Instabilities, nulls and subpulse drift in radio pulsars

This paper continues a previous study of neutron stars with positive polar-cap corotational charge density in which free emission of ions maintains the surface electric field boundary condition E . B = 0. The composition of the accelerated plasma on any subset of open magnetic flux lines above the polar cap alternates between two states: protons or positrons and ions, of which the proton state cannot support electron-positron pair creation at higher altitudes. The two states coexist at any instant of time above different moving elements of area on the polar cap and provide a physically consistent basis for a description of pulse nulls and subpulse drift. In the latter case, it is shown that the band separation P-3 is determined not by the E x B drift velocity, as is generally assumed, but by the diffusion time for protons produced in reverse-electron showers to reach the region of the atmosphere from which they are accelerated. An initial comparison is made with the survey of subpulse drift published by Weltevrede et al.