On Alfvénic Slow Wind: A Journey From the Earth Back to the Sun

Abstract

Comparative studies of fast and slow solar wind streams performed over the past decades have illustrated several differences between the plasma regimes for these different flows, examples including features such as temperatures, particle distribution function anisotropies, and the nature of the embedded turbulence, specifically the Alfvénicity of the fluctuations. Though this two-state classification of the solar wind primarily based on flow speed has been widely adopted, more in depth studies have found that slow solar wind should be further categorized, flow speed not being a sufficient descriptor of the plasma state. Within this framework, slow solar wind streams with a strong Alfvénic character have been identified and characterized, showing that in many ways they resemble fast solar wind. The similarities between fast and slow Alfvénic wind regimes have been explained in terms of a similar solar origin, with the latter corresponding to slow winds emanating from rapidly diverging low-latitude small coronal holes. The aim of this review is to describe the state of art of our understanding of Alfvénic slow solar wind streams. The results presented cover observations performed at different heliocentric distances spanning from Wind at L1 to Helios and Parker Solar Probe in the inner heliosphere, as well as a discussion of their source regions.