1999ApJ...524.1030L

The inner region of the accretion disk onto a rotating magnetized central star (neutron star, white dwarf, or T Tauri star) is subjected to magnetic torques that induce warping and precession of the disk. The origin of these torques lies in the interaction between the surface current on the disk and the horizontal magnetic field (parallel to the disk) produced by the inclined magnetic dipole: the warping torque relies on the radial surface current generated by the twisting of the vertical field threading the disk, while the precessional torque relies on the azimuthal screening current due to the diamagnetic response of the disk. Under quite general conditions, there exists a magnetic warping instability in which the magnetic torque drives the disk plane away from the equatorial plane of the star toward a state in which the disk normal vector is perpendicular to the spin axis. Viscous stress tends to suppress the warping instability at large radii, but the magnetic torque always dominates as the disk approaches the magnetosphere boundary. The magnetic torque also drives the tilted inner disk into retrograde precession (opposite to the rotation of the disk) around the stellar spin axis. Moreover, resonant magnetic forcing on the disk can occur, which may affect the dynamics of the disk. The magnetically driven warping instability and precession may be related to a number of observational puzzles. Examples include (1) spin evolution of accreting X-ray pulsars: it is suggested that the observed torque reversal of the disk-fed magnetized neutron stars is associated with the wandering of the inner disk around the preferred perpendicular state; (2) quasi-periodic oscillations (QPOs) in low-mass X-ray binaries: the magnetic torque induces disk tilt, making it possible to explain the observed low-frequency QPOs using disk precession; (3) superorbital periods in a number of X-ray binaries as a result of warped, precessing disks; and (4) photometric period variations of T Tauri stars.