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.
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