RESIDUAL ENERGY IN MAGNETOHYDRODYNAMIC TURBULENCE

There is mounting evidence in solar wind observations and in numerical simulations that kinetic and magnetic energies are not in equipartition in magnetohydrodynamic (MHD) turbulence. The origin of their mismatch, the residual energy E-r = E-v - E-b, is not understood well. In the present work this effect is studied analytically in the regime of weak MHD turbulence. We find that residual energy is spontaneously generated by turbulent dynamics, and it has a negative sign, in good agreement with the observations. We find that the residual energy condenses around k(parallel to) = 0 with its k(parallel to)-spectrum broadening linearly with k(perpendicular to), where k(parallel to) and k(perpendicular to) are the wavenumbers parallel and perpendicular to the background magnetic field, and the field-perpendicular spectrum of the residual energy has the scaling E-r(k(perpendicular to)) alpha k(perpendicular to)(-1) in the inertial interval. These results are found to be in agreement with numerical simulations. We propose that residual energy plays a fundamental role in Alfvenic turbulence and it should be taken into account for the correct interpretation of observational and numerical data.