摘要：Aims. The aim of this paper is to understand the formation of the O i line at 7772 Å in the solar chromosphere.
Methods. We used SST/CRISP observations to observe O i 7772 Å 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.
Results. 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 Å 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 O i. Both have a small but non-negligible influence on the line intensity.
