Aging of anisotropy of solar wind magnetic fluctuations in the inner heliosphere

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 integral length scale (lambda) anisotropy associated with the turbulent magnetic fluctuations, with respect to the aging of fluid parcels traveling away from the Sun, and according to whether the measured lambda is principally parallel (lambda(parallel to)) or perpendicular (lambda(perpendicular to)) to the direction of a suitably defined local ensemble average magnetic field B-0. 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, and estimate lambda with help of two different proxies for both Helios data sets. We find that close to the Sun, lambda(parallel to) < lambda(perpendicular to). This supports a slab-like spectral model, where the population of fluctuations having wave vector k parallel to B-0 is much larger than the one with k-vector perpendicular. A population favoring perpendicular k-vectors would be considered quasi-two dimensional (2D). Moving toward 1 AU, we find a progressive isotropization of lambda and a trend to reach an inverted abundance, consistent with the well-known result at 1 AU that lambda(parallel to) > lambda(perpendicular to), usually interpreted as a dominant quasi-2D picture over the slab picture. Thus, our results are consistent with driving modes having wave vectors parallel to B-0 near Sun, and a progressive dynamical spectral transfer of energy to modes with perpendicular wave vectors as the solar wind parcels age while moving from the Sun to 1 AU.