  We present detailed semi-empiric models for rotational modulations observed
in ultraviolet wind lines of B0.5 supergiant HD 64760. We model the Rotational
Modulation Regions (RMRs) with advanced 3-D radiative transfer calculations in
the stellar wind and quantitatively fit the time-evolution of the Si IV 1395
resonance line. We find that the RMRs are due to linearly-shaped narrow
sector-like density enhancements in the equatorial wind. Unlike the Co-rotating
Interaction Regions (CIRs) which produce Discrete Absorption Components in the
line, the RMRs do not spread out with larger distance above the stellar
surface. The detailed best fit shows that the RMRs of HD 64760 have maximum
density enhancements of ~17% above the surrounding smooth wind density, about
twice smaller than the hydrodynamic models of CIRs that warp around the star.
The semi-empiric 3-D transfer modelling reveals that the narrow spoke-like RMRs
must co-exist with broader and curved large-scale CIR wind density structures
in the equatorial plane of this fast rotating Ib-supergiant.
