The Shape of Pulsar Polar Caps
  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. 
P. N. Arendt, Jr., and J. A. Eilek