A bistable pulsar magnetosphere: nulls and mode changes

Abstract

It is shown that the ion-proton magnetosphere is unstable in a limited area of the P −Ṗ 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.

Complex emission patterns: fluctuations and bistability of polar-cap potentials

“…In the case of nulls, the primary change may be above the band, whereas in mode-changes it is likely to be outside the observable band in a region of larger Φ ref with only secondary changes above the band. Periodicity is the only difference from the mode-change and null mechanisms described previously (Jones 2018(Jones , 2020. Pulse sequences observed by Yan et al have a well-defined LRFS frequency but do not appear as a high-Q system.…”

“…Thus in the limit of large Φ ref an inverse Compton scattering process may support self-sustaining pair creation. A previous publication (Jones 2018) has described this state for a symmetric polar cap. Outward plasma consists of protons, and small fluxes of positrons and low-energy secondary pairs.…”

A model of an ion-proton radio pulsar polar cap

“…A previous paper (Jones 2018) noted that the giant dipole state is also a source of neutrons at an approximately equal rate to that of protons which, through capture γ-rays, would be a source of pair creation above the polar cap in neutron stars with suitable magnetic field. This indicated the possibility of a self-sustaining inverse Compton scattering process which would be a further mechanism for nulls and mode-changes.…”

“…In a previous paper (Jones 2018) we have expressed the view that this remarkable lack of progress is a direct consequence of assumptions about electron-positron pair production above polar caps for which there is no physical basis. More generally, recent work on the equatorial current sheet has emphasized that the force-free model of the magnetosphere close to the light cylinder does not require electron-positron production anywhere in the magnetosphere to maintain a stable net charge on the star (see Contopoulos 2016).…”

“…This is difficult to reconcile with E × B motion above the polar cap. Mode-changes have been considered in a previous publication (Jones 2018). It may be possible to show that sub-pulse drift can be maintained in a linear and quasi-stable manner starting from a suitable initial state, but this has not yet been achieved.…”