Magnetic field dragging and the vertical structure of thin accretion discs

The presence of an imposed vertical magnetic field may drastically influenc e the structure of thin accretion discs. If the field is sufficiently stron g, the rotation law can depart from the Keplerian one. We consider the stru cture of a disc for a given eddy magnetic diffusivity but neglect details o f the energy transport. The magnetic field is assumed to be in balance with the internal energy of the accretion flow. The thickness of the disc as we ll as the turbulent magnetic Prandtl number and the viscosity, alpha, are t he key parameters of our model. The calculations show that the radial veloc ity can reach the sound speed for a magnetic disc if the thickness is compa rable to that of a non-magnetic one. This leads to a strong amplification o f the accretion rate for a given surface density. The inclination angle of the magnetic field lines can exceed the critical value 30 degrees (required to launch cold jets) even for a relatively small magnetic Prandtl number o f order unity. The toroidal magnetic fields induced at the disc surface are smaller than predicted in previous studies.