Hybrid Numerical Simulations of Pulsar Magnetospheres

TL;DR

This paper presents a hybrid simulation approach to study particle acceleration in pulsar magnetospheres, revealing the necessity of additional positronic currents from the stellar surface and providing analytic models for particle orbits and magnetosphere structure.

Contribution

It introduces a self-consistent hybrid method combining force-free electrodynamics with particle acceleration, advancing understanding of charge replenishment in pulsar equatorial current sheets.

Findings

Charge flow from polar caps is insufficient to sustain ECS currents.

Additional positronic currents originate from the stellar surface.

Magnetosphere structure approaches ideal force-free configuration for high pair-formation multiplicity.

Abstract

We continue our investigation of particle acceleration in the pulsar equatorial current sheet (ECS) that began with Contopoulos (2019) and Contopoulos & Stefanou (2019). Our basic premise has been that the charge carriers in the current sheet originate in the polar caps as electron-positron pairs, and are carried along field lines that enter the equatorial current sheet beyond the magnetospheric Y-point. In this work we investigate further the charge replenishment of the ECS. We discovered that the flow of pairs from the rims of the polar caps cannot supply both the electric charge and the electric current of the ECS. The ECS must contain an extra amount of positronic (or electronic depending on orientation) electric current that originates in the stellar surface and flows outwards along the separatrices. We develop an iterative hybrid approach that self-consistently combines ideal force-free electrodynamics in the bulk of the magnetosphere with particle acceleration along the ECS. We derive analytic approximations for the orbits of the particles, and obtain the structure of the pulsar magnetosphere for various values of the pair-formation multiplicity parameter kappa. For realistic values kappa >> 1, the magnetosphere is practically indistinguishable from the ideal force-free one, and therefore, the calculation of the spectrum of high-energy radiation must be based on analytic approximations for the distribution of the accelerating electric field in the ECS.