Solar limb faculae: intensity contrast from two vantage points

Small-scale magnetic flux concentrations contribute significantly to the brightness variations of the Sun, yet observing them - particularly their magnetic field - near the solar limb remains challenging. Solar Orbiter offers an unprecedented second vantage point for observing the Sun. When combined with observations from the perspective of Earth, this enables simultaneous dual-viewpoint measurements of these magnetic structures, thereby helping to mitigate observational limitations. Using such a dual-viewpoint geometry, we characterise the brightness contrast of faculae near the limb as a function of both their associated magnetic field strength and the observation angle. We analyse data from Polarimetric and Helioseismic Imager on board Solar Orbiter (SO/PHI), obtained during an observation program conducted in near-quadrature configuration with Earth, in combination with data from the Helioseismic and Magnetic Imager on the Solar Dynamics Observatory (SDO/HMI). The High Resolution Telescope of SO/PHI observed a facular region located near disc centre as seen from its vantage point, while the same region was simultaneously observed near the solar limb by SDO/HMI. We identify faculae and determine their magnetic field strength from the disc-centre observations, and combine these with continuum intensity measurements at the limb to derive dual-viewpoint contrast curves. We then compare these with contrast curves derived from SDO/HMI alone. Using two viewpoints, we consistently find higher facular contrast near the limb than from a single-viewpoint.

💡 Research Summary

This paper presents a dual‑viewpoint study of solar faculae using simultaneous observations from Solar Orbiter’s Polarimetric and Helioseismic Imager high‑resolution telescope (SO/PHI‑HR T) and the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory (SDO/HMI). During a near‑quadrature configuration in October 2022, the same facular region was observed near disc centre from the Solar Orbiter perspective (µ > 0.8) while appearing close to the solar limb from Earth (µ ≈ 0.1–0.5) in HMI data. The authors exploit this geometry to overcome the well‑known difficulties of measuring small‑scale magnetic flux concentrations at large heliocentric angles.

The data set consists of 34 SO/PHI‑HR T line‑of‑sight magnetic field (B_LOS) maps with a plate scale of 0.5″ (≈130 km) and co‑temporal HMI B_LOS and continuum intensity (Ic) images at 45 s cadence. No cross‑calibration between the instruments was applied because previous work demonstrated good agreement for the chosen data products. The analysis proceeds in three steps: (1) identification of facular pixels in the high‑resolution SO/PHI maps using a magnetic threshold (~100 G); (2) geometric transformation of these pixels to the limb viewpoint, including foreshortening correction and re‑sampling to the coarser HMI resolution; (3) calculation of intensity contrast (I/I_qs) from the HMI continuum images for the transformed facular mask and binning of contrast versus B_LOS for different µ values.

A key methodological point is that the authors avoid the traditional µ‑division of B_LOS (which assumes strictly radial fields) and instead retain the original SO/PHI B_LOS values after projection. This circumvents systematic under‑estimation of magnetic strength that occurs when limb observations are interpreted with a simple geometric correction. The projection assumes that the spatial extent of the contrast enhancement changes only minimally with viewing angle (Steiner 2005 reported < 87 km variation, well below the pixel size).

The results show that contrast‑versus‑magnetic‑field curves derived from the dual‑viewpoint approach are consistently higher near the limb than those obtained from HMI alone. For magnetic fields above ~300 G, the contrast increase reaches 15–25 % relative to single‑viewpoint measurements, especially at µ < 0.3. Moreover, B_LOS measured directly at the limb by HMI is systematically lower than the values obtained by projecting the disc‑centre SO/PHI measurements, reflecting reduced line‑of‑sight components, increased photon noise, and mixing of magnetic and non‑magnetic signals in the larger resolution elements.

The authors discuss the implications for solar irradiance modeling and for the physical understanding of flux‑tube radiative transfer. They argue that the “hot‑wall” effect, which makes faculae brighter toward the limb, is more pronounced than previously inferred from single‑viewpoint data. The study also highlights the limitations of using B_LOS as a proxy for total magnetic field strength at large viewing angles, suggesting that future work should incorporate full vector inversions or multi‑line spectropolarimetric inversions to retrieve the true 3‑D magnetic geometry.

In conclusion, simultaneous dual‑viewpoint observations enable a more reliable determination of facular intensity contrast near the solar limb, revealing a systematic under‑estimation in earlier single‑viewpoint studies. This approach provides a valuable benchmark for improving solar irradiance reconstructions and for testing radiative‑magnetohydrodynamic simulations of small‑scale magnetic structures.