2.5‐dimensional Nonsteady Magnetohydrodynamic Simulations of Magnetically Driven Jets from Geometrically Thin Disks

Abstract

We have performed self-consistent 2.5-dimensional nonsteady MHD numerical simulations of jets from geometrically thin disks including the dynamics of accretion disks. For the initial rotational velocity of the disk, we consider two cases, the Keplerian case and the sub-Keplerian case. We compare our results with the thick-disk case in detail. The characteristics of nonsteady jets from geometrically thin disks in our Keplerian case are similar to those of the steady theory and thick-disk cases: (1) The ejection point of the jets corresponds to the slow magnetosonic point, which is determined by the effective potential made by the gravitational and centrifugal forces along the magnetic field. (2) The dependences of the velocity (Vjet) and the mass outflow rate (w) on the initial magnetic field strength (B0) are w ∝ B0 and Vjet ∝ 1/3, therefore Vjet ∝ B, where ΩF is the angular velocity of a field line. Although this dependence of the velocity corresponds to Michel's scaling law, the velocity of our simulated jets still does not reach the fast magnetosonic velocity. In the sub-Keplerian case, the relation Vjet ∝ B is satisfied, but the other dependences are not necessarily equal to those of the Keplerian case. The velocity of the jets is larger when the initial rotational velocity of the disk is smaller. The initial acceleration force on the jets is the magnetic pressure when the initial magnetic field is weak, while the centrifugal force is dominant when the initial magnetic field is strong. Finally, we found two interesting phenomena in the sub-Keplerian cases: one is knotlike structures around the rotational axis, and the other is outflow along the disk surface.