Mass accretion processes in magnetic fields - Formation of quasi-Keplerian discs

An axisymmetric, steady-state solution is obtained for geometrically thin accretion discs in external magnetic fields. The main features of this solution are as follows. The azimuthal velocity of a disc plasma is somewhat reduced from the Keplerian value owing to the pressure effect enhanced by the inclusion of a magnetic field. The magnetic stress takes the place of viscous stress in the standard disc model, and extracts angular momentum from the disc. About a half of the gravitational energy is released in the disc, through the Joule dissipation and the work done against the pressure force. The vertical flow and current are also included in the calculation. Analogously to the parameter α in the standard model, the author's solution contains essentially one parameter Δ which specifies the size of the electrical resistivity.