Three-dimensional density and compressible magnetic structure in solar wind turbulence

Owen W Roberts,Yasuhito Narita,C.-Philippe Escoubet

doi:10.5194/angeo-36-527-2018

Abstract

Abstract. The three-dimensional structure of both compressible and incompressible components of turbulence is investigated at proton characteristic scales in the solar wind. Measurements of the three-dimensional structure are typically difficult, since the majority of measurements are performed by a single spacecraft. However, the Cluster mission consisting of four spacecraft in a tetrahedral formation allows for a fully three-dimensional investigation of turbulence. Incompressible turbulence is investigated by using the three vector components of the magnetic field. Meanwhile compressible turbulence is investigated by considering the magnitude of the magnetic field as a proxy for the compressible fluctuations and electron density data deduced from spacecraft potential. Application of the multi-point signal resonator technique to intervals of fast and slow wind shows that both compressible and incompressible turbulence are anisotropic with respect to the mean magnetic field direction P⟂≫P∥ and are sensitive to the value of the plasma beta (β; ratio of thermal to magnetic pressure) and the wind type. Moreover, the incompressible fluctuations of the fast and slow solar wind are revealed to be different with enhancements along the background magnetic field direction present in the fast wind intervals. The differences in the fast and slow wind and the implications for the presence of different wave modes in the plasma are discussed. Keywords. Interplanetary physics (MHD waves and turbulence)

Highlights

The solar wind is a collisionless, magnetised plasma originating from the Sun and is often observed to be in a state of fully developed turbulence (Tu and Marsch, 1995; Bruno and Carbone, 2013; Alexandrova et al, 2013)
Application of the multi-point signal resonator technique to intervals of fast and slow wind shows that both compressible and incompressible turbulence are anisotropic with respect to the mean magnetic field direction P⊥ P and are sensitive to the value of the plasma beta (β; ratio of thermal to magnetic pressure) and the wind type
In this paper we present a new study of the threedimensional structure of the turbulence in the solar wind, and this will be investigated using the multi-point signal resonator technique (MSR) technique (Narita et al, 2011a, c, 2014)

Summary

Introduction

The solar wind is a collisionless, magnetised plasma originating from the Sun and is often observed to be in a state of fully developed turbulence (Tu and Marsch, 1995; Bruno and Carbone, 2013; Alexandrova et al, 2013). The multi-point signal resonator technique (MSR; Narita et al, 2011c) is a derivative of wave telescope/k-filtering methods which estimate the four-dimensional power distribution in wave-number space P (ωsc, k) with an improved signal-to-noise ratio This improvement is essential for investigating the morphology at small scales. Several studies on the solar wind have been performed by looking at the peaks in the four-dimensional spectra revealing a strong wavevector anisotropy at MHD and proton kinetic scales in the incompressible magnetic field (Sahraoui et al, 2010b; Narita et al, 2011c; Roberts et al, 2013; Roberts and Li, 2015; Perschke et al, 2016; Narita and Motschmann, 2017), as well as in the compressible fluctuations (Roberts et al, 2017b). The ability to estimate the power in threedimensional space, as well as collapsing the power into onedimensional spectra as a function of wave number, allow us to make a detailed study of both the power anisotropy and the anisotropy of the spectral index

Data and methodology

Anisotropy

One-dimensional spectra

Conclusion