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In incorporating the effect of atmospheric turbulence in the broadening ofspectral lines, the so-called radial-tangential macroturbulence (RTM) model hasbeen widely used in the field of solar-type stars, which was devised from anintuitive appearance of granular velocity field of the Sun. Since this modelassumes that turbulent motions are restricted to only radial and tangentialdirections, it has a special broadening function with notably narrow width dueto the projection effect, the validity of which has not yet been confirmed inpractice. With an aim to check whether this RTM model adequately represents theactual solar photospheric velocity field, we carried out an extensive study onthe non-thermal velocity dispersion along the line-of-sight (V_los) byanalyzing spectral lines at various points of the solar disk based onlocally-averaged as well as high spatial-resolution spectra, and found thefollowing results. First, the center-to-limb run of V_los derived fromground-based low-resolution spectra is simply monotonic with a slightlyincreasing tendency, which contradicts the specific trend (an appreciable peakat theta~45 deg) predicted from RTM. Second, the V_los values derived from alarge number of spectra based on high-resolution space observation revealed tofollow a nearly normal distribution, without any sign of peculiar distributionexpected for the RTM case. These two observational facts indicate that theactual solar velocity field is not such simply dichotomous as assumed in RTM,but directionally more chaotic. We thus conclude that RTM is not an adequatemodel at least for solar-type stars, which would significantly overestimate theturbulent velocity dispersion by a factor of ~2. The classical Gaussianmacroturbulence model should be more reasonable in this respect.

Does the radial-tangential macroturbulence model adequately describe the spectral line broadening of solar-type stars?