Are There Waves in Magnetized Compressible Turbulence in Space and Astrophysical Plasmas

Turbulence is ubiquitous throughout magnetized plasmas in the Universe. Past research has made great progress on understanding the nature of MHD turbulence, including its anisotropy and its impact on regulating the transport of energetic particles. When the plasma beta is low and turbulent Mach number becomes large, turbulence becomes compressible, characterized by enhanced density fluctuations. Using extensive 3D MHD simulations and the 4D (spatio-temporal) FFT analysis, the nature of compressible MHD turbulence has been examined in detail (Gan, Li, et al. 2022, ApJ, 926:222). Two approaches are used to determine the presence of eigenwave modes – Alfven, Fast and Slow waves, namely the mode decomposition based on spatial variations only and the spatio-temporal 4D FFT analysis of all fluctuations. The latter method enables us to quantify fluctuations that satisfy the dispersion relation of Alfven and compressible modes with finite frequency. Overall, the fraction of fast modes identified via the spatio-temporal 4D FFT approach in total fluctuation power is either tiny with nearly incompressible driving or ∼2% with highly compressible driving. The majority of the turbulence energy resides in the low frequency region with large perpendicular wavenumber. In addition, we will present similar analysis of kinetic simulations of compressible turbulence. The transition from the wave turbulence to strong turbulence in the spatio-temporal domain will also be discussed. These results could have significant implications for understanding the compressible fluctuations in space and astrophysical plasmas, and its impact for heating and accelerating charged particles in such compressible turbulence.