Synthetic glitch and structure parameters computed with MESA and ADIPLS. The MESA models are the same as those shown in Fig. 4. The glitch amplitude 𝒜_gl and period 𝒢_gl are shown on the (a) and (b) panels, respectively, while the amplitude H_HeII and the width b_HeII of the HeII zone in the Γ₁ profile are exhibited on the (c) and (d) panels, respectively. The label matches that of Fig. 4. In panel b, the additional solid light grey line is the modulation period expected from the location of the HeII ionisation zone t_HeII, which is computed according to Eq. (13).