Characteristics of X-class flares of solar cycles 23 and 24 in X-ray and EUV bands

The characteristics of X-class flares of solar cycles 23 and 24 are examined based on the soft X-ray (0.1–0.8 nm) and EUV (26–34 nm) light curves. First, the commonly used correction factors for the center-to-limb variation of EUV flux are compared. Subsequently, enhancements in X-ray and EUV levels are used to calculate the relative apportionment of flare energy released in these two bands. The X-ray to EUV proportion of energy is found to be larger in the high X-class flares (X9.0 and stronger) as compared to the flares weaker than X9.0. The X-ray and EUV light curves revealed that the time of impulsive and thermal EUV peaks differed from X-ray peak by ∼0–25 min. The time differences had a broad range for weaker flares but reduced to within ∼7 min for high X-class flare events. Therefore, the present study brings out the general energetics and temporal evolution of phases of the X-class flares. Implications of the findings on solar flare studies and flare impact on terrestrial atmosphere are discussed.