| Abstract
| - We obtain contemporaneous observations of the surface and corona of AB Doradus (AB Dor), a young single cool star, using ground-based circularly polarized spectra from the Anglo-Australian Telescope and X-ray light curves and spectra from the Chandra satellite. The ground-based data are used to construct surface magnetic field maps, which are extrapolated to produce detailed models of the quiescent corona. The X-ray data serve as a new test for the validity of these coronal models. We find that AB Dor's X-ray corona must be concentrated close to its surface, with a height, H∼ 0.3-0.4 R*; this height is determined by the high coronal density and complex multipolar magnetic field from the surface maps. There is also significant correlation between the positions of surface and coronal active longitudes as determined from the surface spot and magnetic field maps and the X-ray light curve. At this epoch (2002 December) AB Dor appears to possess one very large active longitude region, covering almost half the star; displaying enhanced activity in the form of large dark spots, strong magnetic fields and chromospheric emission. This is unusual as previous surface maps of AB Dor typically display more active regions that span a wider range of longitudes. Finally, the level of rotational modulation and shape of the X-ray light curve depend on the distribution of magnetic field in the obscured hemisphere (AB Dor is inclined by 60°). The models that best reproduce the rotational modulation observed in the contemporaneous Chandra X-ray light curve and spectra require the magnetic field in the obscured hemisphere to be of the same polarity as that in the observed hemisphere. The Sun shows different behaviour, with the leading polarity reversed in the opposite hemisphere. The X-ray observations provide a unique constraint on the magnetic structure in the obscured hemisphere.
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