Synthesizing Sun-as-a-star flare spectra from high-resolution solar observations

¹ Centre for Mathematical Plasma Astrophysics, KU Leuven, Celestijnenlaan 200B, B-3001, Leuven, Belgium
² Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482, Potsdam, Germany
³ Inst. for Solar Physics, Dept. of Astronomy, Stockholm University, Albanova University Centre, SE-106 91, Stockholm, Sweden
⁴ Astronomical Institute, Slovak Academy of Sciences, Tatranská Lomnica, 059 60, Slovakia
⁵ ETH Zürich, Institute for Particle Physics and Astrophysics, Wolfgang-Pauli-Strasse 27, 8093, Zürich, Switzerland
⁶ Istituto Ricerche Solari Aldo e Cele Daccó (IRSOL), Locarno, Switzerland
⁷ Centre of Scientific Excellence – Solar and Stellar Activity, University of Wrocław, 50137, Wrocław, Poland
⁸ Astronomical Institute of the Czech Academy of Sciences, 251 65, Ondřejov, Czech Republic
⁹ Institute of Theoretical Astrophysics, University of Oslo, P.O. Box 1029 Blindern, N-0315, Oslo, Norway
¹⁰ Rosseland Centre for Solar Physics, University of Oslo, P.O. Box 1029 Blindern, N-0315, Oslo, Norway
¹¹ Department of Physics and Astronomy, George Mason University, Fairfax, VA, 22030, USA
¹² Heliophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD, 20771, USA
¹³ National Solar Observatory, 3665 Discovery Drive, Boulder, CO, 80303, USA
¹⁴ Evgeni Kharadze Georgian National Astrophysical Observatory, Mount Kanobili, 0301, Abastumani, Georgia
¹⁵ Instituto de Astrofísica de Canarias, E-38205, La Laguna, Tenerife, Spain
¹⁶ Universidad de La Laguna, Dept. Astrofísica, E-38206, La Laguna, Tenerife, Spain
¹⁷ Northumbria University, NE1 8ST, Newcastle upon Tyne, UK
¹⁸ Instituto de Astrofísica e Ciências do Espaço, CAUP, Universidade do Porto, Rua das Estrelas, 4150-762, Porto, Portugal
¹⁹ Departamento de Fisica e Astronomia, Faculdade de Ciencias, Universidade do Porto, Rua do Campo Alegre, Porto, Portugal
²⁰ Institut für Sonnenphysik (KIS), Georges-Köhler-Allee 401 A, Freiburg i.Br., Germany
²¹ Institute of Physics, University of Graz, Universitätsplatz 5, 8010, Graz, Austria
²² Department of Astronomy and Astrophysics at Space Research Center, Ilia State University, Tbilisi, Georgia
²³ Astronomical Institute of the University of Bern, Sidlerstrasse 5, 3012, Bern, Switzerland
²⁴ Plasma Dynamics Group, School of Electrical and Electronic Engineering, University of Sheffield, Sheffield, S1 3JD, UK

^⋆ Corresponding author: apietrow@aip.de

Received: 30 March 2025
Accepted: 9 July 2025

Abstract

Context. Spatially resolved observations of the Sun and the astronomical sample size of stellar bodies are the respective key strengths of solar and stellar observations. However, the large difference in object brightness between the Sun and other stars has led to distinctly different instrumentation and methodologies between the two fields.

Aims. We produced and analyzed synthetic full-disk spectra derived from 19 small area field-of-view optical observations of solar flares acquired by the Swedish 1-m Solar Telescope (SST) between 2011 and 2024. These were used to investigate what can and cannot be inferred about physical processes on the Sun from Sun-as-a-star observations.

Methods. The recently released Numerical Empirical Sun-as-a-Star Integrator (NESSI) code provides synthetic full-disk integrated spectral line emission based on smaller field-of-view input while accounting for center-to-limb variations and differential rotation. We used this code to generate pseudo-Sun-as-a-star spectra from the SST observations.

Results. We show that limited-area solar observations can be extrapolated to represent the full disk accurately in a manner close to what is achievable with Sun-as-a-star telescopes. Additionally, we identify nine spectral features, four of which are caused by instrumental effects. Most notably, we find a relation between the heliocentric angle of flares and the width of the excess emission left by them as well as a source of false positive coronal mass ejections-like signatures, and we defined an energy scaling law based on chromospheric line intensities that shows that the peak flare contrast roughly scales with the square root of the bolometric energy.

Conclusions. The presented method of creating pseudo-Sun-as-a-star observations from limited field-of-view solar observations allows for the accurate comparison of solar flare spectra with their stellar counterparts while allowing for the detection of signals at otherwise unachievable noise levels.