Evaluation of the Short-term Periodicities in the Flare Index between the Years 1966-2002

The short-term periodicities of the flare index are investigated in detail using Fourier and wavelet transforms 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). The most pronounced power peaks were found by the Fourier transform to be present at 25.6, 27.0, 30.2, and 33.8 days. The wavelet transform results show that the occurence of periodicities of flare index power is highly intermittent in time. A comparison of the results of the Fourier tra nsform and the time-period wavelet transform of the flare index time series has clari fied the importance of different periodicities, whether they are or are not the har monics of the basic ones, as well as the temporal location of their occurrence. Because we found that the modulation of the flare index due to the 27-day solar rotation is more pronounced during the declining portion of solar cycle than during the rising portion.