Particle-in-cell simulations of the twisted magnetospheres of magnetars

TL;DR

This paper uses particle-in-cell simulations to explore how twisted magnetic fields in magnetar magnetospheres generate plasma, sustain electric currents, and evolve over time through resistive untwisting and particle acceleration.

Contribution

It provides the first direct numerical simulation of plasma creation and evolution in a twisted magnetar magnetosphere, revealing self-regulation of electric discharge and long-term magnetic untwisting.

Findings

Formation of an electric gap with unscreened electric field

Self-regulation of accelerating voltage at pair creation threshold

Gradual resistive untwisting of the magnetosphere and shrinking hot spots

Abstract

The magnetospheres of magnetars are believed to be filled with electron-positron plasma generated by electric discharge. We present a first direct numerical experiment showing how the plasma is created in an axisymmetric closed magnetosphere. The $e^\pm$ discharge occurs in response to twisting of the magnetic field lines by a shear deformation of the magnetar surface, which launches electric currents into the magnetosphere. The simulation shows the formation of an electric "gap" with unscreened electric field ($\mathbf{E}\cdot \mathbf{B}\neq 0$) that continually accelerates particles along the magnetic field lines and sustains pair creation. The accelerating voltage is self-regulated to the threshold of the $e^\pm$ discharge. It controls the rate of energy release and the lifetime of the magnetic twist. The simulation follows the global evolution of the twisted magnetosphere over a long time and demonstrates its gradual resistive untwisting. A vacuum cavity forms near the star and expands, gradually erasing magnetospheric electric currents $j$. The active j-bundle shrinks with time and its footprints form shrinking hot spots on the magnetar surface bombarded by the created particles.