N-S Asymmetry and Solar Cycle Distribution of Superactive Regions from 1976 to 2017

There were 51 superactive regions (SARs) during solar cycles (SCs) 21-24. We divided the SARs into SARs(1), which produced extreme space weather events including >= X5.0 flares, ground level events (GLEs), and super geomagnetic storms (SGSs, Dst < -250 nT), and SARs(2), which did not produce extreme space weather events. The total number of SARs(1) and SARs(2) are 31 and 20, respectively. The statistical results showed that 35.5%, 64.5%, and 77.4% of the SARs(1) appeared in the ascending phase, descending phase, and in the period from two years before to the three years after the solar maximum, respectively, whereas 50%, 50%, and 100% of the SARs(2) appeared in the ascending phase, descending phase, and in the period from two years before to the three years after the solar maximum, respectively. The total number of SARs during an SC has a good association with the SC amplitude, implying that an SC with a higher amplitude will have more SARs than that with a lower amplitude. However, the largest flare index of a SAR within an SC has a poor association with the SC amplitude, suggesting that a weak cycle may have a SAR that may produce a series of very strong solar flares. The analysis of the north-south asymmetry of the SARs showed that SARs(1) dominated in the southern hemisphere of the sun during SCs 21-24. The SAR(2) dominated in the different hemispheres by turns for different SCs. The solar flare activities caused by the SARs with source locations in the southern hemisphere of the sun were much stronger than those caused by the SARs with source locations in the northern hemisphere of the sun during SCs 21-24.