Flare Index Variability in the Ascending Branch of Solar Cycle 23

A brief description and the results of the determination of the flare index as a measure of solar activity on the ascending branch of cycle 23 up to 31 December 2000 are presented. The patterns of similar activity indices that arise under different physical conditions during the rising activity phase are compared with the flare index. All studied solar indices rise more slowly in the current cycle than the last one, except the total solar irradiance. The intermediate-term periodicities in the daily flare index data were studied using the Fourier transform, and it was found that the 35-, 62-, 116-, 198-, and 276-day periodicities are in operation during the ascending branch of cycle 23. Contrary to the previous three cycles, 155-, 73-, or 51-day periodicities were not detected by the Fourier transform in this branch of the cycle. The wavelet transform results show that the occurrence of flare index power is highly intermittent in time, and the most pronounced power peaks were found to be present at 35 days (the temporal locations at 1998.3, 1999.9, and 2000.5 years), 116 days (the temporal interval 1999.7–2000.4 years) and 276 days period (interval roughly from 1999.5 until the end of the flare index data). Comparison of the Fourier and wavelet transform results has clarified the importance of different periodicities, whether they are (62, 198 days) or are not (35, 116, 276 days) the harmonics of the basic ones, as well as the temporal location of their occurrence.