Solar flux atlases The new HARPS-N quiet Sun benchmark and continuum normalisation of the Ca II H & Klines

Context. Solar flux atlases observe the spatially integrated light from the Sun, treating it as a star. They are fundamental tools for gaining insight into the composition of the Sun and other stars. They are utilised as reference material for a wide range of solar applications such as stellar chemical abundances, atmospheric physics, stellar activity, and radial velocity signals. Aims. We provide a detailed comparison of solar activity reported in some of the well-known solar atlases against the new High Accuracy Radial velocity Planet Searcher for the Northern hemisphere (HARPS-N) Quiet Sun (QS) and Measured Activity (MA) atlases published, for the first time, in this work. Methods. Ten of the widely used individual spectral lines from each flux atlas were selected to compare solar activity based on three methods: (1) equivalent widths; (2) a novel activity measure introduced in this work and referred to as the activity number; and (3) bisectors and radial velocity. Results. The significantly smaller activity levels measured in the MA atlas, compared to the other atlases, relative to the QS atlas, underscores the dominance of instrumental effects over solar activity in their impact on spectral lines, which cannot be corrected through simple line convolution to match resolutions of other atlases. Additionally, our investigation unexpectedly revealed a substantial intensity shift in the CaiiH&K lines of other atlases compared to our HARPS-N atlases, likely resulting from the assumptions made when applying normalisation techniques for the early Kitt Peak atlases. Conclusions. With an average spot number of zero, our QS atlas is well suited to serve as an absolute benchmark atlas representative of solar minimum for the visible spectrum, which other atlases can be compared against. Our recommendations going forward include: (1) the publication of a detailed log along with the observations to include exact dates and indications of solar activity; and (2) given the dominance of instrumental effects over variations caused by activity, quiet Sun reference atlases must be constructed with the same instruments to ensure high precision.