Left and center panels: kernels ⟨ K_ur ⟩ and ⟨ K_uθ ⟩ for the r and θ components of the flow. Point r₁ is at the north pole (photosphere) and point r₂ is at 45° latitude. The values of the kernels are scaled by the sound speed c and are saturated at 1/400 of the maximum value. The ray path connecting the two points is shown (thick black line) as well as the computational boundary (black half circle). Right panel: comparison between the travel times computed directly from the cross-covariance function (curves, see Eq. (25)) and those computed from the sensitivity kernels (“+” symbols, see Eq. (90)). The blue curve is for the travel times measured from the pole to latitude 45°, the red curve for the travel times measured in the opposite direction. The accuracy of the travel times is approximately 10^-3 s.