# Convective blueshifts in the solar atmosphere: III. High-accuracy observations of spectral lines in the visible

## TL;DR

This study provides high-precision measurements of convective blueshifts in the solar atmosphere, revealing their center-to-limb variation and dependence on line depth, which aids in refining solar atmospheric models and exoplanet detection methods.

## Contribution

It offers the first high-accuracy, systematic spectroscopic observations of convective blueshifts across the solar disk using laser frequency comb calibration.

## Key findings

- Convective blueshift varies systematically from disk center to limb.
- Line asymmetries depend on line depth and position on the solar disk.
- Synthetic models show significant deviations from observed blueshift patterns.

## Abstract

Convective motions in the solar atmosphere cause spectral lines to become asymmetric and shifted in wavelength. For photospheric lines, this differential Doppler shift varies from the solar disk center to the limb. Precise and comprehensive observations of the convective blueshift and its center-to-limb variation improve our understanding of the atmospheric hydrodynamics and ensuing line formation, and provide the basis to refine 3D models of the solar atmosphere. We performed systematical spectroscopic measurements of the convective blueshift of the quiet Sun with the Laser Absolute Reference Spectrograph (LARS) at the German Vacuum Tower Telescope. The spatial scanning of the solar disk covered four radial (meridional and equatorial) axes. The high-resolution spectra of 26 photospheric to chromospheric lines in the visible range were calibrated with a laser frequency comb to absolute wavelengths at the 1m/s accuracy. Applying ephemeris and reference corrections, the bisector analysis provided line asymmetries and Doppler shifts with an uncertainty of only few m/s. To allow for a comparison with other observations, we convolved the results to lower spectral resolutions. Typically, a blueshifted "C"-shaped curve at disk center transforms into a less blueshifted "\"-shape toward the solar limb. The comparison of all lines reveals the systematic dependence of the convective blueshift on the line depth. Synthetic models yield considerable deviations from the observed center-to-limb variation. The obtained Doppler shifts of the quiet Sun can serve as an absolute reference for other observations, the relative calibration of Dopplergrams, and the necessary refinement of atmospheric models. Based on this, the development of high-precision models of stellar surface convection will advance the detection of (potentially habitable) exoplanets by radial velocity measurements.