Since the plasma surrounding a high energy astrophysical object has extremely high energy, relativistic effects need to be taken into account in order to understand the phenomena occurring there. Basic properties of a relativistic plasma have been unresolved yet. We study some relaxation processes occurring in relativistic plasmas using theory, modeling, and numerical simulations.

Parametric instabilities of a relativistic nonlinear Alfven wave

In the vicinity of some high energy astrophysical objects, very large amplitude relativistic Alfven waves are though to be generated. Generally speaking, when wave amplitude becomes large, a variety of side band waves are generated through wave-wave interactions. This process is called parametric instabilities. We were successful in solving the dispersion relation of parametric instabilities of a relativistic Alfven wave and conducted numerical simulations to examine long time evolution of the instabilities. We found that type of parametric instability changes depending on the initial energy and frequency of a parent Alfven wave. It was also found that acceleration efficiency of particles is extremely enhanced when initial wave energy becomes large. We have discussed the relativistic effect playing crucial roles in these phenomena. 

 Spatio-temporal evolution of magnetic field in a parametric instability and some typical particle orbits

Synchrotron maser instability

In some astrophysical objects accompanying extremely high intensity emission such as pulsar and fast radio burst etc., it is thought that collective motions of relativistic plasma lead to coherent emissions of electromagnetic waves. Synchrotron maser instability has been paid attention as a concrete mechanism of the emissions. It occurs when plasma particles have relativistic free energy perpendicular to the ambient magnetic field. In the course of the instability gyro phases of cyclotron motion of particles are synchronized. It results in the generation of coherent electromagnetic wave emission and acceleration of high energy particles. We have discussed the details of long time evolution of the instability using numerical simulation.

Spatio-temporal evolution (left) and omega-k spectrum (right) of magnetic field