Quantitative estimates of the slab and 2‐D power in solar wind turbulence using multispacecraft data

Abstract

[1] A quantitative estimate of the field-aligned anisotropy of magnetohydrodynamic inertial range turbulence is obtained by comparing multispacecraft data with a fully threedimensional turbulent magnetic field numerical model. The simulated turbulence is a superposition of slab fluctuations parallel to the mean field and 2-D fluctuations perpendicular to it. This model is fitted to spatial autocorrelation functions, computed from measurements made by the four Cluster spacecraft in the solar wind, by varying the slab-2-D fraction. The mean power in the 2-D fluctuations, 81 ± 3%, is consistent with solar wind fluctuations being anisotropic with energy mainly in wave vectors perpendicular to the large-scale mean magnetic field. Eight intervals of multipoint magnetic field data are analyzed, and a large degree of variation in the anisotropy estimates about the mean is observed. The origin of this variability is unclear. However, variations in the anisotropy estimates are correlated between different field components.Citation: Osman, K. T., and T. S. Horsbury (2009), Quantitative estimates of the slab and 2-D power in solar wind turbulence using multispacecraft data,

Anisotropy in solar wind plasma turbulence

“…Moreover, in the inner heliosphere, there is only weak distance or wind speed dependence in the results, at least towards the high-frequency end of the inertial range [45,46]. Slab + 2D-type fits similar to equation (3.1) have been widely used [48][49][50]53,[60][61][62] and these also report high values for the inertial range two-dimensional fraction, e.g. 70-100%.…”

Transport of solar wind fluctuations: A two-component model

“…On the other hand, observations also suggested that turbulence was present and may have contributed to heating [e.g., Coleman, 1968]. Subsequent observational studies indicated that at magnetohydrodynamic (MHD) scales there are at least two distinct types of fluctuation and that the wave-like energy may be a minor component Bieber et al, 1996;Milano et al, 2004;Dasso et al, 2005;Horbury et al, 2005Horbury et al, , 2008Podesta, 2009;Osman and Horbury, 2009;Narita et al, 2010]. This encouraged development of more complete transport theories for the energy-containing range quantities [e.g., Tu and Marsch, 1993;Matthaeus et al, 1994Matthaeus et al, , 1996Matthaeus et al, , 1999Matthaeus et al, , 2004Zank et al, 1996;Smith et al, 2001Smith et al, , 2006Isenberg et al, 2003Isenberg et al, , 2010aIsenberg, 2005;Breech et al, 2005Breech et al, , 2008Yokoi and Hamba, 2007;Usmanov and Goldstein, 2010;Ng et al, 2010], in which turbulence properties are built in, contrasting with WKB theory, in which the waves are noninteracting at leading order.…”

“…Note that the partitioning of the total fluctuation energy into its Z 2 and W 2 components must also be specified. Here we assume that 80% of the fluctuation energy is in the quasi-2-D component [Bieber et al, 1996;Osman and Horbury, 2009]. This percentage of 2-D fluctuations is most appropriate to slow wind .…”

“…Observational studies at 1 AU suggest that the quasi-2-D energy is ∼80% of the total fluctuation energy [Bieber et al, 1996;Horbury et al, 2008;Osman and Horbury, 2009;MacBride et al, 2010], and we use a similar partitioning to set Z 2 and W 2 here.…”

Anisotropy of the magnetic correlation function in the inner heliosphere

“…Equation (1) can be ignored, and then R(r, τ) becomes a function of r alone. A computation of the correlation function taking into account the pure spatial lag, using simultaneous observations from two spacecraft located at a proper spatial separation, have been done recently by the first time [16,17,18]; a good agreement with the classical techniques assuming the Taylor hypothesis, analyzing single spacecraft observations as done in this work, has been found [19]. In this fashion we employ a Blackman-Tukey technique to compute each correlation function R I in the same way as done in [20].…”