The first four ground-level enhancements in the 1940s: investigation, digitization and analysis of forgotten data

Intense solar eruptions occasionally accelerate solar energetic particles (SEPs) and trigger ground-level enhancements (GLEs). Among the 77 known GLEs, the first four #1- 4, in the 1940s, took place before the advent of the standard neutron monitors (NMs) and were missing from the International GLE Database (IGLED). This data gap challenged their quantification. To overcome this difficulty, we systematically gathered, digitized and quantified contemporaneous cosmic-ray records pertaining to these GLEs. These data allow us to reconstruct the GLE temporal evolution with 1-15 minute resolutions and broaden their geographical coverage to a global scale. GLEs #1 and #3 exhibited gradual increases in their rise times, measured at 45±15 and 105±15 minutes, respectively. By contrast, GLEs #2 and #4 both exhibited abrupt increases of 15±15 minutes. We also compared integral ionization increase on the standard ionization chambers (ICs) and their local geomagnetic cut-off rigidities Pc to qualitatively compare GLE spectral hardness: Our result indicates that their spectra are extremely hard for GLEs #2 and #4 and mildly hard for GLEs #1 and #3. GLE #3 showed the greatest integral ionizations for polar detectors among them. This article is part of the Theo Murphy meeting issue 'Radiocarbon and cosmic radiation events'.