Relationship between SXR peak intensity of extreme solar flare (largest intensity within a year, >X10 in the previous category, https://www.sws.bom.gov.au/Educational/2/3/9) and integrated SEP fluence above 200 MeV (F₂₀₀ [cm⁻²], Kovaltsov et al., 2014) for the same year of the extreme solar flare, for the period from 1976 to 2020 CE (circles). The flare scale for the period 1976–2017 CE is recalibrated (i.e., the previous SXR peak intensity × 1.43) (Cliver et al., 2022). Estimations of SXR peak intensity for the Carrington events are reported in Hayakawa et al. (2023a). The arrow indicates the upper limit of F₂₀₀ for the Carrington event (see text).