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arXiv · 2607.29455

The Fe I 4377 {\AA} Line as a Solar Faculae Indicator: Insights from Spectral Ratio Analysis

Katlyn L. Hobbs·Christopher A. Watson·Jean C. Costes·Yvonne Unruh·Dana Clarice Yaptangco·Krishnamurthy Sowmya·Mitchell E. Young·Ernst J. W. de Mooij·Alexander G. M. Pietrow·Pál Váradi Nagy·Alexander I. Shapiro·Veronika Witzke·Federica Rescigno·Ryan A. Rubenzahl·Megan Bedell·Andrew Collier Cameron·Xavier Dumusque·Sean M. O'Brien·Benjamin M. J. Cadell·Baptiste Klein·Niamh Mallaghan·Niamh K. O'Sullivan·Toby Rodel·Sara Tavella
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Abstract

Faculae are a dominant source of stellar activity noise in radial velocity measurements, yet their low contrast and broad surface distribution make them difficult to track in disc-integrated observations. We apply Spectral Ratio Analysis (SRA) to HARPS-N Sun-as-a-star observations to isolate and characterize the spectral imprint of facular regions over rotational timescales. The resulting SRA spectra show coherent, line-dependent variability sensitive to surface magnetic activity, with the Fe I 4377 Angstrom line exhibiting a particularly strong diagnostic response to facular coverage. We interpret the observed signatures using two complementary synthetic frameworks: composite PHOENIX spectra, from which we derive best-fit facular temperature contrasts in the range 200-400 K, and MPS-ATLAS spectra synthesized using MURaM simulations of the quiet Sun including a small-scale dynamo and magnetically-enhanced facular analogues with initial mean vertical magnetic fields of 100G, 200G, and 300G. Both approaches are benchmarked against facular filling factors measured from Solar Dynamics Observatory (SDO) disc-resolved images. We find good agreement between SDO-measured and SRA-inferred filling factors using the Fe I 4377 Angstrom line, with Pearson R coefficients of 0.587-0.927 across models and timescales. The estimated filling factors track the solar activity cycle, rising from ~1.5% at lower activity to ~5.5% at higher activity, consistent with SDO-measured filling factors. These results demonstrate that SRA offers a means to reliably track surface magnetic activity in disc-resolved spectra, which is necessary for mitigating the effects of activity on RV characterization of exoplanet masses and atmospheres at modern precision.

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Katlyn L. Hobbs, Christopher A. Watson, Jean C. Costes, Yvonne Unruh, Dana Clarice Yaptangco, Krishnamurthy Sowmya, Mitchell E. Young, Ernst J. W. de Mooij, Alexander G. M. Pietrow, Pál Váradi Nagy, Alexander I. Shapiro, Veronika Witzke, Federica Rescigno, Ryan A. Rubenzahl, Megan Bedell, Andrew Collier Cameron, Xavier Dumusque, Sean M. O'Brien, Benjamin M. J. Cadell, Baptiste Klein, Niamh Mallaghan, Niamh K. O'Sullivan, Toby Rodel, Sara Tavella. 2026-07-31. The Fe I 4377 {\AA} Line as a Solar Faculae Indicator: Insights from Spectral Ratio Analysis. https://doi.org/10.1093/mnras%2Fstag1462

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