Power maps (each panel in logarithmic scale) at the 3, 5 and 7 min period bands of the DS at 0.35 Å (top row) and 0.70 Å (bottom row) from the Hα line center (power in arbitrary units). Overlayed are the contours of the βTH with the bold contour defining the height of 1.6 Mm. The space between consecutive contours, in βTH, is 0.32 Mm (see text). In color in the on-line edition of the paper.