Absorption profiles in the Mg i line calculated with the fluxes averaged over transit and post-transit observations of HD 209458b. The observed profile (Vidal-Madjar et al. 2013) is displayed as a blue histogram with a resolution of 20 km s^-1, while the solid black line corresponds to the theoretical global best-fit profile. The associated with this absorption profile is 225.9 (357 data points), and the χ² for all observations is 802.4 for 1067 degrees of freedom.