A matter of class and flare

A. G. M. Pietrow

doi:10.1051/0004-6361/202660960

A matter of class and flare | ResearchHub

Context . Solar active regions are the source of virtually all significant space weather events. It is therefore important to characterize the relationship between their observable properties and flare production. Aims . The flare productivity of solar active regions is analyzed across solar cycles 23–25 as a function of magnetic complexity, sunspot area, and solar-cycle phase. Additionally, the power-law index of the peak flux frequency distribution is estimated. Methods . The GOES soft X-ray event reports matched with daily Solar Region Summaries were used to create a robust active region catalog. This catalog was supplemented with the 50-year HEK flare catalog and the newly released STIX flare catalog from Solar Orbiter to estimate the power-law index. Results . Magnetically complex active regions ( βγ , βδ , and βγδ ) produce the majority of M- and X-class flares while comprising only ∼13% of all daily active-region observations, showing that larger flares disproportionately occur in larger area sunspots. The peak flux frequency distribution of all three catalogs can be represented with a consistent power-law index of α = 2.02 ± 0.01. However, a class-by-class analysis of the C/X and M/X flare ratios reveals that the distribution steepens significantly toward higher energies. We fit this steepening as a lognormal tail and extrapolated two decades beyond the X-class threshold, which yielded a recurrence time of 370 −159 +292 yr for an X100-class (“Carrington-type”) superflare, compared to only ∼26 yr from a naive single power-law extrapolation. Conclusions . While α ≈ 2 provides a useful global description of the flare frequency distribution, the steepening area-productivity relation and the divergence between the derived C/X and M/X slopes both point to a deficit of the most energetic flares relative to a single power-law extrapolation. This implies that extreme events such as Carrington-type flares are considerably rarer than a fixed power law would predict. Our extrapolated recurrence time is in much closer agreement with independent literature estimates.

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A matter of class and flare

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