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 ^ { \pm } 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 Alfvén waves .
These waves are ducted along the tube and converted to electromagnetic waves , providing a new mechanism for pulsar radiation .