  We present numerical magnetohydrodynamic (MHD) simulations of the effect of
stellar dipole magnetic fields on line-driven wind outflows from hot, luminous
stars. Unlike previous fixed-field analyses, the simulations here take full
account of the dynamical competition between field and flow, and thus apply to
a full range of magnetic field strength, and within both closed and open
magnetic topologies. A key result is that the overall degree to which the wind
is influenced by the field depends largely on a single, dimensionless, `wind
magnetic confinement parameter', $\eta_{\ast}$ ($ = B_{eq}^2 R_\ast^2/{\dot M}
v_\infty$), which characterizes the ratio between magnetic field energy density
and kinetic energy density of the wind. For weak confinement $\eta_{\ast} \le
1$, the field is fully opened by the wind outflow, but nonetheless for
confinements as small as $\eta_{\ast}=1/10$ can have a significant
back-influence in enhancing the density and reducing the flow speed near the
magnetic equator. For stronger confinement $\eta_{\ast} > 1$, the magnetic
field remains closed over a limited range of latitude and height about the
equatorial surface, but eventually is opened into a nearly radial configuration
at large radii.
