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
\alpha=j/c\rho_GJ, where \rho_GJ is the corotation charge density. The
customary assumption \alpha\approx 1 is not supported by recent simulations of
pulsar magnetospheres and we study polar caps with arbitrary \alpha. We argue
that no significant activity is generated on field lines with 0<\alpha<1.
Charges are extracted from the star and flow along such field lines with low
energies. By contrast, if \alpha>1 or \alpha<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.Comment: 11 pages, accepted to ApJ