Magnetized Current Filaments: Evolution and single particle dynamics
Weibel Instability is considered as a promising candidate to explain the origin of strong sub-equipartition magnetic fields in Gamma Ray Bursts (GRBs) and Supernova Remnants (SNRs). These instabilities arise when relativistic outflows from accreting black holes interact with the ambient medium. The radiation from these systems can be attributed to the motion of the constituent particles. Recent observations have reported significant fractions of polarized radiation. Understanding single particle dynamics holds the key to decode radiation and its polarization properties. In most of the cases, the interstellar medium is magnetized. Hence, studying Weibel Instability in magnetized plasmas is important. With the availability of high intensity lasers and e-e+ fireball beams in conventional accelerators, it is possible to study the physics of such astrophysical processes in scaled laboratory experiments.
The movie shows a two dimensional particle in cell (PIC) simulation of a cold relativistic e-p+ plasma flowing perpendicular to the simulation plane through a background plasma of identical composition at rest but with small temperature (60eV). The system has an initial magnetization of 0.05 with magnetic field pointing along the direction of flow. A transverse magnetic field perturbation in such a system grows as a result of Weibel Instability (WI) where a fluctuating magnetic field bends the electron trajectories in such a a way that it gives rise to current filaments which further amplifies the perturbation. The electron current attains saturation much before the ions play any role due to their small mass.
In the animation shown here, the isosurfaces describe the distribution of the magnetic field energy with their height being proportional to the magnitude. The electrons are described by spheres and the color scale their energies. Trajectories of 64 electrons are shown in order to describe their motion at the single particle level. They can be seen undergoing EXB drift at the edges of the filaments, while the ones inside gyrate under the influence of initial magnetic field.
The filaments with parallel currents attract each other and merge to form larger filaments. The rate of merging decreases with time. Furthermore, filamentation of electrons give rise to charge separation which in turn induces an electrostatic field. The electric and magnetic fields are perpendicular to each other. The electric and the magnetic fields arising from current filaments are limited to a loop of thickness of the order of skin depth (c/⍵pe) surrounding the current filament. The electric and and axial magnetic field together induce an EXB drift in the electrons that move in the loop surrounding the current filaments. This induces an azimuthal rotation in the electron fluid, while inside the filament, the electrons perform Larmor gyrations under the influence of the initial magnetic field.
To reference this movie, use Sinha, U. et al., Magnetized Weibel filaments as a source of circularly polarized light, Bul. Am. Phys. Soc., 57th Annual Meeting of the APS DPP, 60, PO7.00012 (2015), manuscript in preparation.