Top: θ_c;m as a function of R_o and Λ_E for non-axisymmetric retrograde (m = 1) and prograde (m =  −1) waves. On one hand, the corner region for which R_o ≤ Λ_E/2 corresponds to the domain where and where waves are vertically trapped. On the other hand, the region where θ_c;m = 0 is the one where the MHD TA applies (i.e. |ν_M;m| < 1 and ) and where there is therefore no critical latitude. Finally, the θ_c ≠ 0 region corresponds to the regime where the MHD TA does not apply (i.e. |ν_M;m| ≥ 1 and ). Bottom: same for m = 2 and m =  −2. As emphasised in Sect. 2.4.3 prograde waves are more trapped in an equatorial belt than retrograde ones.