  Analysis of plasma acceleration in pulsars with positive corotational charge
density has shown that any element of area on the polar cap is bi-stable: it
can be in phases either of pure proton emission or of mixed ions and protons
(the ion phase). Ion-phase zones are concentrated near the edge of the polar
cap, and are a physical basis for the coherent radio emission observed as
components within the mean pulse profile. The state of the polar cap is
generally chaotic, but organized linear motion of ion zones in a peripheral
band is possible and is the likely source of sub-pulse drift. It is shown that
several patterns of limited movement are possible and can account for the
varied phenomena observed including mirror and bi-directional drifting.
