  Incompressible and isotropic magnetohydrodynamic turbulence in plasms can be
described by an exact relation for the energy flux through the scales. This
Yaglom-like scaling law has been recently observed in the solar wind above the
solar poles observed by the Ulysses spacecraft, where the turbulence is in an
Alfv\'enic state. An analogous phenomenological scaling law, suitably modified
to take into account compressible fluctuations, is observed more frequently in
the same dataset. Large scale density fluctuations, despite their low
amplitude, play thus a crucial role in the basic scaling properties of
turbulence. The turbulent cascade rate in the compressive case can moreover
supply the energy dissipation needed to account for the local heating of the
non-adiabatic solar wind.
