Strong coupling of Alfvén and fast modes in compressible relativistic magnetohydrodynamic turbulence in magnetically dominated plasmas

Abstract

In this paper, we report our detailed analysis of the new strong-coupling\nregime between Alfv\\'en and fast modes in Poynting-dominated plasma turbulence,\nreported in our previous work Takamoto & Lazarian (2016), which is an important\neffect for many relativistic plasma phenomena, and calls for new theories of\nPoynting-dominated MHD turbulence. We performed numerical simulations of\nrelativistic MHD turbulence in isothermal plasmas, and analyzed the ratio of\nfast to Alfv\\'en mode energy. We found that the increase of the fast mode with\nthe background $\\sigma$-parameter can be observed even in isothermal plasma,\nshowing that such a phenomena is universal in trans-Alfv\\'enic turbulence in\nPoynting-dominated plasmas. To study the detailed energy conversion process, we\nalso performed a series of simulations of decaying turbulence injecting pure\nAlfv\\'en, fast, and slow modes, respectively, and investigated the development\nof the mode conversion from the each mode. We also found that the mode\nconversion between Alfv\\'en and fast modes is nearly insensitive to the\nbackground temperature. Finally, we report a result of a simulation with\ninitially fast mode dominated turbulence. It developed into a temporally\nstrong-coupling regime, which is a strong evidence for the existence of our\nsuggesting strong-coupling regime of fast and Alfv\\'en modes. Our result\nsuggests that the strong turbulence in Poynting-dominated plasma is very\ndifferent from that in the non-relativistic plasma. It will also give an\nimportant guidance to studies of particle acceleration and non-thermal photon\nemission from Poynting-dominated plasma.\n