# Solar Center--Limb Variation of the Strengths of Spectral Lines: Classification and Interpretation of Observed Trends

## TL;DR

This study analyzes the center-limb variation of spectral line strengths in the solar spectrum, revealing that temperature sensitivity and population stage of elements primarily influence observed trends, with comprehensive data made publicly available.

## Contribution

It provides a detailed classification and interpretation of spectral line strength variations across the solar disk, linking them to line properties and population stages, and offers extensive online data resources.

## Key findings

- Center-limb variation correlates with temperature sensitivity of lines.
- Line behavior depends on whether species are in minor or major population stages.
- Data includes equivalent widths, turbulence, abundances, and formation depths.

## Abstract

The equivalent widths (W) of 565 spectral lines in the wavelength range of 4690--6870A were evaluated at 31 consecutive points from the solar disk center (mu=cos(theta)=1) to near the limb (mu=0.25) by applying the synthetic spectrum-fitting technique, in order to clarify the nature of their center--limb variations, especially the observed slope differing from line to line and its interpretation in terms of line properties. We found that the distribution of the gradient beta (= -dlog W/dlog mu) well correlates with that of dlog W/dlog T index, which means that the center-to-limb variation of W is determined mainly by the T-sensitivity of individual lines because the line-forming region shifts towards upper layers of lower T as we go toward the limb. Further, the key to understanding the behavior of dlog W/dlog T (depending on the temperature sensitivity of number population) is whether the considered species is in minor population stage or major population stage, by which the distribution of beta is explained in terms of differences in excitation potential and line strengths. All the center--limb data of equivalent widths (as well as line-of-sight turbulent velocity dispersions, elemental abundances, and mean line-formation depths derived as by-products) along with the solar spectra used for our analysis are made available as on-line materials.