Polar-cap accelerator and radio emission from pulsars

Abstract

Electric currents j flow along the open magnetic field lines from the polar caps of neutron stars. Activity of a polar cap depends on the ratio α=j/cρ_GJ, where ρ_GJ is the corotation charge density. The customary assumption α\approx 1 is not supported by recent simulations of pulsar magnetospheres and we study polar caps with arbitrary α. We argue that no significant activity is generated on field lines with 0<α<1. Charges are extracted from the star and flow along such field lines with low energies. By contrast, if α>1 or α<0, a high voltage is generated, leading to unsteady e^+- discharge on a scale-height smaller than the size of the polar cap. The discharge can power observed pulsars. Voltage fluctuations in the discharge imply unsteady twisting of the open flux tube and generation of Alfven waves. These waves are ducted along the tube and converted to electromagnetic waves, providing a new mechanism for pulsar radiation.