A Decade of Solar High-Fidelity Spectroscopy and Precise Radial Velocities from HARPS-N

TL;DR

This paper reports a comprehensive optimization of the ESPRESSO data-reduction software for HARPS-N solar observations, including correcting TH-AR lamp aging effects, curating thorium lines, refining flat-fielding, and removing Ca II H&K ghosts. Through stringent data-curation and novel drift- and blaze-correction methods, the authors deliver a decade-long, high-fidelity solar RV dataset comprising 109,466 spectra with a median photon-noise precision of $0.28\,\mathrm{m\,s^{-1}}$ and a long-term RV precision of $0.41\,\mathrm{m\,s^{-1}}$ after activity modeling. They compare HARPS-N with NEID, finding broadly consistent short-term performance and revealing a residual trend in RV differences that highlights the challenge of long-term instrument stability. The curated data are publicly released, providing a valuable resource for solar/stellar activity studies and future extreme-precision RV work. The work demonstrates that careful calibration, line selection, and activity modeling can push RV precision toward the regime needed to detect Earth-like signals, and sets a benchmark for ongoing calibration efforts across EPRV spectrographs.

Abstract

We recently released 10 years of HARPS-N solar telescope and the goal of this manuscript is to present the different optimisations made to the data reduction, to describe data curation, and to perform some analyses that demonstrate the extreme RV precision of those data. By analysing all the HARPS-N wavelength solutions over 13 years, we bring to light instrumental systematics at the 1 m/s level. After correction, we demonstrate a peak-to-peak precision on the HARPS-N wavelength solution better than 0.75 m/s over 13 years. We then carefully curate 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 correct the curated data for spurious sub-m/s RV effects caused by erroneous instrumental drift measurements and by changes in the spectral blaze function over time. After curation and correction, a total of 109,466 HARPS-N solar spectra and respective RVs over a decade are available. The median photon-noise precision of the RV data is 0.28 m/s and, on daily timescales, the median RV rms is 0.49 m/s, similar to the level imposed by stellar granulation signals. On 10-year timescales, the large RV rms of 2.95 m/s results from the RV signature of the Sun's magnetic cycle. When modelling this long-term effect using the Magnesium II activity index, we demonstrate a long-term RV precision of 0.41 m/s. We also analysed contemporaneous HARPS-N and NEID solar RVs and found the data from both instruments to be of similar quality and precision, with an overall RV differece rms of 0.79 m/s. This decade of high-cadence HARPS-N solar observations with short- and long-term precision below 1 m/s represents a crucial dataset to further understand stellar activity signals in solar-type stars , and to advance other science cases requiring such an extreme precision.

Figures (13)

• Figure 1: Top: Median drift of all TH lines as a function of time. This tracks the drift of HARPS-N over time, which stays below 1 km s$^{-1}$ despite several drastic instrumental interventions. Middle: Median behaviour of the AR lines as a function of time once the drift of TH lines has been corrected for.This correction is applied locally on different regions of the detector to be less sensitive to LSF variations (see text). Bottom: TH flux ratio, i.e. integrated flux of the TH lamp with respect to a reference, as a function of time. We see that between JD = 2458500 and 2459002 (16 January 2019 to 2 June 2020) the TH flux ratio reached very high values, which will have an impact on wavelength solution derivation. The thick red dashed vertical lines correspond, chronologically to the change of the HARPS-N focus, the change of the TH-AR HC lamp used for wavelength solution and the change of the detector cryostat. The thin red vertical dashed lines correspond to sudden changes in the flux of the TH-AR HC lamp (see bottom panel), and the black vertical dashed lines correspond to detector warm-ups. More details about those events are given in Table \ref{['app:intervention']}
• Figure 2: Histogram of the daily RV rms for the HARPS-N solar data presented in this paper, compared with the same statistics from the NEID data published in Ford:2024aa for comparison. We note that the NEID data correspond to the 5-minute binned NEID solar data, rvs_binned_5.csv, available here https://zenodo.org/records/13363762.
• Figure 3: Top: Comparison between the $\log{R'_{HK}}$ activity index derived from HARPS-N and the linearly rescaled Bremen Composite Mg2 activity index. Bottom: Correlation between those two activity indices, that demonstrate their very strong relation.
• Figure 4: Top: RVs without and with the correction for blaze variation, smoothed using a 30-day moving average to mitigate stellar activity signals due to the Sun's rotation (in blue and orange, respectively). The red and purple dots corresponds to the best-fitted linear model of the Mg2 activity index to those two different time-series. Middle: RV residuals after subtracting the best Mg2 activity index linear model. The vertical lines highlight the change of the broken solar feed fibre, the change of the TH-AR HC lamp used for wavelength solution and the change of the detector cryostat (see legend). The green region highlights a period when the flux of the TH-AR HC lamp was larger than 2.5 times it nominal value (see Fig. \ref{['fig:th_ar_drift']}). Bottom: RV residuals after subtracting a linear model including the Mg2 activity index, two offsets for the broken fibre and the cryostat change and a scaling factor for the AR1 to TH ratio.
• Figure 5: RVs of the carefully curated HARPS-N solar data as a function of time. The three important gaps highlighted by red regions correspond to a broken fibre guiding light from the solar telescope to the HARPS-N calibration unit, a moment in time where the TH-AR HC lamp was out of specification to provide extremely precise wavelength solutions and a problem of alignment between the guiding camera tracking the Sun and the solar telescope itself. The inset focuses on nearly 200 days of solar RVs just after solar maximum, where rotational modulation, induced by active regions co-moving with the solar surface, is very strong.