| Title: |
Synthesizing Sun-as-a-star flare spectra from high-resolution solar observations.
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| Authors: |
DE WILDE, M. - PIETROW, A. G. M.** - DRUETT, M. K. - PASTOR YABAR, A. - KOZA, Julius - KONTOGIANNIS, Ioannis - ANDRIIENKO, O. - BERLICKI, A. - BRUNVOLL, A. R. - DE LA CRUZ RODRIQUEZ, J. - THOEN FABER, J. - JOSHI, R. - KURIDZE, David - NOBREGA-SIVERIO, D. - ROUPPE VAN DER VOORT, L. - RYBAK, Jan - SCULLION, Eamon - SILVA, A. M. - VASHALOMIDZE, Zurab - VINCENTE AREVALO, A. - WISNIEWSKA, Aneta - YADAV, R. - ZAQARASHVILI, Teimuraz V. - ZBINDEN, J. - OYRE, E. S.
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| Image & caption: |
Please click on image thumbnail to get more info:
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| Image caption::
| Comparison of localized flare profiles and disk-integrated Sun-as-a-star profiles. Left: A GONG Hα line core image showing a large solar flare (red box) within a field of view (FOV) typical of high-resolution solar observations. Right: The Ca II 8542 Å, Hα, and Ca II K line profiles at different locations: within the FOV in a quiet region (blue), integrated over the quiet-Sun disk for the Sun as a star (orange), and on the flare ribbon (green). The scaled flare (red dashed) is a simple ratio scaling of the FOV area to the total disk area, while the NESSI profile is the result of injecting the average flare profile (green) into a full-disk quiet-sun profile (blue). |
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| Description: |
In this article, we have developed and applied a method for generating synthetic Sun-as-a-star spectra from limited field-of-view solar observations. Using 19 small-area optical flare datasets obtained with the Swedish 1-m Solar Telescope (SST) between 2011 and 2024, we calculated full-disk integrated spectra with the recently released Numerical Empirical Sun-as-a-Star Integrator (NESSI), which accounts for center-to-limb variations and differential rotation. We published these pseudo–Sun-as-a-star spectra and used them to investigate which physical processes on the Sun can—and cannot—be reliably inferred from stellar-style observations.
We show that small-area solar observations can be extrapolated with high accuracy to represent full-disk emission at noise levels comparable to those achieved by dedicated Sun-as-a-star instruments. We discovered nine distinct spectral features in the synthetic spectra, four of which originate from instrumental effects. Most notably, we identified a relationship between a flare’s heliocentric angle and the width of its resulting excess emission. We also uncovered a mechanism capable of producing false positive, CME-like spectral signatures in Sun-as-a-star data. In addition, we derived an energy–intensity scaling law based on chromospheric line emission, demonstrating that the peak flare contrast scales approximately with the square root of the bolometric flare energy.
Overall, we have shown that our method enables precise comparisons between solar and stellar flare spectra and permits the detection of signals that would otherwise remain inaccessible due to noise limitations. |
| Reference: |
Astronomy and Astrophysics, 2025, vol. 700, article no. A275, p. 1-21. ISSN 0004-6361. |
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