Particle-in-cell simulations of plasma shocks and instabilities in relativistic jets

Particle-in-cell (PIC) simulations of collisionless shock formation and instability growth, such as the Weibel instability, the kinetic Kelvin-Helmholtz instability (kKHI), and the Mushroom instability (MI), have been performed to study magnetic field generation, particle acceleration, and emission of radiation with applications to astrophysical plasma jets, like those of active galactic nuclei (AGN) and gamma-ray bursts (GRBs).

Nishikawa et al. (2016) were the first to present results of numerical PIC simulations of global relativistic jets containing helical magnetic fields. The presence of helical magnetic fields in the jets is suggested by twisted structures that have been observed in many AGN jets, from sub- to kiloparsec scales.

The main result obtained by Nishikawa et al. (2016) has revealed that new types of shocks, similar to recollimation shocks attained in relativistic magnetohydrodynamic (MHD) simulations, occur when a relativistic plasma jet contains a helical magnetic field. In the (e--p+) jet case (left figure), recollimation-like shocks are developed. In the (e--e+) jet case (right figure), small recollimation structures are initially formed, and after instabilities have grown, the currents expand outside the jet and the current density becomes turbulent.

These studies were extended by Nishikawa et al. (2020) to the nonlinear stage of the growth of kinetic instabilities to explore acceleration of particles via magnetic reconnection in relativistic jets that contain initial helical magnetic fields, using a larger simulation system. In the nonlinear stage, the helical magnetic fields are untangled under the action of reconnection-like processes and disappear. The disappearance of the helical fields generates magnetic field islands, which then interact with each other producing turbulent magnetic fields. In these turbulent fields, further reconnection events take place. Then, the turbulent reconnection explosively converts the magnetic field energy into kinetic energy of the plasma, so that the jet electrons are rapidly accelerated. The ambient electrons entrained in the relativistic jet are strongly accelerated as well.

ISI papers:

Non-ISI papers:

Talks: