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In order to map the spatial structure of a stellar atmosphere using Doppler imaging techniques, observations of a rapidly-rotating star must be obtained at all of its rotational phases. In order to discriminate temporaral variability (e.g. flares) from phase-locked variability (produced by magnetically-active regions on the stellar surface), observations must be obtained over at least two rotational cycles. Using the International Ultraviolet Explorer, we observed the bright RS~CVn-type system HR~1099 (V711~Tau) continuously for two contiguous orbital/rotational cycles (period$\sim$2.83~days) in December~1992.
These observations were coordinated with the 1992 campaign for Multi-Site Continuous Spectroscopy (MUSICOS). The purpose of this campaign was to coordinate a network of ground-based telescopes distributed around the Earth so that moderate and high-resolution spectroscopy could be obtained pseudo-continuously for several days. Supporting photometric and radio observations also were obtained simultaneously.
We are using the IUE high-dispersion spectra of the Mg~II~h and k lines to map the spatial structure of the stellar chromosphere and to study atmospheric dynamics during flares. In conjunction with the IUE low-dispersion spectra, we will be able to model the radial structure of the chromosphere. We present preliminary results showing the rotational modulation of the ultraviolet line fluxes and of the high-dispersion line profiles. At least 3 transition-region flares occurred within this 6-day interval, and our observations constrain both the rise and decay phases of these flares. The effect of a non-uniform atmospheric structure is subtle, but it is visible in the observed line profiles.
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