a) Horizontal averages and for f-mode-filtered HMI observations as functions of solar latitude, averaged over 336 × 8 h datasets of about 180 × 180 Mm² and four angles β. b) Horizontal average ⟨ τ^oiτ^ac ⟩ for different data: HMI at full resolution (0.5 arcsec px^-1), MDI full-disk data (2.0 arcsec px^-1), and HMI data spatially averaged over 4 × 4 px (after remapping) and convolved with a Gaussian with 2.4 Mm FWHM to match the MDI sampling and PSF.