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. The Solar Orbiter mission 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 characterised the brightness contrast of faculae near the limb as a function of both their associated magnetic field strength and the observation angle. We analysed data from the Polarimetric and Helioseismic Imager on board the Solar Orbiter (SO/PHI), obtained during an observation programme 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 the disc centre as seen from its vantage point, while the same region was simultaneously observed near the solar limb by SDO/HMI. We identified faculae and determine their magnetic field strength from the disc-centre observations, and combined these with continuum intensity measurements at the limb to derive dual-viewpoint contrast curves. We then compared 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. A comparison of the facular line-of-sight magnetic field derived from limb observations with that derived from disc-centre observations (and re-projected to the limb) reveals significant differences between the two. Co-temporal observations of limb faculae from a second, disc-centre viewpoint enable a more precise determination of their associated magnetic field.
Albert et al. (2025) studied this question.