Long- and Mid-Term Variations of the Soft X-ray Flare Type in Solar Cycles

I.M. Chertok and A.V. Belov

Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation (IZMIRAN), Troitsk, Moscow, Russia

Abstract.  By data from the GOES satellites in the 18 Å wavelength range for cycles 2324 and partly for cycles 2122, we compare mean temporal parameters (rising, decay times, duration) and fraction of the impulsive short-duration events (SDE) and gradual long-duration events (LDE) among C- and M1.0 class flares. It is found that the fraction of the SDE (including spike) M1.0-class flares in cycle 24 exceeds that in cycle 23 by all three temporal parameters at the maximum phase and by the decay time at the ascending cycle phase. However, cycles 23 and 24 do not differ by fraction of the SDE C-class flares. The temporal parameters of SDEs, their fraction and, consequently, relationship between the SDE and LDE flares do not remain constant, but reveal stable and regular changes within individual cycles and during the transition from one cycle to another. In different cycles and at the separate phases of individual cycles, these changes have a character of the pronounced, large-amplitude "quasi-biennial" oscillations (QBOs). In different cycles and at the separate phases of individual cycles, such QBOs are superimposed on various systematic trends displayed by the analyzed temporal flare parameters. In cycle 24, the fraction of the SDE M1.0-class flares from the N- and S- hemispheres displays the most pronounced synchronous QBOs. The QBO amplitude and general variability of the intense M1.0-class flares almost always significantly exceeds that of the moderate C-class flares. The revealed ordered quantitative and qualitative variations of the flare character in the course of the solar cycles are discussed within the framework of the concept that the SDE flares are associated mainly with small sunspots (including those in developed active regions) and that small and large sunspots behave differently during cycles and form two distinct populations for which their dynamos, acting in the outer and inner layers of the convective zone, are responsible.

Figure 1s.  A series of seven similar LDE M- and X-class flares originated in AR11429 during the period of 2-13 March 2012


Figure 2s.  A series of four identical impulsive (spike-like) M- and X-class flares originated in AR11283 during the period of 6-8 September 2011.


Figure 3s. Histograms characterizing the relative distributions (percent) of the moderate C1.0÷C9.9 class and the most intense ≥M1.0-class flares by the rising time (dt1) at the ascending, maximum and declining phases of cycles 23 and 24.


Figure 4s (2). The same as on Figure 3s but by the flare decay time (dt2).


Figure 5s. The same as on Figures 3s and 4s but by the flare duration (dt).


Preprint.PDF    Solar Physics, Volume 292, Issue 9, paper 144, 2017. DOI: 10.1007/s11207-017-1169-1.



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