Pulsar polar cap

A pulsar polar cap is the region on the stellar surface threaded by magnetic field lines that reach the outer magnetosphere rather than closing close to the star.

For an aligned dipole whose last relevant field line reaches equatorial radius $R_e$, the polar-cap opening angle obeys ${\sin }^2{\theta }_{\ast }=R_{\ast }/R_e$. It is approximately $\sqrt{R_{\ast }/R_e}$ for a small cap.