Magnetically driven jets from accretion disks .2. Nonsteady solutions and comparison with steady solutions

We perform time-dependent one-dimensional (1.5-dimensional) magnetohydrodynamic numerical simulations of astrophysical jets that are magnetically driven from Keplerian disks, in order to study the origin and structure of jets ejected from star-forming regions, close binary systems, and active galactic nuclei. We study the initial-value problem, in which the Keplerian disk threaded by the poloidal magnetic field suddenly begins to rotate and twists the field line, generating ''nonsteady jets'' by the J x B force. This is similar to the problem treated by Shibata & Uchida in their two-dimensional (2.5-dimensional) simulations. The main purpose of this study is to clarify the physical relation between such ''nonsteady jets'' and steady jets, by using one-dimensional (1.5-dimensional) simulations. The one-dimensional (1.5-dimensional) simulation has the merit that we can perform simulations over many disk orbital periods with large computational regions in a wide range of parameters. We find that the jets, which are ejected from the disk, have the same properties as the steady magnetically driven jets: (1) The mass flux of the nonsteady jet strongly depends on the angle between the disk's surface and the magnetic field line. (2) The scaling law known as Michel's solution is also satisfied by the nonsteady jets. (3) The magnetic energy dependence of the mass flux of the nonsteady jet is consistent with that of the steady one. One of the most important findings in this study is that, even when the initial poloidal magnetic field is very weak in the disk [e.g., E(mg) = (magnetic energy/gravitational energy) similar to 10(-6)], a jet with a speed on the order of the Keplerian velocity is produced by the effect of magnetic pressure force in the toroidal fields generated from the poloidal fields by the rotation of the disk. We also find several new nonsteady phenomena, which cannot be found from the steady models but may be important for application: MHD fast and slow shocks inside the jets, and quasi-periodic mass ejections from the disk by large-amplitude Alfven waves.