The Solar Cycle Variations of the Anisotropy of Taylor Scale and Correlation Scale in the Solar Wind Turbulence

TL;DR

This study investigates how the anisotropy of solar wind turbulence varies with the solar cycle, revealing that the Taylor scale anisotropy is more sensitive to solar activity than the correlation scale anisotropy.

Contribution

It provides a comprehensive analysis of the solar cycle dependence of anisotropy in the Taylor and correlation scales of solar wind turbulence using two-point correlation measurements.

Findings

Correlation scale anisotropy is largest along the magnetic field.

Taylor scale anisotropy varies with solar cycle stages.

Taylor scale anisotropy correlates more strongly with solar activity.

Abstract

The field-aligned anisotropy of the solar wind turbulence, which is quantified by the ratio of the parallel to the perpendicular correlation (and Taylor) length scales, is determined by simultaneous two-point correlation measurements during the time period 2001-2017. Our results show that the correlation scale along the magnetic field is the largest, and the correlation scale in the field-perpendicular directions is the smallest, at both solar maximum and solar minimum. However, the Taylor scale reveals inconsistent results for different stages of the solar cycles. During the years 2001-2004, the Taylor scales are slightly larger in the field-parallel directions, while during the years 2004-2017, the Taylor scales are larger in the field-perpendicular directions. The correlation coefficient between the sunspot number and the anisotropy ratio is employed to describe the effects of solar activity on the anisotropy of solar wind turbulence. The results show that the correlation coefficient regarding the Taylor scale anisotropy (0.65) is larger than that regarding the correlation scale anisotropy (0.43), which indicates that the Taylor scale anisotropy is more sensitive to the solar activity. The Taylor scale and the correlation scale are used to calculate the effective magnetic Reynolds number, which is found to be systematically larger in the field-parallel directions than in the field-perpendicular directions. The correlation coefficient between the sunspot number and the magnetic Reynolds number anisotropy ratio is -0.75. Our results will be meaningful for understanding the solar wind turbulence anisotropy and its long-term variability in the context of solar activity.