  The solar wind provides a natural laboratory for observations of MHD
turbulence over extended temporal scales. Here, we apply a model independent
method of differencing and rescaling to identify self-similarity in the
Probability Density Functions (PDF) of fluctuations in 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 rho, energy densities B^2 and rho v^2 as well as MHD-approximated
Poynting flux vB^2 are mono-scaling on the timescales up to ~26 hours. The
single curve, which we find to describe the fluctuations PDF of all these
quantities up to this timescale, is non-Gaussian. We model this PDF with two
approaches-- Fokker-Planck, for which we derive the transport coefficients and
associated Langevin equation, and the Castaing distribution that arises from a
model for the intermittent turbulent cascade.
