Lithium 671-nm line at the polar and equatorial solar limbs

Abstract

The equivalent widths of six weak lines ( 7 Li 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.

Convective differential rotation in stars and planets – I. Theory

“…The baroclinicity λ is in agreement with (albeit uncertain) measurements of the solar pole-equator temperature difference (Teplitskaya et al 2015). Unfortunately while there have been measurements of the meridional circulation in the Sun (Zhao et al 2013;Rajaguru & Antia 2015;Schad & Roth 2020), there are still significant disagreements between the different inversion techniques which make a direct comparison to our theory challenging.…”

“…Some authors argue that thermal wind balance and entropy gradients dominate the solar rotation profile (Miesch et al 2006;Balbus & Schaan 2012), while others have called this into question (Brun et al 2010;Brun & Toomre 2002). Observations suggest that the thermal wind term is substantial, though there remain uncertainties as to its precise contribution (Caccin et al 1976;Rast et al 2008;Teplitskaya et al 2015). Other models suggest that turbulent anisotropy is the most relevant factor in setting the differential rotation (Ruediger 1989;Kueker et al 1993;Kitchatinov 2013), and more complex models with various parameterizations have also been proposed (Tuominen & Ruediger 1989;Kissin & Thompson 2015;Brun & Rempel 2009).…”

Solar dynamo cycle variations with a rotational period

“…The effect is strong near the bottom of the convection zone and it is quenched toward the surface. So the temperature inhomogeneity is rather small at the solar surface Beckers (1960); Miesch et al (2008); Teplitskaya et al (2015). The models for the rotation periods from 1 to 10 days show the stronger latitudinal contrast for the convective velocity RMS variations than the model M25n.…”

Convective Differential Rotation in Stars and Planets I: Theory

“…Some authors argue that thermal wind balance and entropy gradients dominate the solar rotation profile (Miesch et al 2006;Balbus & Schaan 2012), while others have called this into question (Brun et al 2010;Brun & Toomre 2002). Observations suggest that the thermal wind term is substantial, though there remain uncertainties as to its precise contribution (Caccin et al 1976;Rast et al 2008;Teplitskaya et al 2015). Other models suggest that turbulent anisotropy is the most relevant factor in setting the differential rotation (Ruediger 1989;Kueker et al 1993;Kitchatinov 2013), and more complex models with various parameterizations have also been proposed (Tuominen & Ruediger 1989;Kissin & Thompson 2015;Brun & Rempel 2009).…”