Convective shifts of iron lines in the spectrum of the solar photosphere

TL;DR

This study investigates how convective motions in the solar photosphere affect the shifts of iron spectral lines, revealing the dependence on line formation regions and convection dynamics in visible and infrared spectra.

Contribution

It provides a detailed analysis of the convective influence on iron line shifts using 2-D hydrodynamic models, highlighting differences between visible and infrared line formation.

Findings

Weak and moderate lines form deep in granules, contributing to blue shifts.

Strong lines form higher in the upper photosphere, showing reduced blue shifts.

Infrared lines form higher in the atmosphere, explaining decreased blue shifts.

Abstract

The influence of the convective structure of the solar photosphere on the shifts of spectral lines of iron was studied. Line profiles in the visible and infrared spectrum were synthesized with the use of 2-D time-dependent hydrodynamic solar model atmospheres. The dependence of line shifts on excitation potential, wavelength, and line strength was analyzed, along with the depression contribution functions. The line shifts were found to depend on the location of the line formation region in convective cells and the difference between the line depression contributions from granules and intergranular lanes. In visible spectrum the weak and moderate lines are formed deep in the photosphere. Their effective line formation region is located in the central parts of granules, which make the major contribution to the absorption of spatially unresolved lines. The cores of strong lines are formed in upper photospheric layers where is formed reversed granulation due to convection reversal and physical conditions change drastically there. As a consequence the depression contributions in the strong line from intergranular lanes with downflows substantially increase. This accounts for smaller blue shifts of strong lines. In infrared spectrum the observed decrease in the blue line shifts is explained by the fact that their effective line formation regions lie higher in the photosphere and extend much further into the reversed granulation region due to the line opacity rise with the increase of line wavelength. Additionally the effective line formation depths of the synthesized visible and infrared Fe I lines and their dependence on line parameters is discussed.