  Rotation distorts the vacuum magnetic field of a pulsar from that of a simple
dipole. The effect is particularly strong close to the light cylinder, but also
affects the field close to the stellar surface. We find the shape and locus of
the field lines which just close at the light cylinder. Their footpoints define
the pulsar polar cap. We find this cap is asymmetric and distorted. We also
find that the polar cap is not strongly elliptical. This result disagrees with
calculations based on the non-rotating dipole field. We present our numerical
results, and discuss consequences for interpretation of mean profiles and for
pulsar statistics.
