On the Difficulty of Launching an Outflow from an Accretion Disk

We solve for the local vertical structure of a thin accretion disk threaded by a poloidal magnetic field. The angular velocity deviates from the Keplerian value as a result of the radial Lorentz force, but is constant on magnetic surfaces. Angular momentum transport and energy dissipation in the disk are parametrized by an alpha-prescription, and a Kramers opacity law is assumed to hold. We also determine the stability of the equilibria with respect to the magnetorotational (or Balbus-Hawley) instability. If the magnetic field is sufficiently strong, stable equilibria can be found in which the angle of inclination, i, of the magnetic field to the vertical at the surface of the disk has any value in the range [0,90 degrees). By analyzing the dynamics of a transonic outflow in the corona of the disk, we show that a certain potential difference must be overcome even when i > 30 degrees. We determine this potential difference as a function of i for increasing values of the vertical magnetic field strength. For magnetorotationally stable equilibria, the potential difference increases faster than the fourth power of the magnetic field strength, quickly exceeding a value corresponding to the central temperature of the disk, and is minimized with respect to i at i approximately equal to 38 degrees. We show that this property is relatively insensitive to the form of the opacity law. Our results suggest that an additional source of energy, such as coronal heating, may be required for the launching of an outflow from a magnetized disk