Fit of isothermal models to the transmission spectrum of MASCARA-2b in the Na I region. The vertical scale is atmospheric altitude in [km] assuming a planet-to-star radius ratio of . In red we show a model at the equilibrium temperature (T_eq = 2260 K), which is adjusted to the continuum. One other model is adjusted to the lines core of each line using the data encompassed for a 0.3 Å bandwidth centered on the lines peak (shown in blue). For the D₂ line the best-fit model is at T = 4240 ± 200 K and for the D₁ line is at T = 4180 ± 310 K, i.e., the same temperature taking into account the uncertainties. In the top panel we show both Na I lines and the continuum surrounding. In the bottom panels we show each D₂ (left) and D₁ (right) lines zoomed. Data and models are presented binned by 10 pixels.