Magnetohydrodynamical Winds from an Accretion Disk: Driving by the Magnetic Pressure and Tension of Toroidal Magnetic Fields Magnetohydrodynamical Winds from an Accretion Disk--Driving by the Magnetic Pressure and Tension of Toroidal Magnetic Fields

In order to describe the magnetohydrodynamical (MHD) winds/jetsfrom a geometrically thin accretion disk with rotation, MHD equations are written in orthogonal curvilinear coordinates with axes parallel and perpendicular to the streamlines, respectively. In contrast to the current models of MHD winds/jets, only the toroidal magnetic fields are takeninto consideration. It is demonstrated using a simple model that steady rotating wind-type MHD jets can be accelerated by the magnetic pressure and tension of toroidal magnetic fields. When the streamlines are diverging, MHD winds are a centrifugally-driven type, like that from a rotating star without a magnetic field. In such a case, the magnetic pressure initiateswind near the disk, although the magnetic tension acts to suppress any acceleration of the flow. For almost vertical streamlines, the magnetic pressure of the toroidal fields drives steady jets which have a terminal speed comparable to the rotational speed of the disk. Finally, if the Alfven speed near the disk is significantly larger than the Keplerian speed, the streamlines converge towards the rotation axis. In such a case, it is expected that highly collimated jets are driven by the magnetic tension of toroidalfields.