Heights of formation of Fe I photospheric lines

The determination of the location of spectral lineforming layers by means of line-depression contribution functions is checked by observational tests. The method is based on the assumption that the Doppler velocities derived from line bisectors at a given position in the profile of a spectral line are related to the actual bulk velocity at a particular height in the atmosphere. For a set of six magnetically sensitive and nonsensitive Fe I lines (543.45, 557.61, 630.15, 630.25, 649.50, 649.45 nm) the fluctuations of Doppler velocities along the slit of the spectrograph were determined at various ∆λ from the line centers i.e. at various heights in the photosphere. Correlations of fluctuations allow us to find those parts in line profiles which are formed at identical heights in the photosphere. These experimental results are compared with theoretical estimates based on line depression contribution functions. The agreement is good and best results are found for nonmagnetic strong, but still unsaturated lines. The results support the hypothesis that it is possible to ascribe a single height point in the solar atmosphere to a given position in a spectral line profile.