Expansion of a mildly relativistic hot pair cloud into an electron-proton plasma

TL;DR

This study uses 2D PIC simulations to explore how a hot, relativistic electron-positron pair cloud expands into an ambient plasma, revealing mechanisms for proton acceleration, magnetic field generation, and the slowing of pair outflow.

Contribution

It demonstrates how solitary waves and instabilities facilitate energy transfer, ion acceleration, and magnetic field creation in pair-loaded astrophysical plasma expansions.

Findings

Protons are accelerated to near 1 MeV via solitary waves.

A dilute pair outflow reaches nonrelativistic expansion speeds.

An instability at the cloud front leads to plasma magnetization.

Abstract

The expansion of a charge-neutral cloud of electrons and positrons with the temperature 1 MeV into an unmagnetized ambient plasma is examined with a 2D particle-in-cell (PIC) simulation. The pair outflow drives solitary waves in the ambient protons. Their bipolar electric fields attract electrons of the outflowing pair cloud and repel positrons. These fields can reflect some of the protons thereby accelerating them to almost an MeV. Ion acoustic solitary waves are thus an efficient means to couple energy from the pair cloud to protons. The scattering of the electrons and positrons by the electric field slows down their expansion to a nonrelativistic speed. Only a dilute pair outflow reaches the expansion speed expected from the cloud's thermal speed. Its positrons are more energetic than its electrons. In time an instability grows at the front of the dense slow-moving part of the pair cloud, which magnetizes the plasma. The instability is driven by the interaction of the outflowing positrons with the protons. These results shed light on how magnetic fields are created and ions are accelerated in pair-loaded astrophysical jets and winds.