Center-to-limb variations of solar active regions

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Abstract
Accurate modeling of stellar active regions (ARs) remains a major bottleneck for radial-velocity and transmission-spectroscopy studies aimed at finding Earth-like planets. While much effort has been devoted to AR modeling, their center-to-limb variations (CLVs) have been largely overlooked. We took a step toward remedying this by measuring the CLV of the 6173 Å continuum intensity for sunspots (the whole spot, separate umbrae, and penumbrae), faculae, network, and the quiet Sun using the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO). This study is based on four simple round α -sunspots and their surroundings, as well as one strongly evolving region. After correcting for stray light, we find that relative to the quiet Sun, all components except for the umbra display reduced darkening toward the limb. Additionally, strongly evolving ARs do not appear to display significantly altered CLV profiles compared to stable ARs. Faculae and network show contrast enhancements that peak near µ ≈ 0.3 before declining toward the limb, reaching maxima of approximately 4% and 2%, respectively, in contrast excess relative to the quiet Sun, while the spot-to-quiet-Sun contrast rises to approximately 15% near the limb. For both types of AR, this change in CLV behavior near the limb is likely related to the 3D structure of the ARs and the rapidly changing viewing geometry. This behavior is not captured by synthetic CLVs based on PHOENIX and ATLAS model atmospheres with solar values and a different effective temperature, underscoring the need for more realistic treatments of stellar activity.

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