Residual energy in magnetohydrodynamic turbulence and in the solar wind
  Recent observations indicate that kinetic and magnetic energies are not in
equipartition in the solar wind turbulence. Rather, magnetic fluctuations are
more energetic and have somewhat steeper energy spectrum compared to the
velocity fluctuations. This leads to the presence of the so-called residual
energy E_r=E_v-E_b in the inertial interval of turbulence. This puzzling effect
is addressed in the present paper in the framework of weak turbulence theory.
Using a simple model of weakly colliding Alfv\'en waves, we demonstrate that
the kinetic-magnetic equipartition indeed gets broken as a result of nonlinear
interaction of Alfv\'en waves. We establish that magnetic energy is indeed
generated more efficiently as a result of these interactions, which proposes an
explanation for the solar wind observations.
