a) Time variation of 3 min oscillation power integrated in the range of periods from 1.5 min to 3.5 min in the sunspot umbra. The signal is measured in the 304 Å emission intensity. The digits label the time intervals of the enhanced power (see also Fig. 1b). b) Fourier power spectrum of oscillations of the 304 Å emission intensity integrated over the umbra. The 3 min power and spectral power density are given in arbitrary units normalised to the highest values.