Top panels: images of NOAA 11131 at 1700 Å (photosphere), 304 Å (chromosphere) and 171 Å (corona) on December 8, 2010, at 02:30 UT. Bottom panels: spatial distribution of 3-min oscillations, obtained by filtering in the 5.2–9.5 mHz band. Thin dotted lines indicate the umbra-penumbra boundary at the photospheric level. The thick dashed line in the narrowband image at 304 Å shows the slit of the time-distance plot.