Cluster observation of magnetohydrodynamic turbulence in the plasma sheet boundary layer

Y Narita

doi:10.1186/s40623-016-0442-0

Abstract

Measurement of turbulent magnetic field is presented from the Earth magnetotail crossing of the Cluster spacecraft on August 25, 2006, as an ideal case study of magnetohydrodynamic turbulence in the plasma sheet boundary layer on a spatial scale of about 10,000 km. The fluctuation energy of the magnetic field is evaluated in both the frequency and wavevector domains. The observed plasma sheet turbulence event shows anisotropy in the wavevector domain with a spectral extension perpendicular to the mean magnetic field. The analyses of the dispersion relation and phase speed diagrams indicate that the coherent wave components should be regarded as a set of the linear-mode waves and the other fluctuation components in magnetohydrodynamics. Although the magnetic field fluctuation amplitudes are sufficiently small compared to the large-scale field strength, there is no clear indication of the linear-mode dominance in the plasma sheet. As a lesson, magnetohydrodynamic turbulence must be modeled by including both linear-mode waves and nonlinear wave components such as sideband waves.

Highlights

Plasma sheet surrounding a current sheet occur in many places in space plasmas such as planetary magnetospheres, heliosphere, and solar atmosphere, whenever a shear or anti-parallel magnetic field geometry appears
The ISEE-3 observations show turbulent activities in the distant geomagnetic tail from 60 to 240 Re (Tsurutani et al 1984), where 1 Re = 6378 km is the Earth radius, and a broadband spectrum which extends to a power-law spectrum at frequencies higher than 0.1 Hz with both incompressible
The Multi-point Signal Resonator (MSR) method was developed for studying waves and turbulence using four-point magnetic field data and makes extensive use of the 12-by-12 covariance matrix by combining the minimum variance projection with the Multiple Signal Classification (MUSIC) algorithm

Summary

Introduction

Plasma sheet surrounding a current sheet occur in many places in space plasmas such as planetary magnetospheres, heliosphere, and solar atmosphere, whenever a shear or anti-parallel magnetic field geometry appears. The goal of this manuscript is to observationally evaluate the spatio-temporal properties of magnetohydrodynamic turbulence in the plasma sheet boundary layer in the Earth magnetotail at a distance of about 16 Re such as the energy spectrum and the wave characteristics (linear modes or sideband waves, dispersion relations, propagation directions, and phase speed distribution) using the measurement by the Cluster mission (Escoubet et al 2001).

Results

Conclusion