Magnetic Correlation Lengths in the Turbulent Solar Wind

We analyze the evolution of the interplanetary magnetic field spatial structure by examining the heliospheric autocorrelation functions (R) based on "in situ" observations and using classical single-spacecraft techniques. Correlation lengths (λ) derived from R can be viewed as a measured of the integral scale of the turbulence. We focus on how evolves the anisotropy of λ with respect to the aging of fluid parcels traveling away from the Sun, and according to the anisotropy of the measured λ, distinguishing mainly the values parallel (λ∥) or perpendicular (λ⊥) to the direction of the local average magnetic field B0. We find that close to the Sun, λ∥ < λ⊥, supporting a slab-like spectral model, and a progressive isotropization of λ with a trend to reach an inverted relation, while moving towards higher heliodistances, supporting dynamical spectral transfer of energy. We also present preliminary results on the probability distribution functions (PDFs) of λ from an analysis at different heliodistances from the Sun using observations from Helios, ACE and Ulysses. Advances made to better characterize the nature of interplanetary fluctuations, as those presented here, has significant implications in space and astrophysical plasmas, including scattering and transport of charged energetic particles, and theories of MHD turbulence.