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The Sun's spectrum varies with center-to-limb distance. This variation isgoverned by the underlying temperature-density structure of the solaratmosphere. To explore the spectrally resolved center-to-limb variation (CLV)we make use of two spectral atlases recorded with the Fourier transformspectrometer (FTS) at the McMath-Pierce facility at Kitt Peak. One spectralatlas obtained 10 arcsec inside the solar limb was recorded in 1978-79 as partof the first survey of the Second Solar Spectrum, while the other atlas is thewell used reference NSO/Kitt Peak FTS atlas for the disk center. Both atlasesrepresent fully resolved spectra without any spectral stray light. We thenconstruct an atlas of the limb/disk-center ratio between the two spectra overthe wavelength range 4084-9950 \AA. This ratio spectrum, which expresses theCLV amplitude relative to the continuum, is as richly structured as theintensity spectrum itself, but the line profiles differ greatly in both shapeand amplitude. It is as if we are dealing with a new, unfamiliar spectrum ofthe Sun, distinctly different from both the intensity spectrum (which we hererefer to with the acronym SS1) and the linear polarization of the Second SolarSpectrum (for which we use acronym SS2). In analogy we refer to the new ratiospectrum as SS3. While there is hardly any resemblance between SS3 and SS2, weare able to identify a non-linear mapping that can translate SS1 to SS3 in thecase of weak to medium-strong spectral lines that are mainly formed in LTE(being directly coupled to the local temperature-density structure). Thisnon-linear mapping is successfully modeled in terms of two free parameters thatare found to vary approximately linearly over the entire wavelength rangecovered. These parameters and the various SS3 line profiles provide a novel,rich set of observational constraints on model atmospheres.

FTS atlas of the Sun's spectrally resolved center-to-limb variation