Particle acceleration in colliding flows: Binary star winds and other double-shock structures.

A shock wave propagating perpendicularly to an ambient magnetic field accelerates particles considerably faster than in the parallel propagation regime. However, the perpendicular acceleration stops after the shock overruns a circular particle orbit. At the same time, it may continue in flows resulting from supersonically colliding plasmas bound by a pair of perpendicular shocks.

First-Principles Fermi Acceleration in Magnetized Turbulence.

This Letter provides a concrete implementation of Fermi's model of particle acceleration in magnetohydrodynamic (MHD) turbulence, connecting the rate of energization to the gradients of the velocity of magnetic field lines, which it characterizes within a multifractal picture of turbulence intermittency. It then derives a transport equation in momentum space for the distribution function. This description is shown to be substantiated by a large-scale numerical simulation of strong MHD turbulence.

Saturation of the asymmetric current filamentation instability under conditions relevant to relativistic shock precursors.

The current filamentation instability, which generically arises in the counterstreaming of plasma flows, is known for its ability to convert the free energy associated with anisotropic momentum distributions into kinetic-scale magnetic fields. The saturation of this instability has been extensively studied in symmetric configurations where the interpenetrating plasmas share the same properties (velocity, density, temperature). In many physical settings, however, the most common configuration is that of asymmetric plasma flows.

Physics of relativistic collisionless shocks. III. The suprathermal particles.

In this third paper of a series, we discuss the physics of the population of accelerated particles in the precursor of an unmagnetized, relativistic collisionless pair shock. In particular, we provide a theoretical estimate of their scattering length l_{scatt}(p) in the self-generated electromagnetic turbulence, as well as an estimate of their distribution function. We obtain l_{scatt}(p)≈γ_{p}ε_{B}^{-1}(p/γ_{∞}mc)^{2}c/ω_{p}, with p the particle momentum in the rest frame of the shock front, ε_{B} the strength parameter of the microturbulence, γ_{p} the Lorentz factor of the background plasma relative to the shock front, and γ_{∞} its asymptotic value outside the precursor.

Physics of relativistic collisionless shocks. II. Dynamics of the background plasma.

In this second paper of a series, we discuss the dynamics of a plasma entering the precursor of an unmagnetized, relativistic collisionless pair shock. We discuss how this background plasma is decelerated and heated through its interaction with a microturbulence that results from the growth of a current filamentation instability in the shock precursor. We make use, in particular, of the reference frame R_{w} in which the turbulence is mostly magnetic.

Physics of relativistic collisionless shocks: The scattering-center frame.

In this first paper of a series dedicated to the microphysics of unmagnetized, relativistic collisionless pair shocks, we discuss the physics of the Weibel-type transverse current filamentation instability that develops in the shock precursor, through the interaction of an ultrarelativistic suprathermal particle beam with the background plasma. We introduce in particular the notion of the "Weibel frame," or scattering center frame, in which the microturbulence is of mostly magnetic nature. We calculate the properties of this frame, using first a kinetic formulation of the linear phase of the instability, relying on Maxwell-Jüttner distribution functions, then using a quasistatic model of the nonlinear stage of the instability.

Physics of Weibel-Mediated Relativistic Collisionless Shocks.

We develop a comprehensive theoretical model of relativistic collisionless pair shocks mediated by the current filamentation instability. We notably characterize the noninertial frame in which this instability is of a mostly magnetic nature, and describe at a microscopic level the deceleration and heating of the incoming background plasma through its collisionless interaction with the electromagnetic turbulence. Our model compares well to large-scale 2D3V particle-in-cell simulations, and provides an important touchstone for the phenomenology of such plasma systems.