What determines the departure from equipartition of energy in Alfvénic fluctuations in solar wind streams? Insights from Solar Orbiter observations

The very first observations by Mariner 5 highlighted the presence of Alfvénic fluctuations in the solar wind identified as nearly incompressible fluctuations accompanied by large correlations between velocity and magnetic field components as predicted by the magnetohydrodynamics (MHD) theory. Since then, Alfvénic fluctuations have been observed to be ubiquitous especially in high-speed solar wind streams, but are also in some cases in slow wind streams, which may in turn exhibit a strong Alfvénic character. The so-called Alfvénic slow wind resembles the fast wind in many aspects, but may also differ from it. Indeed, recent observations performed by Solar Orbiter have shown that the fast wind may display a strong Alfvénic content of the fluctuations than the one observed in the Alfvénic slow wind, especially closer to the Sun.In this context, Solar Orbiter offers a unique opportunity to study the origin and radial evolution of the Alfvénic solar wind. In this particular study, we present a comparative study between different Alfvénic streams, both fast and slow, at different heliocentric distances, focusing on the characterization of Alfvénicity of different streams with particular reference to the energy balance of the fluctuations.The aim of this work is to deepen our understanding of what are the mechanisms responsible for the evolution of Alfvénicity in solar wind fluctuations and to understand better to what extent the two solar wind regimes show different Alfvénic content of the fluctuations and eventually evolve in a different way.