Solar limb faculae: intensity contrast from two vantage points

TL;DR

The paper tackles the challenge of measuring facular brightness contrast near the solar limb and its dependence on magnetic-field strength. It leverages dual-viewpoint observations from SO/PHI-HRT (disc-centre) and SDO/HMI (near-limb) to build contrast curves as a function of magnetic-field strength and viewing angle, using re-projection to align the two perspectives. Key findings include higher facular contrast toward the limb in the dual-viewpoint setup and the observation of polarity reversal at the limb, consistent with height-dependent expansion of flux tubes; disc-centre-derived magnetic fields help reduce limb biases and improve facular maps. The work advances limb diagnostics for solar irradiance modeling and bright-feature physics, though it is limited by statistics from a single-region dataset, underscoring the need for broader samples in future studies.

Abstract

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.

Figures (14)

• Figure 1: Sketch of the observing geometry between the Solar Dynamics Observatory (SDO) and Solar Orbiter for the observations used in the study. The coordinate system is with Earth and Sun at fixed positions, as seen from above. While SDO observes the Sun from the Earth's perspective, on the horizontal black line, Solar Orbiter varies its view-angle between the two red lines, moving from right to left (i.e. approaching the Sun-Earth line). The spacecraft illustrations are a courtesy of ESA and NASA.
• Figure 2: The first and last data set pairs used in this study, from 20 and 22 October 2022 (A and B, respectively). Top: $B{LOS}$ and $\mu$ of a facular region near disc centre (a and b). Bottom: $B{LOS}$ and $\mu$ of the same region, as seen simultaneous by SDO/HMI (c and d). The contour shows the location of the SO/PHI field of view, and the $\mu$ in the SDO/HMI view-point is shown only for the contoured region.
• Figure 3: Co-observation of $B_{LOS}$ in the same region at the limb (SDO/HMI, left), and at disc centre (SO/PHI-HRT, right), from the first data set pair in our study. The SO/PHI observation time was 2022-10-20T21:15:46, which corresponds to 2022-10-20T21:20:28 at Earth after accounting for light travel time (in UTC). The SDO/HMI observation time was 2022-10-20T21:19:58 UTC.
• Figure 4: Sub-region of the first dataset pair in the time series used in the study, showing: $B_{LOS}$ derived from the limb observation by SDO/HMI (first panel); the same data divided by $\mu$ ($B_{LOS}/\mu$, second panel), and $B_{LOS}$ derived from disc centre by SO/PHI-HRT, divided by the $\mu$ value of the disc-centre view and re-projected to the limb to match the reference frame of SDO/HMI (third panel).
• Figure 5: The data combined in our analysis from different viewing angles. Left: $B_{LOS}$ derived at disc centre from SO/PHI-HRT observations, normalised by the local $\mu$ at which it was observed, and reprojected to the limb. Right: the intensity contrast is computed from SDO/HMI $I_c$ data at the limb. Contours: we identify facular pixels at disc centre in SO/PHI-HRT observations, then re-project this map to the limb. The same sub-region is shown as in Fig. \ref{['Fig:BLOS_compared']}.