Structure of a collisionless pair jet in a magnetized electron-proton plasma: flow-aligned magnetic field

TL;DR

This study uses PIC simulations to analyze how a guiding magnetic field influences the formation and structure of a collisionless pair jet in an electron-proton plasma, revealing complex shock and particle acceleration phenomena.

Contribution

It provides new insights into the jet structure, shock formation, and particle acceleration mechanisms in a magnetized collisionless plasma with a guiding magnetic field.

Findings

A jet forms with an outer cocoon separated by an electromagnetic piston.

A supercritical fast magnetosonic shock is expected to form beyond simulation time.

Positron escape upstream leads to MeV positron excess ahead of the jet.

Abstract

We study the effect a guiding magnetic field has on the formation and structure of a pair jet that propagates through a collisionless electron-proton plasma at rest. We model with a PIC simulation a pair cloud with the temperature 400 keV and mean speed 0.9c. The cloud propagates through a spatially uniform, magnetized and cool ambient electron-proton plasma that is at rest. Its mean velocity vector is aligned with the background magnetic field. A jet forms in time. Its outer cocoon consists of jet-accelerated ambient plasma and is separated from the inner cocoon by an electromagnetic piston with a thickness that is comparable to the thermal gyroradius of jet particles. A supercritical fast magnetosonic shock will form between the pristine ambient plasma and the jet-accelerated one on a time scale that exceeds our simulation time by an order of magnitude. The inner cocoon is pair plasma that lost its directed flow energy while it swept out the background magnetic field. A beam of electrons and positrons moves along the jet spine at its initial speed. Its electrons are slowed down and some positrons are accelerated as they cross the jet's head. The latter escape upstream along the magnetic field, which yields an excess of MeV positrons ahead of the jet. Some of the protons, which were located behind the electromagnetic piston at the time it formed, are accelerated to MeV energies