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.
X -3 single-spacecraft techniques, and estimate λ with help of two different proxies for both Helios datasets. We find that close to the Sun, λ < λ ⊥ . This supports a slab-like spectral model, where the population of fluctuations having wavevector 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 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 B 0 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.
The solar wind (SW) is a natural laboratory to study magnetohydrodynamic (MHD) turbulence, being the most completely studied case of turbulence in astrophysics, and the only one extensively and directly studied using in situ observations. Here we focus on the dynamical development of anisotropy in the SW magnetic fluctuations, a well-known property of an MHD system with a mean magnetic field (B 0 ). Understanding the nature and origin of this anisotropy is of relevance not only for the study of turbulence itself, but also because magnetic fluctuations directly influence the transport (i.e., acceleration and scattering) of solar and galactic energetic particles in the solar wind.
A magnetized turbulent MHD system will develop anisotropies with respect to the mean magnetic field B 0 . This was originally demonstrated in laboratory experiments [Robinson and Rusbridge, 1971;Zweben et al., 1979] and was also well documented in analytical, numerical and observational studies [Shebalin et al., 1983;Montgomery, 1982;Oughton et al., 1994;Goldreich and Sridhar , 1995]. The simplest models commonly used for the description of anisotropic SW fluctuations are the “slab” model, where fluctuations have wave vectors parallel to B 0 , and the “2D” model, where fluctuations have wave vectors perpendicular to B 0 [e.g., Matthaeus et al., 1990;Oughton et al., 1994;Tu and Marsch, 1993;Bieber et al., 1994Bieber et al., , 1996]]. These models are of course greatly oversimplified but provide a useful parametrization of anisotropy in SW turbulence. In analytical calculations a two component decomposition of this type, having no oblique wave vectors, provides great simplifcations, for example in scattering and transport theory [e.g., Matthaeus et al., 1995;Shalchi et al., 2004]. However, conceptually it is often advantageous to think of the
two components as “quasi-slab” and “quasi-2D”, meaning that according to some specified scheme based on time scales, angle, etc., all wavevector contributions are grouped into these two categories. This is the approach we adopt here, and hereafter we shall refer to the two relevant populations as simply slab or 2D components.
Anisotropies have been widely investigated at 1 astronomical unit (AU) for more than 20 years, using single spacecraft techniques [e.g., Belcher and Davis, 1971;Matthaeus et al., 1990;Tu and Marsch, 1995;Milano et al., 2004;Dasso et al., 2005], that means, analyzing under the Taylor frozen-in hypothesis [Taylor , 1938], spatial structures from time series measured by one spacecraft. Recently, multi spacecraft studies at 1 AU have validated the main results on anisotropic turbulence obtained from single spacecraft observations [Matthaeus et al., 2005;Dasso et al., 2008;Weygand et al., 2009;Osman and Horbury, 2007;Matthaeus et al., 2010].
As a consequence of these numerous studies, a common assumption is that the SW at 1 AU, contains a major population of 2D fluctuations and a minor slab component.
However, single spacecraft studies have shown that, when subdividing the sample into fast SW (speeds larger than 500 km/s) and slow SW (speeds lower than 400 km/s), the former contains more slab-like than 2D-like fluctuations, while in the latter it is the other way round [Dasso et al., 2005]. This result has recently been confirmed by means of multi spacecraft techniques [Weygand et al., 2011], where the slow SW was defined as having speeds below 450 km/s and the fast SW above 600 km/s. Thus, all these results at 1 AU motivate the following questions: Is the distinct relative population of fluctuations in fast and slow SW streams a consequence of intrinsic differences in the fluctuation properties that are established at the coronal sources? Or,
if we assume that a high-speed stream will arrive at 1 AU more quickly than a low-speed stream, thus revealing “younger” states in the evolution of interplanetary turbulence, are these differences a consequence of the dynamical evolution of the turbulence from the Sun to 1 AU? In the latter case, this issue becomes relevant to questi
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