  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.
