Magnetohydrodynamic waves and instabilities in homogeneous gyrotropic ultrarelativistic plasma

In some astrophysical systems the ionized gas may be of such high temperature and so strongly magnetized that relativistic effects and pressure anisotropy must be considered in the magnetohydrodynamic (MHD) model. This paper gives an overview of the characteristics of linear MHD waves and instabilities in homogeneous ultrarelativistic plasmas with gyrotropic pressure. The energy closure is the double-polytropic laws with two polytropic exponents, γ_∥ and γ_⊥, and for the adiabatic and monatomic cases, the polytropic values (γ_∥, γ_⊥) are respectively (3, 2) and (2, 1.5) for nonrelativistic and ultrarelativistic plasmas. In this formulation, the general dispersion relations can conveniently be reduced to isotropic and/or nonrelativistic limits. Slow waves are found to exhibit some anomalies due to the pressure anisotropy in that they may possess a positive density-magnetic field correlation such as for fast waves and may possibly travel faster than intermediate waves. They may also develop a mirror instability, as well as a new type of compressible fire-hose instability that for a certain parameter regime may grow faster than the standard incompressible fire hose. Both the fire-hose and mirror instability criteria are found to have the same forms of β_∥ - β_⊥ > 2 and γ _∥β_∥ < β_⊥ ²/(2 + γ_⊥β_⊥), respectively, as for nonrelativistic plasma, although the growth rates may be significantly modified by the relativistic effect.