Unipolar Induction Star-Disk Interaction
Some critical comments on the prevailing model of star-disk interaction are made, in particular, on the rotating nature of the magnetic field lines and on the application of the magnetohydrodynamic frozen-field theorem to the disk plasma. As an alternative, a unipolar induction model is proposed, where the magnetic field is stationary in space and the stellar unipolar electric field $\vec E_{*}$ on the surface is uploaded to the magnetosphere. Through the Poynting vector, the star and the magnetosphere form a coupled system where the total angular momentum, consisting the mechanical one of the star, the electromagnetic one of the magnetosphere, and the mechanical one of the plasma in the magnetosphere, is conserved. The stellar interaction with the accretion disk is through the projection of the unipolar electric field $\vec E_{*}$ onto the disk via the equipotential field lines, generating a disk current and consequently toroidal fields with opposite signs on both sides of the disk, with return current loops via the stellar surface. As a result, magnetic flux is added to the magnetospheric field in the northern and southern hemispheres with the disk current sheet as the boundary condition. This makes the star-disk system an astrophysical site where intense magnetic fields are generated through the rotational energy of the star. Angular momentum extraction from this star-disk system happens as the magnetic flux in the magnetosphere increases to the point that exceeds the current carrying capacity of the disk, leading to a mega scale magnetic eruption sending Poynting fluxes to space either isotropically or beamed.