Energy transfer in Hall-MHD turbulence: cascades, backscatter, and dynamo action

Scale interactions in Hall magnetohydrodynamics (MHDs) are studied using both the mean field theory derivation of transport coefficients, and direct numerical simulations in three space dimensions. In the magnetically dominated regime, the eddy resistivity is found to be negative definite, leading to large-scale instabilities. A direct cascade of the total energy is observed, although as the amplitude of the Hall effect is increased, backscatter of magnetic energy to large scales is found, a feature riot present in MHD flows. The coupling between the magnetic and velocity fields is different than in the MHD case, and backscatter of energy from small-scale magnetic fields to large-scale flows is also observed. For the magnetic helicity, a strong quenching of its transfer is found. We also discuss non-helical magnetically forced Hall-MAD simulations where growth of a large-scale magnetic field is observed.