Left and center panels: kernels ⟨ K_ur ⟩ and ⟨ K_uθ ⟩ for the r and θ components of the flow, using all azimuthal components | m | ≤ m_max = 35. The separation distance between points r₁ and r₂ is 42°; the center point is located at a latitude of 40°. The ray path connecting the two points is shown in black. The right panel shows the convergence of the travel times as a function of m_max, for the north-south (blue) and south-north (green) travel directions. The red curve shows their difference.