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

Abstract

We present the results of 2.5-dimensional MHD simulations of jet formation by magnetic accretion disks in which both ejection andaccretion of disk plasma are included self-consistently. Although thejets in nonsteady MHD simulations have often been described as transientphenomena resulting from a particular choice of initial conditions, wefound that the characteristics of the nonsteady jets are very similar tothose of steady jets: (1) The ejection point of the jet, whichcorresponds to the slow magnetosonic point in steady MHD jet theory, isdetermined by the effective potential resulting from gravitational andcentrifugal forces along a field line. (2) The dependences of thevelocity (vz) and mass outflow rate (Ṁw) onthe initial magnetic field strength are approximatelyṀw~B0andvz~((Ω2FB20)/(Ṁw))1/3~B1/30, where B0 is the initial poloidal magnetic field strengthand 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 steadysolutions, although their explanation is a little more complicated inthe 2.5-dimensional case, because of an avalanche-like accretion flowthat is present. The dependence of the accretion rate(Ṁa) on the initial field strength is given byṀa~Bb0 where b ~ 1.4 from thesimulations and b ~= 2 from the semianalytical results. We also confirmthat the velocity of the jet is of order the Keplerian velocity of thedisk for a wide range of parameters. We conclude that the ejectionmechanism of nonsteady jets found in the 2.5-dimensional simulations canbe understood using the steady state theory even when nonsteadyavalanche-like accretion occurs along the surface of the disk.Nevertheless, it must be stressed that the jet and accretion never reacha steady state in our simulations, in which the back-reaction of the jeton the disk is included self-consistently.