Tuesday, December 8, 2020
Many important turbulent plasma environments are characterized by a low ratio of the electron plasma pressure to magnetic energy density (electron beta). Examples are the ionosphere, the Earth's magnetosheath, the solar corona and some instances of the solar wind. At scales below the electron skin depth in plasmas with low electron beta, the dominant low-frequency plasma modes are inertial Alfvén waves, whose turbulent cascade is governed by the existence of two ideal invariants: energy and generalized kinetic helicity. Turbulent dynamics in the presence of two invariants is poorly understood in both plasmas and non-ionized fluids. We argue that, in inertial Alfvénic turbulence, both energy and generalized kinetic helicity exhibit forward cascades. We demonstrate via direct numerical simulations that the two cascades are compatible due to the existence of a strong scale-dependence of the Fourier phase alignment angle between velocity and magnetic field fluctuations, which manages to suppress helicity while allowing the energy cascade to proceed unhindered. We further present numerical evidence that this phenomenon, termed dynamic phase alignment, is at play in the joint forward cascade of energy and helicity in Navier-Stokes turbulence. This supports the tantalizing hypothesis of dynamic phase alignment as a potentially universal mechanism regulating the turbulent dynamics in the presence of a joint direct cascade of two invariants regardless of the details of the physical interactions.