Direct observations of a complex coronal web driving highly structured slow solar wind

The study addresses the origins of the highly structured slow solar wind and its connection to magnetic topology in the middle corona. While fast solar wind (>500 km s−1) emerges from open-field coronal holes, slow solar wind (<500 km s−1) is associated with the coronal streamer belt and exhibits strong internal variability and coronal-like composition, suggesting hotter, denser source regions. Interplanetary measurements indicate two components of slow wind, one highly Alfvénic and one less so. During active periods, small low-latitude coronal holes reshape the streamer belt and warp the heliospheric current sheet (HCS), implying a complex separatrix web (S-web) of magnetic topology. The research question is how the streamer belt and S-web relate to the origin and structure of the slow solar wind, particularly via middle-coronal reconnection processes where magnetic topology transitions from closed to open around 1.5 R⊙. The purpose is to directly observe and model middle-coronal dynamics to test whether they drive structured slow solar wind.

Multiple mechanisms have been proposed for slow solar wind generation and variability. Super-radial expansion within coronal holes and MHD wave turbulence can drive slow wind along open fields, with variations in expansion factor and footpoint field strengths influencing composition and speed. Large-scale transients (coronal mass ejections and streamer blobs) contribute to variability. Interchange reconnection between open and closed fields and reconnection along quasi-separatrix layers and separatrices (S-web reconnection) created by low-latitude coronal holes are widely invoked to produce slow wind and its structure. Models predict persistent reconnection at streamer and pseudostreamer cusps in the middle corona releasing plasma from closed structures into the wind. Despite inferences and modeling, direct middle-coronal observational signatures of S-web reconnection and their direct linkage to slow solar wind structure remained limited prior to this work.

Observations: The team used an off-pointing campaign of GOES/SUVI to directly image the middle corona in EUV, primarily using the 195 Å passband, from 2018-08-07 to 2018-09-13. SUVI produced three-panel mosaics (central Sun-centered plus east/west off-point panels) at ~20 min cadence, image scale 5 arcsec/pixel, with deep exposure processing to reduce noise, stray light, and brightness gradients. Effective EUV structure detection extended to ~2.7 R⊙ (negligible structure above ~3.5–4 R⊙ due to faintness). SUVI observed a pair of near-equatorial, roughly east–west aligned coronal holes with an embedded decaying active region (NOAA 12711) as the system rotated across the disk and appeared at the west and east limbs. To extend to greater heights, SOHO/LASCO C2 coronagraph images (2.5–6 R⊙) were processed (standard reduction, time stacking to 1 h cadence, background subtraction including F-corona, despiking, and radial normalization) and temporally paired with SUVI to form SUVI–LASCO composites showing EUV up to ~2.7 R⊙ and visible-light above. Context data from SDO/AIA (171, 193, 211 Å) and HMI (line-of-sight magnetic field) at 2 h cadence were used to track the CH–AR system’s evolution from 2018-08-08 to 2018-09-09. PFSS extrapolations (source surface at 2 R⊙) using HMI synoptic maps provided polarity inversion lines and large-scale topology context. STEREO-A (near quadrature at −108° ecliptic longitude) provided complementary vantage observations with EUVI 195 Å (inner corona to ~1.7 R⊙), COR-2 (2.5–15 R⊙), and HI-1 (15–84 R⊙). Data from 2018-08-07 to 2018-08-12 and again 2018-10-25 to 2018-10-29 (near PSP’s first perihelion epoch) were processed (standard SECCHI_PREP; F-corona subtraction for COR-2; star suppression in HI-1 level-2). Modeling: Advanced global 3D MHD coronal models (MAS-type) were driven by synoptic HMI photospheric magnetic fields. Magnetic topology was analyzed using the squashing factor Q; synoptic signed log10 Q maps at 3 R⊙ highlighted separatrices and quasi-separatrix layers comprising the S-web, colored by field polarity. 3D volume renders of Q delineated flux domain boundaries and topological complexity around the CH–AR system. Synthetic EUV emission (inner to middle corona) and polarized brightness (extended corona) were forward-modeled for comparison with SUVI–LASCO observations. Modeled radial velocities at 3 R⊙ over the HCS and pseudostreamer arcs were compared with observed outflow speeds of the slow wind streams. Temporal and geometric comparisons were made between observed coronal web dynamics (especially at streamer/pseudostreamer cusps) and modeled S-web structures.

