Time-dependent pair cascades in magnetospheres of neutron stars I. Dynamics of the polar cap cascade with no particle supply from the neutron star surface

TL;DR

This paper develops a first-principles numerical model of electron-positron pair cascades in neutron star magnetospheres, revealing regular, quasi-periodic discharge cycles and potential implications for pulsar radio emission.

Contribution

It introduces a self-consistent kinetic simulation algorithm for pair cascades, applying it to model the Ruderman-Sutherland process in one dimension, highlighting new insights into cascade dynamics.

Findings

Pair creation occurs in regular, quasi-periodic cycles.

Discharges produce coherent superluminal electrostatic waves.

Cascade adjusts to current density via particle trapping and reversal.

Abstract

I argue that the problem of electromagnetically driven electron-positron cascades in magnetospheres of neutron stars must be addressed starting from first principles. I describe a general numerical algorithm for doing self-consistent kinetic simulations of electron-positron cascades - wherein particle acceleration, pair creation and screening of the electric field are calculated simultaneously - and apply it to model the Ruderman and Sutherland cascade in one dimension. I find that pair creation is quite regular and quasi-periodic. In each cycle a blob of ultrarelativistic electron-positron plasma is generated, it propagates into the magnetosphere leaving a tail of less relativistic plasma behind, and the next discharge occurs when this mildly relativistic plasma leaves the polar cap. A short burst of pair formation is followed by a longer quiet phase when accelerating electric field is screened and no pairs are produced. Some of freshly injected electron-positrons pairs get trapped in plasma oscillations creating a population of low energy particles. The cascade easily adjusts to the current density required by the pulsar magnetosphere by reversing some of the low energy particles. Each discharge generates a strong coherent superluminal electrostatic wave, which may be relevant for the problem of pulsar radioemission.