Lithium 671-nm Line at the Polar and Equatorial Solar Limbs

The equivalent widths of six weak lines (7Li I, Si I, Fe I (two lines), Ti I, and CN) at the equatorial and polar limbs are compared. The lithium line is of greatest interest, because the abundance of lithium isotopes on the Sun and stars is closely related to primordial nucleosynthesis and plasma mixing in their interiors. For four of the six lines, the equivalent widths near the equator have been found to differ from those near the pole at a significance level of at least 95%. The ratios of the equivalent widths at the pole and the equator for the Si I and Fe I lines are consistent with the predictions of the elementary theory of the curves of growth for weak photospheric lines and lead to an estimate of the temperature difference ~13 K. The Li I and Ti I lines deviate significantly from the predictions made on the basis of static planeparallel, 1D models under local thermodynamic equilibrium (LTE) conditions. The greatest (among the lines being studied) difference between the equivalent widths (≈4%) found for Li I, the only one obtained essentially at a significance level of ∼100%, could be interpreted as one of the numerous manifestations of granulation dynamics. However, the current status of 3D NLTE simulations for the photosphere does not yet allow the ultimate conclusions to be reached. It may well be that at least part of the observed decrease in the equivalent width of the lithium resonance line at the solar pole is attributable to more efficient lithium depletion in the polar regions.