On the collisional sensitivity of polarised Mg II solar lines

Abstract

Neutral and singly ionised states of the magnesium (Mg) are the origin of several spectral lines that are useful for solar diagnostic purposes. An important element in modelling such solar lines is collisional data of the Mg with different perturbers abundant in the Sun, specially with neutral hydrogen. This work aims at providing complete depolarisation and polarisation and population transfer data for Mg II due to collisions with hydrogen atoms. For this purpose, a general formalism is employed to calculate the needed rates of MgII due to collisions with hydrogen atoms. The resulting collisional rates are then employed to investigate the impact of collisions on the polarisation of 25 Mg II lines relevant to solar applications by solving the governing statistical equilibrium equations within multi-level and multi-term atomic models. We find that the polarisation of some Mg II lines starts to be sensitive to collisions for hydrogen density $n_H \!\gtrsim\!$ 10 $^{14}$ cm $^{-3}$ .