Convective blueshifts in the solar atmosphere, I. Absolute measurements with LARS of the spectral lines at 6302 {\AA}

TL;DR

This study uses high-resolution spectroscopic observations with laser calibration to measure absolute convective blueshifts in the solar photosphere across different disk positions, revealing detailed velocity stratification and limb variation.

Contribution

First precise absolute measurements of convective line shifts in the solar photosphere using laser-calibrated spectra at multiple disk positions.

Findings

Maximum blueshifts of up to -450 m/s at disk center.

Line bisectors change shape from C-shape to \\-shape toward limb.

Blueshift increases slightly away from disk center, more so in less active regions.

Abstract

The solar convection manifests as granulation and intergranulation at the solar surface. In the photosphere, convective motions induce differential Doppler shifts to spectral lines. The observed convective blueshift varies across the solar disk. We focus on the impact of solar convection on the atmosphere and aim to resolve its velocity stratification in the photosphere. We performed high-resolution spectroscopic observations of the solar spectrum in the 6302\,\AA\ range with the Laser Absolute Reference Spectrograph (LARS) at the Vacuum Tower Telescope. A laser frequency comb enabled the calibration of the spectra to an absolute wavelength scale with an accuracy of ${\rm 1\,m\,s^{-1}}$. We systematically scanned the Quiet Sun from disk center to the limb at ten selected heliocentric positions. The analysis included 99 time sequences of up to 20\,min in length. By means of ephemeris and reference corrections, we translated wavelength shifts into absolute line-of-sight velocities. A bisector analysis on the line profiles yielded the shapes and convective shifts of seven photospheric lines. At disk center, the bisector profiles of the iron lines feature a pronounced C-shape with maximum convective blueshifts of up to ${\rm -450\,m\,s^{-1}}$ in the spectral line wings. Toward the solar limb, the bisectors change into a \textbackslash-shape with a saturation in the line core at a redshift of ${\rm +100\,m\,s^{-1}}$. The center-to-limb variation of the line core velocities shows a slight increase in blueshift when departing the disk center for larger heliocentric angles. This increase in blueshift is more pronounced for the magnetically less active meridian than for the equator. Toward the solar limb, the blueshift decreases and can turn into a redshift. Best spectroscopic measurements enabled the accurate determination of absolute convective shifts in the solar photosphere.