Left: travel-time measurements δτ⁽¹⁾(ψ′_ij, t_j), Gaussian smoothed with FWHM = 90 days in time for better visualization, where ψ′_ij ≡ ψ_ij − ψ_0, j is the separation from the central meridian. Middle: fitted background S(ψ′_ij, t_j). Right: difference between the two panels on the left. The horizontal dashed lines in all panels indicate the division between the results from MDI and HMI data; the MDI data used in this work span from May 1996 to April 2010 while HMI data span from May 2010 to April 2017.