Electric Field Screening with Back-Flow at Pulsar Polar Cap

TL;DR

This paper investigates how back-flowing particles at the pulsar polar cap can screen electric fields without requiring pair cascades, providing insights into particle dynamics and radio emission mechanisms.

Contribution

It introduces a new analytical and numerical framework for electric field screening at the neutron star surface considering back-flowing particles, expanding understanding beyond pair cascade models.

Findings

Electric field can be screened without pair cascade if ultrarelativistic particles flow back.

Particles with significant density and mild relativistic temperature can adjust current and charge to screen the field.

Quasi-periodic particle ejection may explain observed radio emissions.

Abstract

Recent $\gamma$-ray observations suggest that the particle acceleration occurs at the outer region of the pulsar magnetosphere. The magnetic field lines in the outer acceleration region (OAR) are connected to the neutron star surface (NSS). If copious electron--positron pairs are produced near the NSS, such pairs flow into the OAR and screen the electric field there. To activate the OAR, the electromagnetic cascade due to the electric field near the NSS should be suppressed. However, since a return current is expected along the field lines through the OAR, the outflow extracted from the NSS alone cannot screen the electric field just above the NSS. In this paper, we analytically and numerically study the electric-field screening at the NSS taking into account the effects of the back-flowing particles from the OAR. In certain limited cases, the electric field is screened without significant pair cascade if only ultrarelativistic particles ($\gamma\gg1$) flow back to the NSS. On the other hand, if electron--positron pairs with a significant number density and mildly relativistic temperature, expected to distribute in a wide region of the magnetosphere, flow back to the NSS, these particles adjust the current and charge densities, so that the electric field can be screened without pair cascade. We obtain the condition for the number density of particles to screen the electric field at the NSS. We also find that in ion-extracted case from the NSS, bunches of particles are ejected to the outer region quasi-periodically, which is a possible mechanism of observed radio emission.