Solar Flux Atlases: The new HARPS-N Quiet Sun Benchmark and Ca II H & K Lines Continuum Normalisation

TL;DR

This paper introduces a new high-precision solar flux atlas from HARPS-N, compares it with existing atlases, and discusses the impacts of instrumental effects and normalization techniques on solar activity measurements.

Contribution

The study provides the first comparison of the HARPS-N Quiet Sun atlas with other solar flux atlases, highlighting instrumental effects and normalization issues affecting spectral line analysis.

Findings

HARPS-N QS atlas shows minimal solar activity, ideal as a solar minimum benchmark.

Instrumental effects dominate spectral line variations over solar activity in some atlases.

Normalization assumptions in early Kitt Peak atlases cause intensity shifts in Ca II H & K lines.

Abstract

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, and are utilized 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 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 that are 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 using three methods: 1) Equivalent Widths 2) Activity number, a novel activity measure which we introduce in this work. 3) Bisectors and radial velocity. Results. The significantly smaller activity 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 Ca ii H & K lines of other atlases compared to our HARPS-N atlases, likely caused by assumptions in normalisation techniques used in 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 that other atlases can be compared against. Our recommendations are 1) publication of a detailed log along with the observations, to include exact dates and indications of solar activity...