Multi-spacecraft measurement of anisotropic power levels and scaling in solar wind turbulence

K T Osman,T S Horbury

doi:10.5194/angeo-27-3019-2009

Abstract

Abstract. Measurements by the four Cluster spacecraft in the solar wind are used to determine quantitatively the field-aligned anisotropy of magnetohydrodynamic inertial range turbulence power levels and spectral indexes. We find, using time-lagged second order structure functions, that the spectral index is near 2 around the field-parallel direction, which is consistent with a "critical balance" turbulent cascade. Solar wind fluctuations are found to be anisotropic with power mainly in wavevectors perpendicular to the mean field, where the spectral index is around 5/3.

Highlights

The solar wind is a natural laboratory for the study of magnetohydrodynamic (MHD) turbulence
The “slab” model is the simplest approximation to solar wind fluctuations, where all excited wavevectors lie parallel to the mean magnetic field direction, k
Direct measurements in the solar wind have shown that turbulent fluctuations are anisotropic with energy mainly in wavevectors perpendicular to the mean magnetic field (Matthaeus et al, 1990; Carbone et al, 1995; Bieber et al, 1996; Dasso et al, 2005; Osman and Horbury, 2007; Horbury et al, 2008; Osman and Horbury, 2009) and that the spectral index is around 5/3 (Matthaeus and Goldstein, 1982; Roberts and Goldstein, 1991; Bruno and Carbone, 2005)

Summary

Introduction

The solar wind is a natural laboratory for the study of magnetohydrodynamic (MHD) turbulence (see Goldstein et al, 1995; Tu and Marsch, 1995; Bruno and Carbone, 2005; Horbury et al, 2005; Sorriso-Valvo et al, 2006, and references therein). The “slab” model is the simplest approximation to solar wind fluctuations, where all excited wavevectors lie parallel to the mean magnetic field direction, k. This produces a one-dimensional spectrum which decays with increasing. The GS95 model predicts an inertial range power spectrum scaling of 5/3 in k⊥ and 2 in k (Boldyrev, 2005) These values were measured by Horbury et al (2008) in the high speed (∼750 km s−1) polar solar wind using 30 days of Ulysses magnetic field data. Our analysis is not restricted by the limitations imposed by GS95, Here we use a multi-spacecraft approach to measure the field-aligned anisotropy of solar wind turbulence power levels and spectral indexes. We measure low speed (∼340 km s−1) solar wind fluctuations using short (of order an hour) intervals of magnetic field data from the four Cluster spacecraft

Multi-spacecraft technique

Results

Conclusions