Abstract
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.
Highlights
Plasma turbulence is a common phenomenon occurring in nature and the turbulence in the heliosphere plays a significant role in several aspects of space plasma behaviors, such as high-energy particle acceleration, solar wind generation, plasma heating, galactic cosmic ray modulation, and solar energetic particle propagation (Kraichnan 1965; Belcher 1971; Matthaeus & Goldstein 1982a,b; Tu & Marsch 1995; Chen 2016; He & Wan 2019)
Based on the simultaneous two-point correlation function measurements, in this work we investigate the solar cycle variations of the anisotropy of the correlation scales and the Taylor scales in the solar wind turbulence
The magnetic field data used in this investigation were measured by the triaxial fluxgate magnetometers on board spacecraft Advanced Composition Explorer (ACE), Wind, and Cluster during the period from January 2001 to December 2017, which covers more than an entire solar cycle
Summary
Plasma turbulence is a common phenomenon occurring in nature and the turbulence in the heliosphere plays a significant role in several aspects of space plasma behaviors, such as high-energy particle acceleration, solar wind generation, plasma heating, galactic cosmic ray modulation, and solar energetic particle propagation (Kraichnan 1965; Belcher 1971; Matthaeus & Goldstein 1982a,b; Tu & Marsch 1995; Chen 2016; He & Wan 2019). According to the slab model, the correlation function decays in the directions parallel to the mean magnetic field, but without field-perpendicular variations. Dasso et al (2005) found that in the fast solar wind and at the larger scale of the inertial range, the correlation scales are longer in the field-perpendicular directions than in the field-parallel direction (slab model), whereas for the slow solar wind situation, the 2D component is predominant. Despite the two-component model is a rather idealized model with drastic approximation, it can present the dominant properties of the solar wind turbulence and provide a useful parameterization for the anisotropy studies (Matthaeus et al 1990; Bieber et al 1996; Oughton & Matthaeus 2005; Dasso et al 2005; Osman & Horbury 2007; Weygand et al 2009, 2011; Horbury et al 2012)
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