Statistical analysis of Soft X-ray solar flares during solar cycles 21, 22 and 23

This paper presents statistical analysis of Soft X-ray (SXR) flares during the period January 1976 to December 2007 covering solar cycles (SCs) 21, 22 and 23. We have analysed north–south (N–S) and east–west (E–W) asymmetry of SXR flares at total (1–90°), low (1–40°) and high (50–90°) latitudes and center meridian distances (CMDs). We have also presented the N–S and E–W asymmetry of different intensity classes (B, C, M and X) during the period of investigation. A slight southern and eastern excess is found after analysis during SCs 21, 22 and 23. N–S distribution shows that the SXR flare events are most prolific in the 11–20° latitude band in the northern and southern hemispheres whereas E–W distribution does not show any prolific band. We found that the N–S and E–W asymmetry which often peaks near the activity minimum is in agreement with the theoretical dynamo models. Our results show that N–S asymmetry is statistically more significant than E–W asymmetry. It is revealed that the SXR flare activity (M and C class flares) during SC 23 is low as compared to the SC 22, indicating the violation of Gnevyshev–Ohl rule. The B class flare activity is higher for SC 23 whereas C, M and X class activities are higher for SC 21. We have also analysed the flare evolution parameters, i.e. duration, rise time, decay time and event asymmetry for total SXR as well as for different classes during last three SCs. The duration, rise time and decay time increase with increasing intensity class. The increase is more pronounced for the duration and decay time than for the rise time for SCs 21 and 22 while for SC 23 it is the same for duration, rise time and decay time. On analyzing event asymmetry indices, we found more positive values during SC 21 (≈64.86%)(≈64.86%) and SC 22 (≈54.31%)(≈54.31%), but for SC 23 we have more negative values (≈48.08%)(≈48.08%). Our study shows that during SC 23 we have more SXR flare events having shorter decay time than the rise time as compared to SCs 21 and 22.