SIMULATIONS OF JET FORMATION FROM A MAGNETIZED ACCRETION DISK

Axisymmetric magnetohydrodynamic (MHD) simulations have been made of t he formation of jets from a Keplerian disk threaded by a magnetic fiel d. The disk is treated as a boundary condition, where matter with high specific entropy is ejected with a Keplerian azimuthal speed and a po loidal speed less than the slow magnetosonic velocity, and where bound ary conditions on the magnetic fields correspond to a highly conductin g disk. Initially, the space above the disk, the corona, is filled wit h high specific entropy plasma in the thermal equilibrium in the gravi tational field of the central object. The initial magnetic field is po loidal and is represented by the superposition of the fields of monopo les located below the plane of the disk. The rotation of the disk twis ts the initial poloidal magnetic field lines, and this twist propagate s into the corona pushing matter into jet-like outflow in a cylindrica l region. After the first ''switch-on wave'', which originates during the first rotation period of the inner radius of the disk, the matter outflowing from the disk starts to flow and accelerate in the z-direct ion owing to both the magnetic and pressure gradient forces. The how a ccelerates through the slow magnetosonic and Alfven surfaces and at la rger distances through the fast magnetosonic surface. The flow velocit y of the jet is approximately parallel to the z-axis, with the collima tion mainly a result of the pinching force of the toroidal magnetic fi eld. The energy Bur of the flow increases with increasing magnetic fie ld strength on the disk. Some of the cases studied have been run for l ong times, 60 rotation periods of the inner radius of the disk, and sh ow indications of approaching a stationary state.