  The magnetic O-star HD191612 exhibits strongly variable, cyclic Balmer line
emission on a 538-day period. We show here that its variable Halpha emission
can be well reproduced by the rotational phase variation of synthetic spectra
computed directly from full radiation magneto-hydrodynamical simulations of a
magnetically confined wind. In slow rotators such as HD191612, wind material on
closed magnetic field loops falls back to the star, but the transient
suspension of material within the loops leads to a statistically overdense, low
velocity region around the magnetic equator, causing the spectral variations.
We contrast such "dynamical magnetospheres" (DMs) with the more steady-state
"centrifugal magnetospheres" of stars with rapid rotation, and discuss the
prospects of using this DM paradigm to explain periodic line emission from also
other non-rapidly rotating magnetic massive stars.
