Excitation properties of the neutral carbon along the lines of sight observed by JT11. The filled coloured circles indicate the f₁ and f₂ column density ratios observed along each line of sight. Each circle is colour-coded according to the total column density of neutral carbon, N(C). The black dots show the f₁ and f₂ column density ratios obtained per velocity interval of 0.5 km s⁻¹. The filled green circle shows the total f₁ and f₂ ratios computed over the entire observational sample. The black curves display the theoretical ratios obtained for homogeneous gas with kinetic temperatures of 10, 30, 100, and 1000 K and thermal pressures varying, along each curve, between 10² and 10⁷ K cm⁻³ (from the bottom left to the top right). The empty white circles indicate integer values of the logarithm of the thermal pressure (see Fig. 3).