Continuum images of the sunspot in NOAA active region 11899. Left image: a single continuum image near the central meridian passage on Nov. 18, 2013. Middle image: an average of the continuum images for the ten days analyzed (Nov. 14–23, 2013). For the left, and middle images: the horizontal size is the same as that of the eventual travel time maps. To identify the umbral–penumbral boundary, a contour of the ten-day average intensity at the level of 0.4 is plotted (red). The penumbral–photospheric boundary is represented from the contour at 0.85 (blue). To derive the intensities, a fit to limb darkening is done with the sunspot excluded and the intensity is normalized to unity for the background photosphere. Right image: also the ten-day average of the continuum intensity, but showing the entire field used to derive the travel-time maps. The locations A and B from Fig. 1 are shown.
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