Solar Physics

, 294:63 | Cite as

Solar Center–limb Variation of the Strengths of Spectral Lines: Classification and Interpretation of Observed Trends

  • Y. TakedaEmail author
  • S. UeNo


The equivalent widths (\(W\)) of 565 spectral lines in the wavelength range of 4690 – 6870 Å were evaluated at 31 consecutive points from the solar disk center (\(\mu \equiv \cos \theta = 1\)) 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 \(\beta \)\((\equiv -\mathrm{d}\log W/ {\mathrm{d}}\log \mu )\) well correlates with that of the \(\mathrm{d} \log W/{\mathrm{d}}\log T\) 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. Furthermore, the key to understanding the behavior of \(\mathrm{d}\log W/{\mathrm{d}}\log T\) (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.


Center–limb observations Spectral line, intensity and diagnostics Spectrum, visible Velocity fields, photosphere 


Disclosure of Potential Conflicts of Interest

The authors declare that they have no conflicts of interest.

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© Springer Nature B.V. 2019

Authors and Affiliations

  1. 1.National Astronomical Observatory of JapanMitaka, TokyoJapan
  2. 2.SOKENDAIThe Graduate University for Advanced StudiesMitaka, TokyoJapan
  3. 3.Kwasan and Hida ObservatoriesKyoto UniversityTakayama, GifuJapan

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