Temporal trend of the δτ_sn (top panel), δτ_ew (middle panel), and δτ′_sn (bottom panel) from MDI (blue circles) and HMI (red crosses) data. These monthly-measured travel-time shifts are antisymmetrized about the equator (for δτ_sn and δτ′_sn) or the central meridian (for δτ_ew), and averaged over the latitude range 20°–35° and the distance range Δ = 6°–42°. The typical standard errors of the mean over the distance range for data points in the top, middle, and bottom panels are 0.26 s, 0.24 s, and 0.34 s, respectively. In between the two vertical dashed lines is the overlap between the MDI and HMI observations. The black solid lines in the top two panels are fitted straight lines for the two individual data sets, while the black solid curve in the bottom panel is a two-year running mean. The fitted slopes are indicated in the lower left corner of the top two panels. We note that the δτ_ew shown in the middle panel have not been shifted by whereas the δτ′_sn in the bottom panel result from subtraction of the -shifted δτ_ew as described in Eq. (1).