The First Four Ground-Level Enhancements in the 1940s: Investigation, Digitisation, and Analysis of Forgotten Data

TL;DR

This study digitized and analyzed the first four ground-level enhancements from the 1940s, reconstructing their temporal evolution and spectral properties to fill historical data gaps and improve understanding of early solar energetic particle events.

Contribution

It provides the first systematic digitization and analysis of the earliest GLEs, reconstructing their temporal profiles and spectral characteristics from previously missing data.

Findings

GLEs #2 and #4 had abrupt increases of 15 minutes.

GLEs #1 and #3 showed gradual increases in 45 and 105 minutes.

Spectral hardness varied, with GLEs #2 and #4 being extremely hard.

Abstract

Intense solar eruptions occasionally accelerate solar energetic particles (SEPs) and can trigger ground-level enhancements (GLEs). Among the 77 known GLEs, the first four GLEs, #1 -- 4 in the 1940s took place before the advent of the standard neutron monitors and were missing from the International GLE Database. This data gap challenged their quantification. To overcome this difficulty, we systematically gathered, digitised, and quantified contemporaneous cosmic-ray records pertaining to these GLEs. These data allow us to reconstruct the temporal evolution, with the 1 -- 15 min resolutions, of these GLEs, 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 min, respectively. In contrast, GLEs #2 and #4 both exhibited abrupt increases of 15 +/- 15 min. We also compared integral ionisation increase on the standard ionisation chambers and their local geomagnetic cutoff rigidities Pc to qualitatively compare these GLE's 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 ionisations for polar detectors among them.