Inertial range of magnetorotational turbulence

Abstract

Accretion disks around compact stars are formed due to turbulence driven by magnetorotational instability. Despite over 30 years of numerous computational studies on magnetorotational turbulence, the properties of fluctuations in the inertial range—where cross-scale energy transfer dominates over energy injection—have remained elusive, primarily due to insufficient numerical resolution. Here, we report the highest-resolution simulation of magnetorotational turbulence ever conducted. Our simulations reveal a constant cross-scale energy flux, a hallmark of the inertial range. We found that as the cascade proceeds to smaller scales in the inertial range, the kinetic and magnetic energies tend toward equipartitioning with the same spectral slope, and slow magnetosonic fluctuations dominate over Alfvénic fluctuations, having twice the energy. These findings align remarkably with the theoretical expectations from the reduced magnetohydrodynamic model, which assumes a near-azimuthal mean magnetic field. Our results provide important implications for interpreting the radio observations by the Event Horizon Telescope.