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

TL;DR

This study analyzes the long- and mid-term variations in soft X-ray flare types across solar cycles, revealing regular quasi-biennial oscillations and differences in flare characteristics linked to sunspot populations.

Contribution

It provides a detailed comparison of flare temporal parameters and their variations across multiple solar cycles, highlighting the presence of quasi-biennial oscillations and their relation to sunspot activity.

Findings

SDE fraction in Cycle 24 exceeds Cycle 23 during maximum and decay phases.

QBOs are prominent in flare parameters across all cycles and phases.

Intensity of M-class flares shows larger variability than C-class flares.

Abstract

Using data from the Geostationary Operational Environmental Satellites (GOES) spacecraft in the 1-8 \AA\ wavelength range for Solar Cycles 23, 24, and part of Cycles 21 and 22, we compare mean temporal parameters (rising, decay times, duration) and the proportion of impulsive short-duration events (SDE) and gradual long-duration events (LDE) among C- and $\geq$M1.0-class flares. It is found that the fraction of the SDE $\geq$M1.0-class flares (including spikes) in Cycle 24 exceeds that in Cycle 23 in all three temporal parameters at the maximum phase and in the decay time during the ascending cycle phase. However, Cycles 23 and 24 barely differ in the fraction of the SDE C-class flares. The temporal parameters of SDEs, their fraction, and consequently the relationship between the SDE and LDE flares do not remain constant, but they reveal regular changes within individual cycles and during the transition from one cycle to another. In all phases of all four cycles, these changes have the character of 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 $\geq$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 $\geq$M1.0-class flares almost always markedly exceeds those of the moderate C-class flares. The ordered quantitative and qualitative variations of the flare type revealed 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.