Correlation plots between the integrated intensities of the Hα, Lyβ and Lyγ lines. The stars represent Hα Meudon/MSDP and SoHO/SUMER observational points between 10″ and 46″ along the SoHO/SUMER slit (see Fig. 2). Other symbols correspond to the theoretical one-dimensional (1D) models at different temperatures (Gouttebroze et al. 1993): +: T = 6000 K,  × : T = 8000 K, ◇: T = 10 000 K,  △ : T = 15 000 K.