The Occurrence of Powerful Flares Stronger than X10 Class in Solar Cycles

Solar flares stronger than X10 (S-flares, >X10) are the highest-class flares that significantly impact on the Sun's evolution and space weather. Based on observations of Geostationary Orbiting Environmental Satellites at soft X-ray wavelength and the daily sunspot numbers (DSNs) since 1975, we obtained some interesting and heuristic conclusions: (1) both S-flares and the more powerful extremely strong flares (ES-flares, >X14.3) mostly occur in the late phases of solar cycles (SCs) and low-latitude regions on the solar disk; (2) similar to X-class flares, the occurrence of S-flares in each SC is somewhat random, but the occurrence of ES-flares seems to be dominated by the mean DSN (Vm) and its rms deviation during the valley phase (Vd) before the cycle: the ES-flare number is strongly correlated with Vd, and the occurrence time of the first ES-flare is anticorrelated with Vd and Vm. These facts indicate that the higher the Vm and Vd, the stronger the SC, the more the ES-flares, and the earlier they occurred. We propose that the Sun may have a low-latitude active zone (LAZ), and most ES-flares are generated from the interaction between the LAZ and the newly emerging active regions. The correlations and the linear regression functions may provide an useful method to predict the occurrence of ES-flares in an upcoming SC, which derives that SC 25 will have about 2 ± 1 ES-flares after the spring of 2027.