Extremely large flares/multiple large flares expected from sunspot groups with large area

The possible occurrence of major space weather events, such as large solar flares within one hundred years, is studied anticipating their effects on our social facilities. However, the continuous soft X-ray (SXR) observation of flares by Geostationary Operational Environmental Satellites (GOES) started in 1975, and the period of data collection is less than 50 years. On the other hand, ground-based sunspot observations have a long history. Their duration of data collection exceeds 100 years. The possibility of the occurrence of extremely large flares is estimated using the daily sunspot area data of individual sunspot groups between 1879 and 2016 using the catalogue complied by the Debrecen Heliophysical Observatory in Hungary and the catalogue updated by Mandal, Krivova, Solanki, Shinha, and Banerjee in 2020. It had become clear that large sunspot groups with the potential to produce Carrington-class flares (areas of more than 3000 MSH) have appeared on a total of 119−139 days between 1879 and 2016, and a sunspot group with the potential to produce an X100-class flare appeared between March and April 1947. According to the past major space weather events, the large sunspot groups caused a series of multiple large flares instead of just one large flare. We tried to estimate the probabilities of occurrence of a SXR flare ≥ X100 for 30-, 50-, and 100-year periods to be 0.70−0.76, 0.87−0.91, and 0.98−0.99, respectively, using the complementary cumulative distribution function (CCDF) of sunspot areas for the 138-year data. Graphical Abstract