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Context. Alfvénic fluctuations are ubiquitous features observed in solar wind, especially in the inner heliosphere. However, strong Alfvénic 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 Alfvénic content of solar wind fluctuations. Particular attention is devoted to the Alfvénic 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 Alfvénic 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 Alfvénic slow solar wind. Results. The statistical analysis highlights similarities between two solar cycles and confirms that the Alfvénic 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 Alfvénic solar wind close to the region where it is generated and accelerated.

We use a 2.5D magnetohydrodynamic model to investigate the propagation of azimuthally driven Alfvén waves with different periods and their interaction with the solar wind. In the absence of waves, the dipole field is stretched into a helmet streamer by the solar wind. The wind speeds near the equator are lower than those in the mid and high latitudes. Magnetic reconnection in the equatorial plasma sheet regularly triggers a breakup and expulsion of a plasmoid. We next inject monochromatic Alfvén waves with a moderate amplitude of 9 km s−1 and a period of τ = 1000 s at the coronal base. A cavity showing features of forward and backward propagating modes is formed. The backward waves are able to accelerate the background plasma at mid and high latitudes through the nonlinear coupling to compressional waves. The size of the cavity increases with the period of the Alfvén waves as long as the outer boundary remains in the sub-Alfvénic wind. When τ = 4000 s, we find enhanced acceleration and heating of the solar wind plasma as well as suppression of the reconnection in the equatorial plasma sheet. The amplitudes of the backward Alfvén waves remain large inside the cavity and modify its size. The cavity ceases to exist as its outer boundary gradually moves into the super-Alfvénic wind and the large amplitude backward waves are swept away by the wind. Results suggest that Alfvén waves with moderate amplitudes can modify the dynamics and the energetics of the solar wind plasma with the embedded magnetic field.