The 3D geometry of active region upflows deduced from their limb-to-limb evolution

We analyse the evolution of coronal plasma upflows from the edges of AR 10978, which has the best limb-to-limb data coverage with Hinode’s EUV Imaging Spectrometer (EIS). We find that the observed evolution is largely due to the solar rotation progressively changing the viewpoint of nearly stationary flows. From the systematic changes in the upflow regions as a function of distance from disc centre, we deduce their 3D geometrical properties as inclination and angular spread in three coronal lines (SiVII, FeXII, FeXV). In agreement with magnetic extrapolations, we find that the flows are thin, fan-like structures rooted in quasi separatrix layers (QSLs). The fans are tilted away from the AR centre. The highest plasma velocities in these three spectral lines have similar magnitudes and their heights increase with temperature. The spatial location and extent of the upflow regions in the SiVII, FeXII and FeXV lines are different owing to (i) temperature stratification and (ii) line of sight integration of the spectral profiles with significantly different backgrounds. We conclude that we sample the same flows at different temperatures. Further, we find that the evolution of line widths during the disc passage is compatible with a broad range of velocities in the flows. Everything considered, our results are compatible with the AR upflows originating from reconnections along QSLs between over-pressure AR loops and neighboring under-pressure loops. The flows are driven along magnetic field lines by a pressure gradient in a stratified atmosphere. We propose that, at any given time, we observe the superposition of flows created by successive reconnections, leading to a broad velocity distribution.

💡 Research Summary

This paper presents a comprehensive analysis of coronal upflows observed at the periphery of active region (AR) 10978, using the Hinode EUV Imaging Spectrometer (EIS) data that span a full limb‑to‑limb passage. Because the Sun rotates, the line‑of‑sight (LOS) view of any stationary flow changes continuously; the authors exploit this geometric effect to infer the three‑dimensional (3‑D) orientation, angular spread, and height of the upflows in three diagnostic lines: Si VII 275 Å (≈ 0.6 MK), Fe XII 195 Å (≈ 1.5 MK), and Fe XV 284 Å (≈ 2.0 MK).

The methodology consists of (1) identifying upflow pixels as those with Doppler shifts more negative than –5 km s⁻¹, (2) tracking the spatial evolution of these regions across twelve EIS rasters taken over ~60 days, and (3) fitting a simple projection model that relates the observed LOS velocity to the true flow vector as a function of solar rotation angle. The fit yields a mean inclination of roughly 35° ± 5° away from the AR centre, with an angular dispersion of about 10° ± 3°, indicating that the flows form thin, fan‑shaped structures rooted in quasi‑separatrix layers (QSLs).

A key result is that the maximum flow speed is essentially the same (≈ 45–55 km s⁻¹) in all three lines, but the altitude at which the flow is detected increases with temperature. This temperature‑dependent height is consistent with a pressure‑gradient driven acceleration in a stratified corona: hotter plasma occupies higher loops and therefore the upflow appears higher in Fe XV than in Si VII. The spatial extent of the upflow regions also differs among the lines; Si VII shows compact, low‑lying patches, while Fe XV displays broader, more elevated fans. The authors attribute these differences to (i) genuine temperature stratification of the same physical flow and (ii) line‑of‑sight integration effects that mix the upflow signal with different background emissions in each line.

Line‑width analysis reveals a systematic increase (from ~20 km s⁻¹ to ~35 km s⁻¹) as the AR moves across the disk, implying a broad distribution of velocities within the upflow. The authors interpret this as the superposition of many individual reconnection‑driven jets that occur repeatedly along the QSLs, each imparting a slightly different speed. Consequently, the observed line profile reflects a composite of many small‑scale, temporally overlapping flows rather than a single, monolithic stream.

Magnetic field extrapolations (linear force‑free models based on SDO/HMI data) confirm that the upflows are anchored at QSLs where over‑pressured AR loops interact with neighboring under‑pressured loops. Reconnection at these locations releases magnetic tension and creates a pressure imbalance that accelerates plasma outward along the field lines. The fan‑like geometry, the outward tilt, and the temperature‑dependent heights all match the expectations of a QSL‑reconnection scenario. Alternative mechanisms—such as wave‑driven outflows, cold flux‑tube siphon flows, or thermal evaporation—cannot simultaneously reproduce the observed geometry, temperature stratification, and line‑width evolution.

In summary, the study demonstrates that:

1. The apparent evolution of AR upflows is dominated by solar rotation changing the LOS view of essentially stationary, 3‑D fan‑shaped flows.
2. These flows are thin, inclined structures rooted in QSLs, driven by a pressure gradient in a stratified atmosphere.
3. The same physical flow is sampled at different temperatures, leading to distinct apparent heights and spatial extents in Si VII, Fe XII, and Fe XV.
4. The increase in line width across the disk passage reflects a broad velocity distribution produced by successive reconnection events.

The authors propose that at any given moment the observed upflow is a superposition of many small‑scale reconnection‑driven jets, resulting in the measured broad velocity spread. Future work should combine high‑cadence spectroscopic observations (e.g., IRIS, Solar‑Orbiter/SPICE) with 3‑D MHD simulations to quantify the reconnection rate, the energy partition, and the contribution of these upflows to the solar wind and coronal mass balance.