Abstract:
We have analyzed the intermediate-term periodicities in soft X-ray flare index (FIsₓR) during solar cycles 21, 22 and 23. Power-spectral analysis of daily FIsₓR reveals a significant period of 161 days in cycle 21 which is absent during cycles 22 and 23. We have found that in cycle 22 periodicities of 74 and 83 days are in operation. A 123-day periodicity has been found to be statistically significant during part of the current solar cycle 23. The existence of these periodicities has been discussed in the light of earlier results.