Despite the extensive knowledge of the characteristics of the coherent radio emission, the mechanism is not understood. The high-energy radiation is incoherent and may be related to the flux of relativistic electrons and positrons in a current sheet at the boundary of the magnetosphere. The radio emission from the polar cap is at lower frequency at larger radii, as the magnetic field lines diverge. The emission may be affected by propagation through the polar cap; refraction along the magnetic field lines may increase the apparent pulse width at lower frequencies.