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Fig. 4

image

Two-skip analysis of ν dependenceof the cross covariance for the umbra (left panels) and quiet Sun (right panels). Gaussian filters with FWHM = 0.8 mHz are applied to the cross covariances. The umbral times are averaged over the full ten days and the quiet sun is averaged over nine days. The grey scale image in the upper left (right) is the ν resolved cross covariance for the umbra (quiet Sun), scaled separately for each ν. The blue and green curves are the results of the Gabor wavelength fitting for τp and τe. In the middle row left (right) plot is shown the envelope of the cross covariance computed from the analytic signal (Bracewell 1965) for the umbra (quiet Sun). Overplotted are the same blue and green curves from the top line. In the lower left, the umbral and quiet Sun τp and τe are shown. The errors are smaller than the symbols. In the lower right, the difference travel times (umbral minus quiet Sun) are shown.

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