Study of major historical GLEs on the basis of historical records 

A methodological study of relativistic solar energetic particles provides the necessary basis to reveal the nature of various processes, such as the production and acceleration of energetic particles at the Sun, their transport in the interplanetary medium, interactions of energetic particles with magnetic fields in the heliosphere, the induced corresponding terrestrial and space weather effects. Following solar eruptive processes, such as solar flares and/or coronal mass ejections solar ions are accelerated to a high-energy range. When the energy of the accelerated solar proton reaches the GeV/n range, it is enough high, so that solar ions generate an atmospheric cascade in the Earth’s atmosphere, whose secondary particles reach the ground, eventually registered by ground-based detectors, such as neutron monitors (NMs). This particular class of events is known as ground-level enhancements (GLEs).   Over several decades, NMs provided the main records allowing analysis of GLEs, namely their spectral and anisotropy characteristics, a procedure requiring complicated modeling and several NM stations (records starting from GLE #5, 23-Feb-1956). However, the first four GLEs were recorded mostly by ionization chambers (ICs), devices with a weaker responses and greater energy threshold compared to NMs. Here, we analyzed several GLEs on the basis of historical records, where data of NMs and IC are both available. As a first step, we derived the GLE protons spectra employing a method verified by space-borne measurements. Secondly, employing a forward modeling, we rescaled the response of the available ICs, that is, we assessed the response function of selected ICs. Finally, employing a procedure similar to GLE analysis using NMs, but here using ICs records, we derived for the first time the spectra of e.g. GLE # 4. We compared the derived spectra with GLE #5 and discussed the obtained results.