Dependence of solar wind power spectra on the direction of the local mean magnetic field

TL;DR

This study uses wavelet analysis of magnetic field data from high-speed solar wind streams to examine how the power spectrum varies with the direction relative to the local mean magnetic field, revealing anisotropic turbulence characteristics.

Contribution

It demonstrates that the magnetic field fluctuation spectrum is approximately azimuthally symmetric and shows a continuous change in power-law exponent with respect to the magnetic field direction.

Findings

Spectra are azimuthally symmetric around the mean magnetic field.

Power-law exponent varies from ~1.6 perpendicular to ~2.0 parallel to the field.

Results are consistent with previous high-latitude observations.

Abstract

(Abridged) Wavelet analysis can be used to measure the power spectrum of solar wind fluctuations along a line in any direction with respect to the local mean magnetic field. This technique is applied to study solar wind turbulence in high-speed streams in the ecliptic plane near solar minimum using magnetic field measurements with a cadence of eight vectors per second. The analysis of nine high-speed streams shows that the reduced spectrum of magnetic field fluctuations (trace power) is approximately azimuthally symmetric about B_0 in both the inertial range and dissipation range; in the inertial range the spectra are characterized by a power-law exponent that changes continuously from 1.6 \pm 0.1 in the direction perpendicular to the mean field to 2.0 \pm 0.1 in the direction parallel to the mean field. The large uncertainties suggest that the perpendicular power-law indices 3/2 and 5/3 are both consistent with the data. The results are similar to those found by Horbury et al. (2008) at high heliographic latitudes.