Characterization of Alfvénic Solar Wind Intervals Observed by SWA onboard Solar Orbiter, with Insights into Their Solar Sources

Fast and slow solar wind exhibit distinct kinetic, compositional, and bulk properties that are related to their solar sources. In recent years, the Alfvénic slow wind has emerged as a third class of solar wind, characterized by speeds typical of nominal slow wind but by several properties commonly associated with fast wind. These include similarities in the solar source, often identified with regions of strongly diverging open magnetic field, challenging the traditional solar wind classification based solely on bulk speed.

The Solar Wind Analyzer (SWA) plasma suite onboard Solar Orbiter provides unique capabilities to investigate how Alfvénic slow wind differs from the fast wind and to relate these differences to their solar sources.

In this study, we present selected examples of Alfvénic solar wind streams observed by SWA. Combined observations from all SWA sensors, together with magnetic field measurements from the Magnetometer (MAG), are used to characterize plasma properties and solar wind fluctuations through spectral analysis. The magnetic connectivity of each stream to its solar source is investigated using Potential Field Source Surface (PFSS) extrapolations combined with ballistic backmapping from the spacecraft and supported by remote-sensing observations.

Our results show that proton velocity distribution functions exhibit anisotropies and field-aligned beams characteristic of Alfvénic streams, while electron pitch-angle distributions display clear strahl populations. Oxygen and carbon charge-state ratios are low in fast wind, while they are higher in Alfvénic slow wind, approaching values typical of standard slow wind. Magnetic connectivity indicates that fast wind originates from a large coronal hole, while Alfvénic slow wind intervals are connected to pseudostreamers with high expansion factors or to coronal holes whose field lines cross pseudostreamer regions that later dissipate.

These findings support the idea that a simple fast/slow wind classification is insufficient to link in situ solar wind properties to their solar sources, and suggest that Alfvénicity is closely related to source-region magnetic topology. In particular, super-radial expansion may play a role in reducing the wind speed while preserving Alfvénic characteristics, setting the conditions for the origin of the Alfvénic slow wind. These results also have implications for the energy balance of solar wind fluctuations observed in situ.