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The field-aligned anisotropy of the solar wind turbulence, which isquantified by the ratio of the parallel to the perpendicular correlation (andTaylor) length scales, is determined by simultaneous two-point correlationmeasurements during the time period 2001-2017. Our results show that thecorrelation scale along the magnetic field is the largest, and the correlationscale in the field-perpendicular directions is the smallest, at both solarmaximum and solar minimum. However, the Taylor scale reveals inconsistentresults for different stages of the solar cycles. During the years 2001-2004,the Taylor scales are slightly larger in the field-parallel directions, whileduring the years 2004-2017, the Taylor scales are larger in thefield-perpendicular directions. The correlation coefficient between the sunspotnumber and the anisotropy ratio is employed to describe the effects of solaractivity on the anisotropy of solar wind turbulence. The results show that thecorrelation coefficient regarding the Taylor scale anisotropy (0.65) is largerthan that regarding the correlation scale anisotropy (0.43), which indicatesthat the Taylor scale anisotropy is more sensitive to the solar activity. TheTaylor scale and the correlation scale are used to calculate the effectivemagnetic Reynolds number, which is found to be systematically larger in thefield-parallel directions than in the field-perpendicular directions. Thecorrelation coefficient between the sunspot number and the magnetic Reynoldsnumber anisotropy ratio is -0.75. Our results will be meaningful forunderstanding the solar wind turbulence anisotropy and its long-termvariability in the context of solar activity.

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