Strongest directly observed Solar Proton Event of 23-Feb-1956: Revised reference for the cosmogenic-isotope method

Solar extreme solar proton events (SPEs) form important radiation hazards for modern technological society. The strongest directly observed SPE took place on 23-Feb-1956 as an up to 5000 % increase of the count rate of ground-based neutron monitors. It was characterized by a very hard energy spectrum and strong particle fluence. On the other hand, as known from indirect proxies (cosmogenic isotopes), several extreme events, one - two orders of magnitude stronger, occurred during the past millennia. In order to study past events, a reference scale needs to be made. The SPE of 23-Feb-1956 is often used as such a reference. Thanks to the recent developments in the methodology of SPE analysis, the spectrum of fluence of the reference event have been revisited and re-assessed with higher precision. Here we present an estimate the sensitivity of the cosmogenic-isotope method to detect extreme SPEs in the past. It is shown that the modern accuracy of the cosmogenic-isotope method to SPEs is insufficient, by an order of magnitude for any single isotope record, to detect the reference event but can resolve events a factor 3 - 4 stronger using a multi-proxy method. This provides a solid basis for research in the field of extreme events, both for fundamental science, namely solar and stellar physics, and practical applications, such as the risk assessments of severe space-based hazards for a modern technological society.