Meridional section of the concordance model of Altair (e.g. Bouchaud et al. 2020) showing the Brunt–Väisälä frequency. Units on the colour bar are cycle/day. Negative values show the convectively unstable regions. The lower plot gives a zoomed-in view of the surface layers and the dashed line shows the T = 50 000 K isotherm around which the second ionisation of helium takes place. Depth is scaled by the polar radius of the model.