  Protons produced in electromagnetic showers formed by the reverse-electron
flux are usually the largest component of the time-averaged polar-cap open
magnetic flux-line current in neutron stars with positive corotational charge
density. Although the electric-field boundary conditions in the corotating
frame are time-independent, instabilities on both medium and short time-scales
cause the current to alternate between states in which either protons or
positrons and ions form the major component. These properties are briefly
discussed in relation to nulling and microstructure in radio pulsars, pair
production in an outer gap, and neutron stars with high surface temperatures.
