  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) or perpendicular (\lambda_perp) to the direction of
a suitably defined local ensemble 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, and estimate \lambda with help of two different
proxies for both Helios datasets. We find that close to the Sun,
\lambda_parallel < \lambda_perp. This supports a slab-like spectral model,
where the population of fluctuations having wavevector k parallel to B0 is much
larger than the one with k-vector perpendicular. A population favoring
perpendicular k-vectors would be considered quasi-two dimensional (2D). Moving
towards 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 > \lambda_perp, 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.
