  We present detailed optical photo-polarimetric observations of the
recently-discovered white dwarf pulsar AR Scorpii. Our extensive dataset
reveals that the polarized emission is remarkably stable and repeatable with
spin, beat and orbital modulations. This has enabled us to construct a simple
geometrical model which assumes that all of the optically polarized emission
emanates from two diametrically opposed synchrotron emission regions on the
white dwarf magnetosphere. We suggest that the observed polarimetric
modulations occur as a result of an enhanced injection of relativistic
electrons into the magnetosphere of the white dwarf as it sweeps past the
M-dwarf. This leads to an increase in synchrotron emission as the injected
electrons accelerate towards each magnetic mirror point close to the magnetic
poles of the white dwarf. Whilst this scenario reproduces the detailed
polarimetric modulations, other suggested scenarios involving emission sites
locked in the white dwarf rotating frame are not ruled out. For example,
pulsar-like particle acceleration as a result of either electric potentials
between the white dwarf and the light cylinder or a striped relativistic
magnetohydrodynamic wind, outside the light cylinder. Previous conclusions that
argued that the observed strong optical beat modulations require that the
optical polarization arises predominantly from or near the M-dwarf are
inconsistent with our observations.
