A decade of solar high-fidelity spectroscopy and precise radial velocities from HARPS-N

¹ Astronomy Department of the University of Geneva 51 ch. des Maillettes 1290 Versoix, Switzerland
² Instituto de Astrofísica e Ciência s do Espaço, Universidade do Porto, CAUP Rua das Estrelas 4150-762 Porto, Portugal
³ Denys Wilkinson Building, Department of Physics University of Oxford OX1 3RH, UK
⁴ MIT Lincoln Laboratory Lexington MA 02421, USA
⁵ INAF – Osservatorio Astronomico di Palermo Piazza del Parlamento 1 90134 Palermo, Italy
⁶ School of Physics & Astronomy, University of Birmingham Edgbaston Birmingham B15 2TT, UK
⁷ INAF – Osservatorio Astrofisico di Torino via Osservatorio 20 10025 Pino Torinese, Italy
⁸ DTU Space, National Space Institute, Technical University of Denmark Elektrovej 328 DK-2800 Kgs. Lyngby, Denmark
⁹ SUPA School of Physics and Astronomy, University of St Andrews North Haugh St Andrews KY16 9SS, UK
¹⁰ Physics Department, University of Warwick Gibbet Hill Road Coventry CV4 7AL, UK
¹¹ Centre for Exoplanets and Habitability, University of Warwick Gibbet Hill Road Coventry CV4 7AL, UK
¹² Fundación Galileo Galilei-INAF Rambla José Ana Fernandez Pérez 7 E-38712 Breña Baja Tenerife, Spain
¹³ Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast Belfast BT7 1NN, UK
¹⁴ Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology Cambridge MA 02139, USA
¹⁵ Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology Cambridge MA 02139, USA
¹⁶ Astrophysics Group, University of Exeter Exeter EX4 2QL, UK
¹⁷ Center for Astrophysics | Harvard & Smithsonian 60 Garden Street Cambridge MA 02138, USA
¹⁸ Space Telescope Science Institute 3700 San Martin Drive Baltimore MD 21218, USA
¹⁹ INAF, Astronomical Observatory of Padua Vicolo dell’Osservatorio 5 I-35122 Padua, Italy
²⁰ Department of Physics and Astronomy ‘Galileo Galilei’, University of Padova Vicolo dell’Osservatorio 3 I-35122 Padova, Italy
²¹ Centro di Ateneo di Studi e Attività Spaziali ‘G. Colombo’, Università degli Studi di Padova Via Venezia 15 IT-35131 Padova, Italy
²² Moses Brown School Providence RI 02906, USA
²³ INAF – Osservatorio Astronomico di Cagliari via della Scienza 5 09047 Selargius, Italy
²⁴ Centre for Astrophysics, University of Southern Queensland Toowoomba QLD 4350, Australia
²⁵ SUPA, Institute for Astronomy, University of Edinburgh, The Royal Observatory Blackford Hill Edinburgh EH9 3HJ, UK
²⁶ Space sciences, Technologies and Astrophysics Research (STAR) Institute, Université de Liège Allée du 6 Août 19C 4000 Liège, Belgium
²⁷ Astrobiology Research Unit, Université de Liège Allée du 6 Août 19C 4000 Liège, Belgium
²⁸ European Southern Observatory, Av. Alonso de Cordova 3107 Vitacura Santiago de Chile, Chile

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Received: 6 September 2025
Accepted: 28 November 2025

Abstract

Context. The HARPS-N solar telescope has been observing the Sun every possible day since the summer of 2015. We have recently released 10 years of these data, which are available online.

Aims. The goal of this paper is to present the different optimisations made to the ESPRESSO data reduction software used to extract the published HARPS-N solar spectra, describe the data curation, and perform some analyses that demonstrate the extreme radial velocity (RV) precision of those data.

Methods. By analysing all of the HARPS-N wavelength solutions over 13 years, we brought to light instrumental systematics at the 1 m s⁻¹ level. We mitigated those systematics by curating the thorium line list used to derive the wavelength solution and applying a correction to the drift of thorium lines induced by the aging of thorium-argon hollow cathode lamps. After optimisation, we demonstrated a peak-to-peak precision on the HARPS-N wavelength solution better than 0.75 m s⁻¹ over 13 years. We then carefully curated the decade of HARPS-N re-reduced solar observations by rejecting 30% of the data affected either by clouds, bad atmospheric conditions, or well-understood instrumental systematics. Finally, we corrected the curated data for spurious sub-meter-per-second RV effects caused by erroneous instrumental drift measurements and by changes in the spectral blaze function over time.

Results. After curation and correction, a total of 109,466 HARPS-N solar spectra and respective RVs over a decade were made available. The median photon-noise precision of the RV data is 0.28 m s⁻¹, and on daily timescales, the median RV rms is 0.49 m s⁻¹, which is similar to the level imposed by stellar granulation signals. On 10 year timescales, the large RV rms of 2.95 m s⁻¹ results from the RV signature of the Sun’s magnetic cycle. Through modelling of this long-term effect using the Bremen composite magnesium II activity index, we demonstrate a long-term RV precision of 0.41 m s⁻¹. We also analysed contemporaneous HARPS-N and NEID solar RVs and found the data from both instruments to be of similar quality and precision. However, an analysis of the RV difference between these two RV datasets over the three available years gave a surprisingly large RV rms of 1.3 m s⁻¹. This variation is dominated by an unexplained trend that could be caused by a different sensitivity to stellar activity of the two datasets. Once this trend was modelled, the overall RV rms for three years reached 0.79 m s⁻¹, and the RV rms during the low-activity phase decreased to 0.6 m s⁻¹, compatible with what is expected from supergranulation.

Conclusions. This decade of high-cadence HARPS-N solar observations with short- and long-term precision below one m s⁻¹ represents a crucial dataset in the pursuit of further understanding the stellar activity signals in solar-type stars and advancing other science cases requiring such extreme precision.