The short-term periodicities of the flare index (FI) which is roughly proportional to the total energy emitted by the flare, are investigated in detail using Fourier transform (FT) for the full-disc and for the northern and the southern hemispheres of the Sun separately over the epoch of almost 4 cycles (1966-2002) which covers 13392 days. Figure 1 shows the power spectra of the three time series of FI calculated for the 289-771 nHz (15-40 days) range with 0.24 nHz interval. The uncertainty in each frequency is ±0.4 nHz due to the 36.5-year data length. The dashed lines indicate the false alarm probability significance levels. In this figure, there are several pronounced power peaks whose significances are enough high. These are at 25.6, 27.0, 30.2 days for the full-disc, 27.0 days for the northern hemisphere and 27.5 and 33.8 days for the southern hemisphere. Since commonly used FT is not able to disclose the possible changes in the periodicities over the period studied, the wavelet transform (WT) was applied to search for temporal variability. The wavelet transform results show that the occurence of periodicities of flare index power is highly intermittent in time. All three FI time series show that the major periods, obtained by Fourier transform are localised in short time intervals (typically up to half of a year).To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html