Compressible Relativistic Magnetohydrodynamic Turbulence in Magnetically-Dominated Plasmas And Implications for A Strong-Coupling Regime

Abstract: In this Letter, we report compressible mode effects on relativistic magnetohydrodynamic (RMHD) turbulence in Poynting-dominated plasmas using 3-dimensional numerical simulations. We decomposed fluctuations in the turbulence into 3 MHD modes (fast, slow, and Alfv\'en) following the procedure mode decomposition in (Cho & Lazarian 2002), and analyzed their energy spectra and structure functions separately. We also analyzed the ratio of compressible mode to Alfv\'en mode energy with respect to its Mach number. We found the ratio of compressible mode increases not only with the Alfv\'en Mach number but with the background magnetization, which indicates a strong coupling between the fast and Alfv\'en modes and appearance of a new regime of RMHD turbulence in Poynting-dominated plasmas where the fast and Alfv\'en modes strongly couples and cannot be separated, different from the non-relativistic MHD case. This finding will affect particle acceleration efficiency obtained by assuming Alfv\'enic critical balance turbulence, and will change the resulting photon spectra by non-thermal electrons.