(a) Profile of the first adiabatic exponent Γ₁ as a function of the normalised acoustic radius in a 1 M_⊙ RGB model computed with MESA at Δν = 1.65 μHz. The parameters of the Γ₁ variations (H_HeII, t_HeII, and b_HeII) are directly shown in the figure. The solid green line is the Γ₁ profile throughout the star. The thick dashed orange line indicates the baseline that connects the local maximum after the dip caused by the second He-ionisation with the Γ₁ profile before the dip. The thin dashed red line gives the fit of the Γ₁ profile with Eq. (12) around the dip. (b) Glitch modulation induced by the second He-ionisation zone in the same model, as in the left panel. The local large separation Δν_n, ℓ is shown in red circles, blue triangles, and green squares for radial, dipole, and quadrupole modes, respectively. The solid green line is the damped oscillator model given by Eq. (10) fitted to the data points. We point out that the data points are plotted at the mean frequencies (ν_n, ℓ + ν_{n + 1, ℓ})/2. The upper x-axis indicates the radial order of ℓ = 0 modes, and the dotted blue line locates the maximum oscillation power.