Speaker
Description
The Sun sporadically produces solar energetic particle (SEP) events that can significantly vary in intensity and spectral hardness. Although the majority of such events miss the Earth or are too weak to be registered by ground-based particle detectors, there are rare occurrences when strong SEP events penetrate deep into the atmosphere and can be observed on the ground. The latter ones are called “Ground-Level Enhancements” (GLEs) because they are seen as enhancements of the count rate over the background in records of ground-based neutron monitors. So far, there have been 77 GLEs registered since the beginning of the 1940s. Starting from GLE #5, they were detected by neutron monitors, digitized and analysed. Those experimental data played a key role in studies of particle acceleration and propagation at and in the vicinity of the Sun. However, the first four events (GLEs #1-4), recorded in the 1940s, were neither registered into the International GLE Database nor analysed in detail , mainly because they were observed by non-standard instruments, and their origin and importance were not fully understood at that time. There were significant difficulties in the interpretation of the data , such as scattered sources, almost always missing tabular data, often insufficient instrumental descriptions, etc. In preparation for future analysis of the events, we identified historical records containing observations of GLEs #1-4, compiled metadata, described the used instruments, digitised cosmic-ray measurements from figures, verified the results, assessed digitisation uncertainties, normalised in a standard for the GLE analysis way, and made the digital data available in the International GLE database (https://gle.oulu.fi) and published as Hayakawa et al. (2026). The results in their finalised form are presented in this contribution.