Alfvénic solar wind intervals observed by Solar Orbiter: Exploiting the capability of the SWA plasma suite and source region investigation

TL;DR

This study harnesses Solar Orbiter's SWA suite (SWA-PAS, SWA-EAS, SWA-HIS) alongside MAG and PFSS-ballistic backmapping to characterize Alfvénic solar wind intervals observed in September 2022. It identifies one fast wind, three Alfvénic slow wind streams (AS1–AS3), and a moderate fast interval (FH), comparing proton VDFs, electron strahl/heats, heavy-ion charge states, and magnetic-turbulence metrics such as $oldsymbol{ abla} ext{C}$, $oldsymbol{ abla} ext{R}$, and the Alfvén ratio $r_A$. The results show that ASW behaves as a slow-speed extension of fast wind with source-region topology (coronal holes, pseudostreamers) strongly influencing Alfvénicity, composition, and fluctuation spectra; AS2 in particular exhibits magnetic-energy imbalance despite sharing a solar source with AS3. The findings challenge a simplistic fast/slow wind taxonomy, reveal significant intra-class variability linked to solar source conditions, and demonstrate the Swift capability of SWA to connect in situ measurements to coronal structures close to their origin. These insights advance our understanding of solar wind acceleration, turbulence, and the role of magnetic topology in shaping Alfvénic fluctuations near the Sun and throughout the inner heliosphere.

Abstract

Fast and slow solar wind have distinct properties linked to their solar sources.Alfvénic 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 Alfvénic slow wind differs from fast wind and relate these differences to their solar origins. In September 2022, Solar Orbiter observed several Alfvénic streams: one fast wind, three Alfvénic 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 Alfvénicity. 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 Alfvénic 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 Alfvénic solar wind streams. Our results reinforce that the simple fast/slow wind classification is inadequate for linking solar wind to sources, and suggest that Alfvénicity relates to the solar source conditions.

Figures (15)

• Figure 1: Long Term Planning (LTP) of Solar Orbiter operations 08 & 09 covering the time interval 2022-06-27 -- 2022-12-26. The figure shows the X-Y projection of the Solar Orbiter orbit (solid black line) in GSE coordinates, along with Parker Solar Probe (purple), Stereo Ahead (green) and Bepi Colombo (yellow). The blue, red and orange segments mark the remote sensing windows of Solar Orbiter. The black square indicates the period start, the triangles the LTP boundaries while the black dots the GAM restrictions. The red oval highlights the interval investigated in this study (adapted from https://2e2.cosmos.esa.int/confluence/display/SOSP/Orbit+Plots)
• Figure 2: Overview of time series of relevant solar wind parameters observed by Solar Orbiter at a heliocentric distance ranging between 0.58 and 0.32 AU. From top to bottom: solar wind bulk speed, $V_{sw}$, in $km~s^{-1}$; heliocentric distance, $R$, in $AU$; radial component of the magnetic field, $B_R$, in $nT$ (blue) and magnetic field magnitude, $B$, in $nT$ (black); number density, $n_p$, in $cm^{-3}$; the angle the magnetic field forms with the velocity field, $\Theta_{BV}$, in degree; v-b correlation coefficient computed at 30 min scale using a running window, $C_{VB}$; the charge state $O^{7+}/O^{6+}$ ratio; the charge state $C^{6+}/C^{5+}$ ratio; plasma $\beta$. The coloured boxes identify the intervals investigated in this study, corresponding to separate streams of fast wind, Alfvénic slow wind (AS1, AS2, AS3), fast wind (Fast) and moderate fast (FH) solar wind.
• Figure 3: SWA-PAS and MAG observations during the "Fast" solar wind interval. Left panel (top to bottom): Energy–time spectrogram of the ion energy differential flux; solar wind velocity (blue) and number density (green); parallel pressure $P_{\parallel}$ (green), perpendicular pressure $P_{\perp}$ (blue), and temperature $T$ (red); and magnetic field vector components in RTN coordinates. The vertical blue line indicates the time of the VDF shown in the right panel. Right panel: Ion velocity distribution function (VDF) in the $\mathbf{V} \times \mathbf{B}$ plane. The X-axis is aligned as closely as possible with the spacecraft–Sun direction. The blue arrow shows the magnetic field vector.
• Figure 4: The same as \ref{['ET_spectrogram_fast']}, but for the slow solar wind case, corresponding to the time interval AS1 (see \ref{['fig00']} ).
• Figure 5: SWA-PAS 2D (upper panel) and 1D (lower panel) ion velocity distribution functions (VDFs) for the selected intervals (see \ref{['fig01']}). The 2D distributions are integrated onto the plane perpendicular to the $\mathbf{V} \times \mathbf{B}$ vector and include the solar wind bulk velocity vector. The bottom row shows the $V_{\parallel}$ cut of each VDF.