The transitions between normal and null states of radio pulsars

It is shown that the null-state solution of a previously published polar-cap model has both a finite surface electric field component parallel with the magnetic flux density and a low acceleration potential difference. The null state is not time-independent, but is stable against a transition to the normal (radio emitting) state if the accelerated ions are not completely ionized by interaction with the thermal radiation field. The properties of the normal-null transitions are considered in detail and order of magnitude estimates of the transition speeds are given. (author)