Extreme solar particle events (ESPEs) are infrequent yet highly potent manifestations of solar activity. These events result in the production of significant amounts of cosmogenic isotopes (CIs): $^{10}$Be, $^{36}$Cl, and $^{14}$C, which are subsequently deposited in natural stratified archives. Analyzing CI measurements from these archives allows us to assess the particle fluxes during ESPEs.
In this study, we introduce a novel approach to reconstruct ESPE fluence (integral flux) by using recent modelling advancements. This method enables the integration of diverse CI data within a single comprehensive model. Within the new approach, ESPE fluence is represented as an ensemble of scaled fluence reconstructions for ground-level enhancement (GLE) events, detected by the neutron monitor network since 1956 and coupled with satellite and ionospheric measurements.
The reconstructed ESPE fluences exhibit a softer spectral shape compared to previous estimates, leading to significantly higher estimates of the low-energy ($E<$100 MeV) fluence. Consequently, ESPEs pose an even greater risk to modern technological systems than previously believed. To facilitate broader applications, the reconstructed ESPE fluences are fitted using a modified Band function which simplifies the utilization of the obtained results in various practical contexts.