• SUVI revealed a highly structured, latitudinally extended coronal web of elongated middle-coronal features above a low-latitude coronal hole–active region (CH–AR) system, visible up to ~2.7 R⊙.
• LASCO showed recurrent, outflowing slow solar wind streams emerging above the tips/cusps of these coronal web structures, often above ~2.65 R⊙. The measured outflow speeds are consistent with slow solar wind speeds.
• Over about five days as the system crossed the west limb, there were persistent interactions and continual rearrangements among middle-coronal structures, including the apparent formation of transient closed loops as initially open structures interacted. Similar dynamics and wind streams were observed when the system crossed the east limb, indicating prevalence and persistence.
• Timing comparisons indicate wind streams emerge when interacting structures converge at the cusps of pseudostreamers and at the HCS, locations predicted by models to host persistent reconnection.
• Global MHD modeling reproduces elongated topological structures consistent with the observed coronal web and shows complex S-web features (warped polarity inversion line and pseudostreamer arcs) surrounding the active region. Synoptic signed logQ at 3 R⊙ reveals longitudinally extended S-web features with embedded smaller-scale cellular separatrices arising from small flux concentrations in coronal holes.
• Forward-modeled EUV and white-light emissions match SUVI–LASCO structures, and modeled radial velocities at 3 R⊙ quantitatively agree with observed wind stream speeds over the HCS and pseudostreamer arcs.
• STEREO-A EUVI/COR-2/HI-1 composites show the continuation of highly structured slow solar wind streams into the heliosphere from the same CH–AR system, providing a linkage from the middle-coronal S-web dynamics to heliospheric structures.
• The observed system corresponds to the region magnetically connected to Parker Solar Probe during its first perihelion, and the results support scenarios where interchange/S-web reconnection can drive Alfvénic structures and potentially contribute to switchback formation.

The observations directly connect middle-coronal magnetic reconnection at S-web separatrices—particularly at streamer and pseudostreamer cusps—to the release and structuring of slow solar wind. By imaging persistent interactions and reconnection-driven rearrangements within the coronal web and correlating them with concurrent slow wind streams, the study provides concrete evidence that S-web dynamics modulate the spatial and temporal structure of slow wind. Modeling confirms that the coronal web corresponds to complex S-web topology and that outflow speeds and morphologies are consistent with reconnection-driven slow wind. The results align with and contextualize in-situ measurements by Parker Solar Probe of structured, often Alfvénic slow wind and switchbacks, suggesting interchange reconnection in S-web regions as a plausible driver of some observed variability. The findings emphasize the critical role of the middle corona, where the transition from closed to open fields occurs, in shaping slow wind and transmitting topological imprints into the heliosphere. Projection effects and single-vantage limitations mean such structures may not always be apparent, underscoring the need for multi-viewpoint observations.

This work identifies and characterizes a complex coronal web in the middle corona as a direct observational imprint of the magnetic S-web and links its dynamic reconnection to the emergence of highly structured slow solar wind streams. The combination of extended-field EUV imaging (SUVI), white-light coronagraphy (LASCO), quadrature observations (STEREO), and global 3D MHD modeling establishes that S-web reconnection at streamer and pseudostreamer cusps actively structures and releases slow wind. While slow wind bulk speeds may depend on coronal hole expansion factors, S-web dynamics imprint magnetic topology and can inject hotter, coronal-composition plasma into the slow wind. Future research should prioritize sustained, higher-resolution, multi-vantage middle-corona imaging coordinated with outer-coronal and in-situ measurements. Upcoming and proposed missions (e.g., PUNCH, ECCCO, SunCET, PROBA3/ASPIICS) and off-ecliptic perspectives, complemented by PSP and Solar Orbiter observations, will help resolve the 3D structure and time dependence of S-web reconnection and its heliospheric consequences.

• SUVI data are limited in spatial resolution, cadence, and sensitivity at higher altitudes; EUV structures fade above ~3.5–4 R⊙, constraining direct imaging of outer coronal dynamics.
• Single vantage point middle-corona imaging introduces projection effects and line-of-sight superposition, obscuring 3D topology and reconnection geometry.
• The study focuses on a specific, well-isolated CH–AR system; while such systems are common, generalization to all slow wind sources may require broader sampling.
• Quantitative plasma diagnostics (e.g., composition, temperature) are indirect; direct spectroscopic constraints in the middle corona are lacking.