# Solar polarimetry through the K I lines at 770 nm

## TL;DR

This study evaluates the K I D1 & D2 lines at 770 nm for solar polarimetry, demonstrating their potential to complement existing spectral lines in the 850 nm window for probing the upper photosphere's magnetic and velocity fields.

## Contribution

It provides a detailed analysis of the formation and sensitivity of K I lines, showing their usefulness in solar atmospheric diagnostics beyond existing spectral lines.

## Key findings

- K I lines form around 500 km in the upper photosphere.
- They are sensitive to line of sight velocity and magnetic field at specific heights.
- They complement 850 nm lines for atmospheric parameter determination.

## Abstract

We characterize the K I D1 & D2 lines in order to determine whether they could complement the 850 nm window, containing the Ca II infrared triplet lines and several Zeeman sensitive photospheric lines, that was studied previously. We investigate the effect of partial redistribution on the intensity profiles, their sensitivity to changes in different atmospheric parameters, and the spatial distribution of Zeeman polarization signals employing a realistic magnetohydrodynamic simulation. The results show that these lines form in the upper photosphere at around 500 km and that they are sensitive to the line of sight velocity and magnetic field strength at heights where neither the photospheric lines nor the Ca II infrared lines are. However, at the same time, we found that their sensitivity to the temperature essentially comes from the photosphere. Then, we conclude that the K I lines provide a complement to the lines in the 850 nm window for the determination of atmospheric parameters in the upper photosphere, especially for the line of sight velocity and the magnetic field.