The crossover to the '1∕f' region of solar wind fluctuations

We present generalized structure function (GSF) analysis of magnetic and velocity field fluctuations measured in situ in the solar wind at ∼ 1 AU with the ACE spacecraft. We decompose the fluctuations into directions parallel and perpendicular to the mean local background magnetic field. GSF plots based on these quantities are shown to provide a clear indicator of the spectral break between the inertial range and “1/f“ range of scaling. In GSF plots, the projected velocity and magnetic field fluctuations track each other closely in the inertial range in both fast and slow solar wind and at solar maximum and minimum. At longer timescales the “1/f” range is easily identified, because the magnetic field fluctuations display a flattening of the GSFs (exponents become zero) consistent with a ∼1/f PSD. The velocity fluctuations on the other hand have GSFs which steepen, consistent with a PSD of ∼1/fα where α as determined by GSF varies with solar wind speed and solar cycle but is closer to two. The extent of th...