A Statistical Analysis of X1- and X2- or Higher-Class Flares during Solar Cycles 21∼23

The X1- and X2- or higher class flares in solar cycles 21, 22, and 23 from 1986 to 2008 have been analyzed statistically in this paper. It is found in the statistical study that the number of the X1-class flares accounted for 52.71% of total X- and higher class flares, while, the number of the X2- and higher class flares accounted for 47.29% of total X- and higher class flares. No matter whether the X1- and X2- or higher class flares, most of them occured in the descending phases of the solar cycles. Moreover, the weaker the intensity of the solar cycle, the higher the ratio of the flares occurred in the descending phase of the solar cycle, and the stronger the intensity of solar flares, the higher the ratio of the flares occurred in the descending phases of the solar cycles. In addition, the phase difference between the peak of the smoothed monthly mean number of sunspots and that of the X-class flares has been calculated, which shows that the smoothed monthly mean number of the X1-class flares had a very noticeable time advance of 1 month with respect to that of sunspots in the cycles 21 and 22, but there was a time lag of 13 months in the cycle 23, while, for the X2- and higher class flares, there was a time lag of 9 months in the cycle 21, but a one-month time advance existed in the cycle 22, and again a time lag of 32 months appeared in the cycle 23.