Master equation theory applied to the redistribution of polarized radiation in the weak radiation field limit. VI. Application to the Second Solar Spectrum of the Na I D1 & D2 lines: convergence

TL;DR

This paper applies a refined numerical method to model the polarization of the Na I D lines in the solar spectrum, demonstrating convergence and discussing limitations due to atmospheric modeling and instrumental effects.

Contribution

It introduces an improved approximation method for partial redistribution calculations, ensuring convergence and reliability in modeling polarized solar spectra.

Findings

Convergence of the numerical results is proven, behaving like a Riemann series.

Discrepancies with observations are attributed to atmospheric model limitations.

Instrumental polarization may influence the observed polarization signals.

Abstract

This paper presents a numerical application of a self-consistent theory of partial redistribution in non-LTE conditions, developed in previous papers of the series. The code was described in a previous paper of this series. However, in that previous paper (number IV of the series), the numerical results were unrealistic. The present paper presents an approximation, which was able to restore the reliability of the outgoing polarization profiles. The convergence of the results is also proved. It is demonstrated that the step increment decreases like 1/N^a, with a > 1, so that the results series behaves like a Riemann series, which is absolutely convergent. However, agreement between the computed and observed linear polarization profiles remains qualitative only. The discrepancy is assigned to the plane parallel atmosphere model, which is insufficient to describe the chromosphere, where these lines are formed. As all the integrals are numerical in the code, it could probably be adapted to more realistic and higher dimensioned model atmospheres. However, it is time consuming for lines having an hyperfine structure as the Na I D lines are. The net linear polarization observed in Na I D1 with the polarimeter ZIMPOL mounted on the McMath-Pierce telescope at Kitt Peak is not confirmed by the present calculations and could be an artefact of instrumental polarization. The presence of instrumental polarization could be confirmed by the higher linear polarization degree observed by this instrument in Na I D2 line center, with respect to the present calculation result, where the magnetic field is not accounted for, when the Hanle effect acts as a depolarizing effect in the Second Solar Spectrum. The observed linear polarization excess is found of the same order of magnitude 0.1% in both line centers, which is also comparable to the instrumental polarization compensation level of this experiment.