A multi-viewpoint comparison of the velocity field of coronal propagating disturbances

Small-scale propagating disturbances (PD) are ubiquitous in the solar corona. Time-Normalised Optical Flow (TNOF) is a method developed for mapping PD velocity fields in time series of Extreme-Ultraviolet (EUV) images. We show PD velocity fields of a quiet Sun (QS) region containing a small coronal hole (CH) and filament channel (FC) jointly observed by Extreme Ultraviolet Imager (EUI) aboard the Solar Orbiter and Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO). The QS observations acquired on 28 October 2023 in 174A channel of High Resolution EUV Imager (HRIEUV) of EUI and 171A channel of AIA are used. During the time of the observations, the separation angle between Solar Orbiter and SDO was approximately 26\de. A novel image alignment analysis shows that the dominant formation heights are 11.4Mm for HRIEUV and 4Mm for AIA. Despite this height difference, the PD velocity fields obtained from the observations from the two instruments are in good agreement across the region. In the QS the median PD speed is around 6.7 and 7.4\kms\ for HRIEUV and AIA respectively, with maximum speeds of around 40\kms. The small equatorial CH is a region dominated by a low temperature of $\approx$0.8MK and is host to high PD speeds, with a median speed of 17\kms. The velocity field bridges coherently across the CH from neighbouring QS regions from east to west, thus the CH must be overlaid by a system of long, low-lying closed magnetic loops. This unexpected configuration is supported by a potential field (PF) magnetic model and may be due to the longevity of the CH, allowing time for interchange reconnection with neighbouring closed-field regions. The FC is observed to be multithermal, with a narrow central strip of high emission at both low (0.8MK) and high (2.5MK) temperatures and low emission at warm (1.2MK) temperature. The FC has PD speeds similar to those of the QS.

💡 Research Summary

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This paper presents a comprehensive multi‑viewpoint analysis of coronal propagating disturbances (PDs) using simultaneous observations from Solar Orbiter’s High‑Resolution EUV Imager (HRIEUV) at 174 Å and SDO’s Atmospheric Imaging Assembly (AIA) at 171 Å on 28 October 2023. The region of interest (ROI) is a quiet‑Sun (QS) area that contains a small equatorial coronal hole (CH) and a quiescent filament channel (FC). Because the two spacecraft were separated by ~26°, the authors first develop a novel image‑alignment technique to determine the dominant formation heights of the two EUV channels. By remapping the images onto spherical surfaces at a range of heliocentric distances and minimizing the translational shift (Δx, Δy) between the two viewpoints using Fourier Correlation Tracking, they find optimal heights of 11.4 Mm for HRIEUV and 4 Mm for AIA. These heights reflect the different temperature response functions of the 174 Å and 171 Å passbands (≈1 MK vs. ≈0.8 MK) and provide a physically motivated basis for subsequent comparison.

With the images aligned in Carrington coordinates, the authors apply the Time‑Normalized Optical Flow (TNOF) method to each time series. TNOF computes a temporally normalised optical‑flow field, yielding a pixel‑by‑pixel velocity vector that captures the motion of faint intensity perturbations. Compared with traditional running‑difference techniques, TNOF is more robust against noise and can resolve subtle flows. The resulting velocity maps reveal three distinct behaviours:

1. Quiet Sun (QS) – The velocity field forms a network of coherent, cell‑like structures whose boundaries (sources) align with the photospheric supergranulation network. Median speeds are 6.7 km s⁻¹ (HRIEUV) and 7.4 km s⁻¹ (AIA), with a maximum of ~40 km s⁻¹. These values are consistent with slow magneto‑acoustic wave interpretations reported in earlier literature.

2. Coronal Hole (CH) – The CH, identified both by intensity thresholds and by a Differential Emission Measure (DEM) analysis (dominant emission at ≈0.8 MK), exhibits markedly higher PD speeds. The median speed is ≈17 km s⁻¹, and the flow vectors run coherently from east to west across the hole. Potential‑field (PF) extrapolation shows that, contrary to the classic picture of an open‑field CH, a system of long, low‑lying closed loops overlays the hole. These loops provide a longer propagation path, explaining the higher speeds.

3. Filament Channel (FC) – DEM results indicate a multithermal structure: a narrow central strand bright at both low (0.8 MK) and high (2.5 MK) temperatures, but faint at intermediate (1.2 MK). Despite this complex thermal profile, the FC’s PD speeds are comparable to the QS (median ≈7 km s⁻¹). The TNOF vectors show PDs flowing from surrounding QS regions into the filament, then aligning along the filament axis. This suggests that the filament’s internal dynamics are driven externally, and that the magnetic configuration is highly non‑potential (barbs‑and‑spine topology) – a feature that the PF model fails to reproduce.

The authors also perform a DEM analysis using all AIA channels and the Solar Iterative Temperature Emission Solver (SITES). Fractional Emission Measure maps confirm the CH’s low‑temperature dominance and the FC’s hot envelope at 2.5 MK, supporting the interpretation of a hot cavity surrounding a cooler filament core.

Key insights emerging from the study are:

• Height‑dependent alignment – By explicitly accounting for different formation heights, the authors achieve sub‑pixel alignment between the two viewpoints, enabling a reliable cross‑instrument comparison of PD velocity fields.
• Velocity‑length correlation – PD speed correlates with the length of the underlying magnetic loops: longer, low‑lying closed loops in the CH produce higher speeds, whereas shorter QS loops and the filament’s finer strands yield slower motions.
• Diagnostic power of TNOF – The method proves capable of extracting consistent velocity fields from instruments with different spatial resolution, cadence, and viewing geometry, reinforcing its utility for future multi‑spacecraft campaigns.
• Magnetic topology probing – In regions where PF extrapolations are insufficient (e.g., the filament), PD flow directions provide indirect constraints on the magnetic field orientation, offering a complementary tool to traditional magnetograms and extrapolation techniques.

The paper concludes that PD velocity mapping, especially when combined with multi‑viewpoint observations and robust alignment methods, can serve as a powerful proxy for coronal magnetic topology. The authors suggest extending the approach to include spectroscopic data (e.g., from Hinode/EIS or Solar‑C) to discriminate between wave‑driven and bulk‑flow interpretations, and to apply it to larger datasets covering active regions and polar coronal holes. This work not only validates the TNOF technique but also opens new avenues for probing the three‑dimensional magnetic architecture of the solar corona.