  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.
