Solar off-limb emission of the OI 7772 \ line
Abstract
The aim of this paper is to understand the formation of the OI line at 7772 \ in the solar chromosphere. We used SST/CRISP observations to observe OI 7772 \ 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 \ transition is populated by a downward cascade from the continuum. We study the effect of Lyman-β pumping and hydrogen collisions between the triplet and quintet system in OI. Both have a small but non-negligible influence on the line intensity.
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