# Solar off-limb emission of the OI 7772 \AA\ line

## TL;DR

This study investigates the formation of the OI 7772 Å line in the solar chromosphere using observations and models, revealing the impact of hydrogen collisions and Lyman-beta pumping on line formation.

## Contribution

It provides new insights into the line formation process by incorporating accurate hydrogen collisional rates and comparing observations with synthetic spectra.

## Key findings

- Dark gap observed in the lower chromosphere due to opacity variations
- Line population influenced by downward cascade from the continuum
- Hydrogen collisions and Lyman-beta pumping affect line intensity

## Abstract

The aim of this paper is to understand the formation of the OI line at 7772 \AA\ in the solar chromosphere. We used SST/CRISP observations to observe OI 7772 \AA\ in several places around the solar limb. We compared the observations with synthetic spectra calculated with the RH code in the one-dimension spherical geometry mode. New accurate hydrogen collisional rates were included for the RH calculations. The observations reveal a dark gap in the lower chromosphere, which is caused by variations in the line opacity as shown by our models. The lower level of the 7772 \AA\ transition is populated by a downward cascade from the continuum. We study the effect of Lyman-$\beta$ pumping and hydrogen collisions between the triplet and quintet system in OI. Both have a small but non-negligible influence on the line intensity.