The magnetic geometry of magnetic-dominated thin accretion disks

The presence of an imposed external magnetic field drastically influences the structure of thin accretion disks. The magnetic field energy is assumed to be in balance with the thermal energy of the accretion flow, and a diffusion approximation simulates the (vertical) energy transport. Our main result is that inside the corotation radius the resulting radial inclination i of the magnetic field lines from the rotation axis easily exceeds the critical value 30 (required to launch cold jets) even for magnetic Prandtl numbers of order unity. The self-consistent consideration of both magnetic field and accretion flow demonstrates only a weak dependence of the inclination angle on the magnetic Prandtl number. The surface values of the toroidal magnetic fields necessary to induce considerably high values for the radial inclination are much smaller than expected. As the innermost part of the disk produces the largest B tor, the largest radial inclination can also be expected there. The idea is therefore supported that the cold jets are basically launched in the central disk area.