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

TL;DR

This study investigates how the anisotropy of magnetic turbulence in the solar wind evolves from near the Sun to 1 AU, revealing a transition from slab-like to more isotropic and quasi-2D turbulence structures.

Contribution

It provides the first comprehensive analysis of the evolution of magnetic fluctuation anisotropy in the inner heliosphere using Helios data, highlighting the transition from parallel to perpendicular dominant wavevectors.

Findings

Near the Sun, _ < _, supporting a slab-like turbulence model.

At 1 AU, _ > _, indicating a shift towards quasi-2D turbulence.

The anisotropy transition suggests energy transfer from parallel to perpendicular wavevectors as solar wind ages.

Abstract

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.