Relation between He i 1083 nm opacity, electron density and the coronal irradiation. Panel a): relative population of the He i 1083 nm lower level, for each pixel at the height in the chromosphere where the relative population has a maximum. Panel b): electron density at the same location as in a). Panel c): J_csw at the same location as in a) and b). Panel d): JPDF of J_csw and the electron density in the chromosphere. Panel d): JPDF of J_csw and the relative population of the He i 1083 nm. Panel f): JPDF of the electron density and the relative population of the He i 1083 nm.