  Historically, the magnetic field geometries of the chemically peculiar Ap
stars were modelled in the context of a simple dipole field. However, with the
acquisition of increasingly sophisticated diagnostic data, it has become clear
that the large-scale field topologies exhibit important departures from this
simple model. Recently, new high-resolution circular and linear polarisation
spectroscopy has even hinted at the presence of strong, small-scale field
structures, which were completely unexpected based on earlier modelling. This
project investigates the detailed structure of these strong fossil magnetic
fields, in particular the large-scale field geometry, as well as small scale
magnetic structures, by mapping the magnetic and chemical surface structure of
a selected sample of Ap stars. These maps will be used to investigate the
relationship between the local field vector and local surface chemistry,
looking for the influence the field may have on the various chemical transport
mechanisms (i.e., diffusion, convection and mass loss). This will lead to
better constraints on the origin and evolution, as well as refining the
magnetic field model for Ap stars. Mapping will be performed using high
resolution and signal-to-noise ratio time-series of spectra in both circular
and linear polarisation obtained using the new-generation ESPaDOnS and NARVAL
spectropolarimeters at the CFHT and Pic du Midi Observatory. With these data we
will perform tomographic inversion of Doppler-broadened Stokes IQUV Zeeman
profiles of a large variety of spectral lines using the INVERS10 magnetic
Doppler imaging code, simultaneously recovering the detailed surface maps of
the vector magnetic field and chemical abundances.
