Solar-cycle Variability of Composite Geometry in the Solar Wind Turbulence

Abstract: The composite geometry and spectral anisotropy of the solar wind turbulence are very important topics in the investigations of solar wind. In this work, we use the magnetic field and plasma data from Wind spacecraft measured during 1995 January to 2023 December, 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 three-dimensional (3D) geometry of the solar wind turbulence. Both the tests reveal that the solar wind turbulence is dominated by the two-dimensional (2D) component (~80% by 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 0.65 (anisotropy test result). This phenomenon suggests that the increasing solar activity (signified by sunspot number) causes increasing slab component in the 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 magnitude and the increased Alfven 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 the solar wind turbulence, and especially of their solar-cycle variability.