Properties of the magnetohydrodynamic turbulent cascade in high latitude solar wind

The occurrence and the nature of a nonlinear energy cascade within the intermediate scales of solar wind Alfvénic turbulence represents an important open issue. Using in situ measurements of fast, high latitude solar wind taken by Ulysses spacecraft at solar minima, we show that the observation of nonlinear energy cascade of imbalanced turbulence is only observed when the solar wind own peculiar properties. These are: reduction of the local correlation between velocity and magnetic field (small cross-helicity); the presence of large scale velocity shears; the extension of the turbulent spectra down to low frequencies. Our observations suggest that the role of the fluid cascade is extremely relevant for the validation of the Yaglom law in solar wind turbulence.