Convective characteristics of Fe I lines across the solar disc

Georg-August Universität Göttingen, Institut für Astrophysik und Geophysik, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
e-mail: momo.ellwarth@uni-goettingen.de

Received: 31 July 2023
Accepted: 9 October 2023

Abstract

Context. Solar convection is visible as a net blueshift of absorption lines, which becomes apparent when observing quiet Sun granulation. This blueshift exhibits variations from the disc centre to the solar limb due to differing projection angles onto the solar atmosphere.

Aims. Our goal is to investigate convective Doppler velocities based on observations from the disc centre to the solar limb. Consequently, we aim to improve our understanding of atmospheric hydrodynamics and contribute to the improvement of solar and stellar atmospheric models.

Methods. We used resolved quiet-Sun spectra to investigate the convective velocity shifts of more than 1000 Fe I lines across multiple centre-to-limb positions on the solar disc. We determined the Doppler velocities with respect to the line depth. Additionally, we calculated the formation temperature and investigated its correlation with Doppler velocities.

Results. The general behaviour of convective line shifts shows a decreasing blueshift as the lines become deeper for all observing positions from the centre to limb. For spectra obtained at the solar limb, even deeper lines exhibit redshifts. We observe a velocity trend for the different observation angles, with a less pronounced convective blueshift towards the solar limb. Convective velocities show a wavelength dependence for each observing angle when analysing on the basis of line depths. We observe a decreasing convective blueshift as the formation temperatures of the lines decrease. The velocity change over temperature ranges proceeds slower towards the solar limb. When investigating Doppler velocities with respect to formation temperature, the disc centre does not exhibit the strongest blueshift.