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Recently Neiner et al. reported that the B2 IV-V star zeta Cas contains aweak magnetic field which varies on the same 5.37 day period found from themodulations of its N V, C IV, and Si IV UV resonance lines. We have studied thetime variable properties of the same resonance lines in greater detail todetermine the physical characteristics of the magnetospheric structureresponsible for them. In our formulation this structure takes the form of anaxisymmetric "disk" similarto those around magnetic He-strong Bp stars. Thisstructure corotates with the star, covering greater or lesser amounts of itsarea during its transit. zeta Cas offers a special case because we observe itfrom a low inclination and yet its magnetic axis is substantially inclined tothe rotation axis. The equivalent width-phase curves show a flat maximum forhalf the cycle, indicating that the disk is extended out of the plane, extendsto the star's surface in the magnetic plane, or both. Synthetic spectra of theline profiles during the maximum and minimum occultation phases can be bestreconciled with a disk geometry in which the resonance lines are formed at aclosed outer edge and along a thin outer layer. We speculate that observed weakredshifted emission is formed in "auroral caps" located near the magnetic polesof the star. We argue that this results from shocks of stagnated wind materialreturning to the star and shocking against the outflowing wind.Comment: 9 pages, 4 figures, accepted by Astronomy & Astrophysics; new version  includes old A&A aa.cls fil

Variations of the ultraviolet resonance lines of the B2 IV-V starζCassiopeiae