Supersonic Magnetohydrodynamic Turbulence at Extreme Reynolds Numbers

Abstract: Supersonic magnetohydrodynamic (MHD) turbulence is a ubiquitous state for many astrophysical plasmas, including thin, cool accretion disks and the interstellar medium of our galaxy. However, even the basic statistics of this type of turbulence remains uncertain. In this talk I will present results from supersonic MHD turbulence simulations at unparalleled resolutions, with plasma Reynolds numbers of over a million and grids up to 10080^3. In the kinetic energy spectrum we find a break between the scales that are supersonic and dominated by kinetic energy, with spectral index -2 (Burgers turbulence), and those that become strongly magnetized and subsonic, with spectral index -3/2. At magnetic Reynolds number of greater than 10^5, we find a power law emerging in the magnetic energy spectrum with spectral index -9/5, unexplained by any modern, asymptotic turbulence theory. On the strongly magnetized scales, the plasma tends to self-organize into locally relaxed states, depleting the nonlinearities and highlighting that plasma relaxation is a fundamental feature of supersonic MHD turbulence. We coin the term competitive relaxation to describe the competition between the turbulent nonlinearities and the depletion of these nonlinearities through relaxation. Both of these aspects challenge profoundly the tenets of magnetohydrodynamic turbulence theories and describe a new paradigm for supersonic turbulence.