Solar-cycle variability of composite geometry in solar wind turbulence

The composite geometry and spectral anisotropy of solar wind turbulence are very important topics in the investigations of solar wind. In this work, we use magnetic field and plasma data from the Wind spacecraft, measured from January 1995 to December 2023, which covers more than two solar cycles, to systematically investigate these subjects in the context of solar-cycle variability. The so-called spectrum ratio test and spectrum anisotropy test are employed to determine the 3D geometry of solar wind turbulence. Both tests reveal that solar wind turbulence is dominated by the 2D component (sim80% by the turbulence energy). More interestingly, we find that the fraction of slab turbulence increases with the rising sunspot number, and the correlation coefficient between the slab fraction and the sunspot number is 0.61 (ratio test result) or 0.69 (anisotropy test result). Moreover, the correlation coefficient between the sunspot number and the averaged slab fraction of the ratio test result and the anisotropy test result is 0.76. This phenomenon suggests that the increasing solar activity (signified by sunspot number) causes an increasing slab component in solar wind turbulence. The relationship between spectral anisotropy and solar activity is discussed and explained. The enhancement of slab fraction is associated with the intensified interplanetary magnetic field (IMF) magnitude and the increased Alfvén speed during the rise phases of the solar cycles. Our findings will be very helpful for achieving a better understanding of the 3D composite geometry and spectral anisotropy of solar wind turbulence and especially of their solar-cycle variability.