  The self-correlation level contours at $10^{10}\ \mathrm{cm}$ scale reveal a
2-D isotropic feature in both the slow solar wind fluctuations and the fast
solar wind fluctuations. However, this 2-D isotropic feature is obtained based
on the assumption of axisymmetry with respect to the mean magnetic field.
Whether the self-correlation level contours are still 3-D isotropic remains
unknown. Here we perform for the first time a 3-D self-correlation level
contours analysis on the solar wind turbulence. We construct a 3-D coordinate
system based on the mean magnetic field direction and the maximum fluctuation
direction identified by the minimum-variance analysis (MVA) method. We use data
with 1-hour intervals observed by WIND spacecraft from 2005 to 2018. We find,
on one hand, in the slow solar wind, the self-correlation level contour
surfaces for both the magnetic field and the velocity field are almost
spherical, which indicates a 3-D isotropic feature. On the other hand, there is
a weak elongation in one of the perpendicular direction in the fast solar wind
fluctuations. The 3-D feature of the self-correlation level contours surfaces
cannot be explained by the existed theory.
