  From Zeeman Doppler images of the young, rapidly-rotating K0 dwarf AB
Doradus, we have created a potential approximation to the observed radial
magnetic field and have evolved it over 30 days due to the observed surface
differential rotation, meridional flow and various diffusion rates. Assuming
that the dark polar cap seen in Doppler images of this star is caused by the
presence of a unipolar field, we have shown that the observed differential
rotation will shear this field to produce the observed high-latitude band of
unidirectional azimuthal field. By cross-correlating the evolved fields each
day with the initial field we have followed the decay with time of the
cross-correlation function. Over 30 days it decays by only 10 percent. This
contrasts with the results of Barnes et al (1998), who show that on this
timescale the spot distribution of He699 is uncorrelated. We propose that this
is due to the effects of flux emergence changing the spot distributions.
