The HARPS-N solar telescope legacy: A decade of Solar High-Fidelity Spectroscopy and Precise Radial Velocities

The solar telescope connected to the HARPS-N spectrograph has been observing the Sun every possible day since the summer of 2015. Such high-cadence, long-baseline data set is crucial to understand spurious radial-velocity signals induced by our Sun but also to characterise better the HARPS-N instrument. This data set includes 10 years of nearly continuous monitoring of our Sun in high-fidelity spectroscopy and precise radial-velocities, with several hours of measurements every possible day. The data were reduced using the ESPRESSO Data Reduction Software version 3.2.5, and post-processed further to assure data quality. All the details related to the reduction and curation of the data can be found in Dumusque et al. 2025 in prep. The median daily radial-velocity precision is 0.49 m/s, and the long-term precision well below the m/s. An exact value for the precision over the long-term is difficult to assess due to the solar magnetic cycle and rotational activity. Modelling this long-term variation thanks to the magnesium II activity index and smoothing the data with a 30-day moving average to mitigate stellar activity at the rotation timescale, we converge to a long-term precision below 0.54 m/s. This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement SCORE No 851555) This work has been carried out within the framework of the NCCR PlanetS supported by the Swiss National Science Foundation under grants 51NF40_182901 and 51NF40_205606.