accretion rate (Ṁa) on the initial field strength is given by Ṁa∝Bb0 where b ~ 1.4 from the simulations and b ≃ 2 from the semianalytical results. We also confirm that the velocity of the jet is of order the Keplerian velocity of the disk for a wide range of parameters. We conclude that the ejection mechanism of nonsteady jets found in the 2.5-dimensional simulations can be understood using the steady state theory even when nonsteady avalanche-like accretion occurs along the surface of the disk. Nevertheless, it must be stressed that the jet and accretion never reach a steady state in our simulations, in which the back-reaction of the jet on the disk is included self-consistently.