Alfv\'enic solar wind intervals observed by Solar Orbiter: Exploiting the capability of the SWA plasma suite and source region investigation

TL;DR

This study uses Solar Orbiter's SWA instrument to analyze Alfvénic slow and fast solar wind streams, revealing their distinct properties, origins, and the limitations of the traditional fast/slow wind classification.

Contribution

It demonstrates the capability of Solar Orbiter's SWA to differentiate Alfvénic slow wind from fast wind and links these streams to their solar sources using magnetic connectivity analysis.

Findings

Alfvénic slow wind shows distinct plasma and magnetic properties.

Magnetic connectivity links wind types to specific solar source regions.

Spectral analysis indicates energy equipartition varies among streams.

Abstract

Fast and slow solar wind have distinct properties linked to their solar sources.Alfv\'enic slow wind complicates the usual speed-based classification, especially at intermediate speeds. Solar Orbiter's Solar Wind Analyzer (SWA) offers unique capabilities to investigate how Alfv\'enic slow wind differs from fast wind and relate these differences to their solar origins. In September 2022, Solar Orbiter observed several Alfv\'enic streams: one fast wind, three Alfv\'enic slow wind (AS1, AS2, AS3), and a moderate fast (FH) interval. We analyze these streams, combining plasma parameters from all SWA sensors with magnetic field measurements from the Magnetometer (MAG). A spectral analysis of magnetic and velocity fluctuations is used to characterize Alfv\'enicity. The magnetic connectivity of each stream to its solar source is examined using Potential Field Source Surface extrapolation combined with ballistic backmapping from the spacecraft. Proton velocity distribution functions show anisotropies and field-aligned beams characteristic of Alfv\'enic streams, while electron pitch-angle distributions reveal clear strahl populations. Oxygen and carbon charge-state ratios are low in fast wind but higher in AS1-AS3, consistent with slow wind origins. Magnetic connectivity suggests the fast wind originates from a large coronal hole; AS1 links to a pseudostreamer with high expansion factor; AS2, AS3, and FH connect to a negative-polarity coronal hole whose field lines cross a pseudostreamer that later dissipates. Spectral analysis indicates near energy equipartition in all intervals except AS2. The combined SWA observations offer key insights into the physical processes shaping Alfv\'enic solar wind streams. Our results reinforce that the simple fast/slow wind classification is inadequate for linking solar wind to sources, and suggest that Alfv\'enicity relates to the solar source conditions.