  It is shown that the ion-proton magnetosphere is unstable in a limited area
of the P - Pdot plane against transitions to a self-sustaining inverse Compton
scattering mode in which the particles accelerated are mainly protons with a
small component of positrons. It is argued that this mode cannot be absolutely
stable. The number density of any outward-moving pair plasma is small and
electron and positron Lorentz factors too high to support growth of any
collective mode capable of exciting normal pulsar coherent radio emission.
Particle fluxes and the position at which they pass through the light cylinder
are mode-dependent and in principle, transitions can be accompanied by changes
in spin-down torque. The properties of the system are discussed in relation to
observations of nulls, mode-changes, and the group of long-term intermittent
pulsars.
