On the slow solar wind with high Alfvenicity: from composition and microphysics to spectral properties

Alfvenic fluctuations are very common features in the solar wind and are found especially within the main portion of fast-wind streams while the slow wind usually is less Alfvenic and more variable. In general, the fast and slow winds show many differences, which span from the large-scale structure to small-scale phenomena, including also a different turbulent behaviour. Recent studies, however, have shown that even the slow wind can sometimes be highly Alfvenic, with fluctuations as large as those of the fast wind. This study is devoted to presenting many facets of this Alfvenic slow solar wind, including for example the study of the source regions and their connection to coronal structures, large-scale properties, and microscale phenomena and also impact on the spectral features. This study will be conducted performing a comparative analysis with the typical slow wind and with the fast wind. It has been found that the fast wind and the Alfvenic slow wind share common characteristics, probably attributable to their similar solar origin, that is coronal-hole solar wind. Given these similarities, it is suggested that in the Alfvenic slow wind a major role is played by the superradial expansion responsible for the lower velocity. Relevant implications of these new findings for the upcoming Solar Orbiter and Solar Probe Plus missions, and more in general for turbulence measurements close to the Sun, will be discussed.