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 turbulence. The origin of their mismatch, the residual
energy E_r=E_v-E_b, is not well understood. In the present work this effect is
studied analytically in the regime of weak magnetohydrodynamic 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 obtain that
the residual energy condenses around k_||=0 with its k_||-spectrum broadening
linearly with k_perp, where k_|| and k_perp 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_perp)\propto k_perp^{-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 correct
interpretation of observational and numerical data.
