Polar caps in the presence of an induction field

TL;DR

This paper investigates the effects of an induction field on pulsar polar caps, showing plasma drift influences charge stability and electron acceleration, aligning with gamma-ray observations of radio-loud pulsars.

Contribution

It demonstrates that the induction field causes plasma drift, affecting charge stability and electron acceleration without altering the radio emission region physics.

Findings

Plasma drift velocity maintains stable charge bands.

Electron injection occurs in the outer magnetosphere gap.

Radio-loud pulsars have positive polar-cap charge density.

Abstract

Following the early paper of Goldreich & Julian (1969), polar-cap models have usually assumed that the closed sector of a pulsar magnetosphere corotates with the neutron star. Recent work by Melrose & Yuen has been a reminder that in an oblique rotator, the induction field arising from the time-varying magnetic flux density cannot be completely screened. The principal consequence is that the plasma does not corotate with the star. Here it is shown that the physics of the polar cap is not changed at the altitudes of the radio emission source. But the presence of a plasma drift velocity in the corotating frame of reference does provide a mechanism whereby the net charge of the star can be maintained within a stable band of values. It also shows directly how electron injection and acceleration occur in the outer gap of the magnetosphere. It is consistent with radio-loud pulsars in the Fermi LAT catalogue of gamma-emitters all having positive polar-cap charge density.