Estimating solar-radiation environment extremes

Context. Extreme solar particle events (ESPEs) were identified almost a decade ago, providing context for super events unleashed by our host star, the Sun. Their assumed solar origin drives the question of their “worst-case” impact, which could be profound, multifaceted, and devastating for our technological society. Aims. Recently, we proposed a methodology that directly relates the soft X-ray flux (FSXR) of the driving solar flare of a solar energetic particle (SEP) event to its “worst-case” integral fluence spectrum. In this study, we aim to put the letter to the stress test. Methods. In this study, we employed our previous method to the ESPEs that have been confirmed in cosmogenic radionuclide records to date, retrieved their “worst-case integral spectrum, and compared the latter to the actual – independently obtained – recent reconstructions based on the radionuclide records. Results. It is shown that our previous method makes it possible to estimate the integral fluence spectra of one of the paleo events, that is, AD774/775, one of the strongest ESPEs found within the cosmogenic radionuclide records so far. We implemented a mean ESPE utilizing four confirmed paleo ESPEs (i.e., AD993/994, AD774/775, 660 BCE, and 7176 BCE) and tested the resulting spectrum against the estimated one. Finally, we tested the same methodology for a series of strong SEP events recorded on the Earth’s surface as ground level enhancement (GLE) events. In all investigated cases, a recent re-calibration of the GOES 1–8 FSXR values is considered. Conclusions. It is shown that the proposed methodology can adequately estimate the “worst-case” integral fluence spectrum for both strong SEP events and ESPEs, quantifying their impact up to an integral energy of ∼E > 1 GeV.