Scaling relations for compressible turbulence in the solar wind

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