Solar wind turbulence: cascade, dissipation and heating

In this paper we show that a direct evidence for the presence of an inertial energy cascade, the most characteristic signature of hydromagnetic turbulence (MHD), is observed in the solar wind. A Yaglom-like scaling relation has been found in the high-latitude data samples measured by the Ulysses spacecraft. Moreover, an analogous scaling law, suitable modified to take into account compressible fluctuations, has been observed in a much more extended fraction of the same data set. Thus, it seems that large scale density fluctuations, despite their low amplitude, play a major role in the basic scaling properties of turbulence. The presence of a nonlinear turbulent magnetohydrodynamic energy cascade provides for the first time a direct estimation of the turbulent energy transfer rate, which can contribute to the in situ heating of the wind. In particular, The compressive turbulent cascade seems to be able to supply the energy needed to account for the local heating of the non-adiabatic solar wind.