Are collisions with neutral hydrogen important for modelling the Second Solar Spectrum of Ti I and Ca II ?

TL;DR

This study assesses how isotropic collisions with neutral hydrogen influence the scattering polarization signals of specific Ti I and Ca II lines, revealing that such collisions have minimal impact on the polarization amplitudes, with some exceptions.

Contribution

It provides a detailed analysis of collisional effects on scattering polarization in solar atmosphere lines, using realistic multilevel atomic models and statistical equilibrium calculations.

Findings

Lower levels of Ti I lines are fully depolarized by elastic collisions.

Upper-level collisional depolarization is negligible for most lines, except a small effect on the Ti I 4536 Å line.

Collisional rates do not affect the Ca II K line polarization, but slightly depolarize the Ca II IR triplet lower levels.

Abstract

The physical interpretation of scattering line polarization offers a novel diagnostic window for exploring the thermal and magnetic structure of the quiet regions of the solar atmosphere. Here we evaluate the impact of isotropic collisions with neutral hydrogen atoms on the scattering polarization signals of the 13 lines of multiplet 42 of Ti I and on those of the K line and of the IR triplet of Ca II, with emphasis on the collisional transfer rates between nearby J-levels. To this end, we calculate the linear polarization produced by scattering processes considering realistic multilevel models and solving the statistical equilibrium equations for the multipolar components of the atomic density matrix. We confirm that the lower levels of the 13 lines of multiplet 42 of Ti I are completely depolarized by elastic collisions. We find that upper-level collisional depolarization turns out to have an unnoticeable impact on the emergent linear polarization amplitudes, except for the ${\lambda 4536$ line for which it is possible to notice a rather small depolarization caused by the collisional transfer rates. Concerning the Ca II lines, we show that the collisional rates play no role on the polarization of the upper level of the K line, while they have a rather small depolarizing effect on the atomic polarization of the metastable lower levels of the Ca II IR triplet.