Smooth high-latitude modification to the background rotation rate, ΔΩ(θ; A), and its effect on the HL1 mode frequency. (a) Functional form of ΔΩ(θ; A) as defined by Eq. (6), symmetric across the equator. (b) Variation of the real part of the mode frequency with A when using Solver 1 (green curve). By construction ω = ω_ref at A = 0. Agreement with the observed eigenfrequency (gray shaded region) is obtained for 10.3 nHz < A/2π < 16.0 nHz (range between the dotted green arrows). The value of A/2π = 13.6 nHz corresponding to the best fit is marked by the dashed green arrow. The dotted black line shows the linear approximation Eq. (7).