Aging of the anisotropy of the magnetic fluctuations in the inner-heliospheric Solar Wind

RUIZ, M.E.; DASSO, S.; W. H. MATTHAEUS; E. MARSCH; J.M. WEYGAND

Trieste

Conferencia; School and Conference on Analytical and Computational Astrophysics; 2011

ICTP

We analyze the evolution of the interplanetary magnetic field spatial structure by examining the inner heliospheric autocorrelation function, using Helios 1 and Helios 2 in situ observations. We focus on the evolution of the anisotropy of the integral length scale (λ) associated with the turbulent magnetic fluctuations, 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 analyze a set of 1065 24-hour long intervals (covering full missions). For each interval, we compute the magnetic autocorrelation function, using classical single-spacecraft techniques. We find that close to the Sun, λ‖ < λ. This supports a slab-like spectral model, where the population of fluctuations having wavevectors k parallel (k‖) to B0 is much larger than the one with k-vectors perpendicular (k). A population favoring k would be considered quasi-two dimensional (2D). Moving towards 1 AU, we find a progressive isotropization of λ and a trend to reach an inverted abundance, consistent with the well-known result at 1 AU that λ‖ > λ, usually interpreted as a dominant quasi-2D picture over the slab picture. Thus, our results are consistent with driving modes having wavevectors parallel to B0 near Sun, and a progressive dynamical spectral transfer of energy to modes with perpendicular wavevectors as the solar wind parcels age while moving from the Sun to 1 AU.