Intermittency and self-similarity in 'natural parameters' in solar wind turbulence

The solar wind provides a natural laboratory for observations of MHD turbulence over extended temporal scales. We identify self-similarity in the probability density functions of fluctuations in certain solar wind bulk plasma parameters as seen by the WIND spacecraft. Whereas the fluctuations of speed v and IMF magnitude B are multi-fractal, we find that the fluctuations in the ion density p, energy densities B-2 and p nu(2) as well as MHD-approximated Poynting flux nu B-2 are self-similar on the timescales up to similar to 26 hours. We argue that these are 'natural parameters' in which to cast the solar wind measurements which are taken in an Eulerian frame of reference. The intermittency of the system is then expressed in these parameters through the non-Gaussian nature of the single curve that describes the fluctuations PDF up to this timescale. Self-similarity implies that a Fokker-Planck model exists for the timeseries and we derive this here along with the associated Langevin equation