Solar wind Alfvenicity during solar cycle 23 and 24 Perspective for future observations with Parker Solar Probe and Solar Orbiter

Context. Alfvenic fluctuations are ubiquitous features observed in solar wind, especially in the inner heliosphere. However, strong Alfvenic fluctuations are recovered in the near-Earth solar wind too, mainly in fast streams, but also in some cases in slow wind intervals, as highlighted in recent studies. Aims. The present study focuses on a statistical comparison between different phases of solar cycles 23 and 24 with regard to the Alfvenic content of solar wind fluctuations. Particular attention is devoted to the Alfvenic slow solar wind, in relation to the solar wind composition and other parameters. Methods. Two-dimensional histograms of the solar wind speed versus the normalized cross-helicity have been used to feature the Alfvenic character of solar wind turbulence on each phase of the solar cycles considered. Moreover, we characterize the different phases of solar cycles by also using composition data. Finally, case studies are discussed to better highlight the similarities and differences between the two solar maxima, which more clearly show a predominance of Alfvenic slow solar wind. Results. The statistical analysis highlights similarities between two solar cycles and confirms that the Alfvenic slow wind is more frequently observed during the maximum of solar activity. The two representative time intervals, containing samples of this solar wind regime, show similar characteristics, with a particular reference to the spectral analysis. Conclusions. This study has important implications for future observations by Parker Solar Probe and Solar Orbiter, devoted to the study of the inner heliosphere inside Mercury's orbit. In fact, both missions will operate up to the maximum of solar cycle 25 which is fast approaching. These unprecedented measurements will then provide insights into the origin and evolution of the Alfvenic solar wind close to the region where it is generated and accelerated.