Energetics of X-Class Flares at the Minima of 22, 23, and 24 Solar Cycles

Abstract

Quite often powerful flares occur during a decrease of solar activity. In this work, we make an attempt to consider X-class flares at the minima of solar activity cycles as a separate group of events. Out of nine X-class flares we selected three such events that occurred at the minima of solar cycles 22–24: SOL1996-07-09T-09:09 (X2.6), SOL2006-12-13T-10:26 (X3.4), and SOL2017-09-06T-11:53 (X9.3); they were well observed by various instruments. All three selected flares are characterized by significant gamma-ray emission, which indicates ion acceleration during the impulsive phase. For the selected events, we estimated the energetics and distribution of the released free energy of the magnetic field between different flare components, including thermal energy and the energy of accelerated electrons and ions. No preferred energy-release channels have been identified for the considered flares at the solar activity minima. It turned out that the amount of magnetic free energy is several times higher than the amount of thermal energy and the energy of accelerated electrons and ions; the possible causes of this imbalance are discussed.