  The properties of inertial and kinetic range solar wind turbulence have been
investigated with the arbitrary-order Hilbert spectral analysis method, applied
to high-resolution density measurements. Due to the small sample size, and to
the presence of strong non-stationary behavior and large-scale structures, the
classical structure function analysis fails to detect power law behavior in the
inertial range, and may underestimate the scaling exponents. However, the
Hilbert spectral method provides an optimal estimation of the scaling
exponents, which have been found to be close to those for velocity fluctuations
in fully developed hydrodynamic turbulence. At smaller scales, below the proton
gyroscale, the system loses its intermittent multiscaling properties, and
converges to a monofractal process. The resulting scaling exponents, obtained
at small scales, are in good agreement with those of classical fractional
Brownian motion, indicating a long-term memory in the process, and the absence
of correlations around the spectral break scale. These results provide
important constraints on models of kinetic range turbulence in the solar wind.
