This work aims to provide key elements to gain better understanding of the formation of the second solar spectrum of the Sr II $lambda$4078 line. We show that in a certain range of hydrogen density, the effect of isotropic collisions between Sr II io
ns with hydrogen atoms is important in determining the polarization of the Sr II $lambda$4078 line. The use of an atomic model that neglects the metastable level 4d of Sr II can induce errors of up to 25 % in the value of the scattering polarization of the solar Sr II $lambda$4078 line.
This paper is dedicated to the assessment of the validity of future coronal spectro-polarimetric observations and to prepare their interpretation in terms of the magnetic field vector.
The physics of the solar chromosphere depends in a crucial way on its magnetic structure. However there are presently very few direct magnetic field diagnostics available for this region. Here we investigate the diagnostic potential of the Hanle effe
ct on the Ba II D2 line resonance polarization for the determination of weak chromospheric turbulent magnetic fields......
Context. Rigorous modeling of the ion{Ba}{ii} ${lambda}4554$ formation is potentially interesting since this strongly polarized line forms in the solar chromosphere where the magnetic field is rather poorly known. Aims. To investigate the role of iso
tropic collisions with neutral hydrogen in the formation of the polarized ion{Ba}{ii} ${lambda}4554$ line and, thus, in the determination of the magnetic field. Methods. Multipole relaxation and transfer rates of the $d$ and p-states of ion{Ba}{ii} by isotropic collisions with neutral hydrogen are calculated. We consider a plane parallel layer of ion{Ba}{ii} situated at the low chromosphere and anisotropically illuminated from below which produces linear polarization in the ${lambda}4554$ line by scattering processes. To compute that polarization, we solve the statistical equilibrium equations for ion{Ba}{ii} levels including collisions, radiation and magnetic field effects. Results. Variation laws of the relaxation and transfer rates with hydrogen number density $n_{textrm {scriptsize H}}$ and temperature are deduced. The polarization of the ${lambda}4554$ line is clearly affected due to isotropic collisions with neutral hydrogen although the collisional depolarization of its upper level $^2P_{3/2}$ is negligible. This is because the alignment of the metastable levels $^2D_{3/2}$ and $^2D_{5/2}$ of the ion{Ba}{ii} are vulnerable to collisions. At the height of formation of the ${lambda}4554$ line where $n_{textrm {scriptsize H}} sim 2 times 10^{14}$ cm$^{-3}$, we find that the neglecting of the collisions induces inaccuracy of $sim$ 25% on the calculation of the polarization and $sim$ 35 % inaccuracy on microturbulent magnetic field determination.
