Three-dimensional Weibel instability in astrophysical scenarios

Abstract

Near equipartition magnetic fields are predicted by gamma ray bursters models and astronomical observations, in general associated with shocks or regions with colliding streams of particles. These scenarios require the conversion of kinetic energy in the outgoing plasma shells into B-fields. How the magnetic fields are generated and how particles are accelerated is still an open question, that can only be definitely addressed via fully kinetic three-dimensional (3D) numerical simulations. These shocks are collisionless because dissipation is dominated by wave–particle interactions, i.e., it is accomplished by particle scattering in turbulent electromagnetic fields generated at the shock front, or equivalently the mean free path is much longer than the shock front thickness (a few collisionless skin depths or a few Larmor radii, in magnetized plasmas). Plasma instabilities driven by streaming particles, such as the Weibel instability, are responsible for the excitation of these turbulent electromagnetic fields. Three-dimensional fully kinetic electromagnetic relativistic particle-in-cell simulations for the collision of two interpenetrating plasma shells were performed using the code OSIRIS [Fonseca et al., Lect. Notes Comput. Sci. 2331, 342 (2002)], showing (i) the generation of long-lived near-equipartition quasistatic (electro)magnetic fields, (ii) nonthermal particle acceleration, and (iii) short-scale to long-scale B-field evolution. These results may be important to understand magnetic field generation and particle acceleration in relativistic collisionless shock fronts, in gamma ray bursters, pulsar winds, and radio supernovae, and open the way to the full 3D kinetic modeling of relativistic shocks.