Electron positron pair flow and current composition in the pulsar magnetosphere

TL;DR

This study uses ab-initio PIC simulations to explore electron-positron pair flow and current structure in pulsar magnetospheres, revealing how plasma distribution and particle trajectories shape the global electromagnetic configuration.

Contribution

It demonstrates that pair production near the neutron star surface suffices to form a force-free magnetosphere, differing from models with widespread particle injection.

Findings

Electrons and positrons follow distinct trajectories, influencing current layers.

Pair production outside the surface is not essential for plasma filling.

Energetic positrons are accelerated near the Y-point.

Abstract

We performed ab-initio Particle-In-Cell (PIC) simulations of a pulsar magnetosphere with electron-positron plasma produced only in the regions close to the neutron star surface. We study how the magnetosphere transitions from the vacuum to a nearly force-free configuration. We compare the resulting force-free like configuration with ones obtained in a PIC simulation where particles are injected everywhere as well as with macroscopic force-free simulations. We found that although both PIC solutions have similar structure of electromagnetic fields and current density distributions, they have different particle density distribution. In fact in the injection from the surface solution, electrons and positrons counterstream only along parts of the return current regions and most of the particles leave the magnetosphere without returning to the star. We also found that pair production in the outer magnetosphere is not critical for filling the whole magnetosphere with plasma. We study how the current density distribution supporting the global electromagnetic configuration is formed by analyzing particle trajectories. We found that electrons precipitate to the return current layer inside the light cylinder and positrons precipitate to the current sheet outside the light cylinder by crossing magnetic field lines contributing to the charge density distribution required by the global electrodynamics. Moreover, there is a population of electrons trapped in the region close to the Y-point. On the other hand the most energetic positrons are accelerated close to the Y-point. These processes can have observational signatures that, with further modeling efforts, would help to distinguish this particular magnetosphere configuration from others.