  We obtain contemporaneous observations of the surface and corona of AB Dor
using ground-based circularly polarised spectra from the Anglo-Australian
Telescope and X-ray data 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.
  The high coronal density and complex multi-polar magnetic field indicate a
compact X-ray corona, which is concentrated close to the surface, with a
height, H~0.3-0.4R*. There is also significant correlation between the surface
and coronal active region locations. At this epoch AB Dor appears to possess
one very large active longitude region; displaying enhanced activity in the
form of large dark spots, strong magnetic fields and chromospheric emission.
  Finally, the level of rotational modulation and shape of the X-ray lightcurve
depend on the distribution of magnetic field in the obscured hemisphere. The
models that best reproduce the rotational modulation observed in the
contemporaneous Chandra X-ray lightcurve 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.
