Theoretical absorption spectra of HD 209458 b around the Na I D2 line, as a function of time and wavelength in the stellar rest frame. The simulated out-of-transit spectrum was used as reference. Upper panel: without a thermosphere. Lower panel: with a thermosphere containing neutral sodium atoms. The atmospheric absorption signature follows the orbital track of the planet represented by green curves. The horizontal orange dashed lines show the contact times of the transit. The y-axis shows time from the centre of the transit.