Comparison between the north-south travel-time perturbations as measured by Gizon et al. (2020; thick solid lines) and the forward travel-time perturbations without (thin solid line) and with (thick dashed line) the magnetic mask. The measurements were averaged over the latitudes λ = 20° −35° in each hemisphere, over the separation distances Δ = 6°–18°, and over consecutive 4-year time intervals. The average over the quiet-Sun period January 2007 to April 2009 was subtracted from the measurements, and the sign was reversed for the purpose of comparison. The corresponding and are used from May 1996 to April 2010, and −τ^AR and from May 2010 to April 2019. For the measurements, the error bars represent the standard error of the mean computed over the latitudinal range of interest. For the forward travel-time perturbations, the standard error of the mean is in the range of 0.001–0.003 s, and is not shown here. For the modeled travel-time perturbations, the errors, not shown here, are about 0.013 s for and 0.009 s for .