Revision of the strongest solar energetic particle event of 23 February 1956 (GLE #5) based on the rediscovered original records

¹ Institute for Space-Earth Environmental Research, Nagoya University, Nagoya 4648601, Japan
e-mail: hisashi@nagoya-u.jp
² Institute for Advanced Research, Nagoya University, Nagoya 4648601, Japan
³ Science and Technology Facilities Council, RAL Space, Rutherford Appleton Laboratory, Didcot, UK
⁴ Nishina Centre, Riken, Wako 3510198, Japan
⁵ Space Physics and Astronomy Research Unit, University of Oulu, PO Box 8000 90014 Oulu, Finland
e-mail: sergey.koldobskiy@oulu.fi
⁶ Sodankylä Geophysical Observatory, University of Oulu, PO Box 8000, 90014 Oulu, Finland
⁷ University of Siedlce, Institute of Mathematics, 08-110 Siedlce, Poland
⁸ Space Research Centre of Polish Academy of Sciences, 00-716 Warsaw, Poland

Received: 22 November 2023
Accepted: 7 January 2024

Abstract

Aims. Intense solar eruptions can produce solar energetic particles (SEPs), potentially detectable by ground-based instruments such as neutron monitors (NMs). These events are called ground-level enhancements (GLEs). The strongest GLE with the hardest known SEP spectrum occurred on 23 February 1956 (conventionally numbered GLE #5), providing a benchmark reference for related studies. However, the existing datasets for GLE #5 were compiled from different sources, often secondary; these datasets exhibited significant discrepancies and internal inconsistencies leading to large uncertainties or biases. Here we resolve the inconsistencies and revisit the reconstructions of the energy spectra and angular characteristics of the SEPs for that event, based on our reanalyses on (somehow forgotten) original contemporary records.

Methods. We collected, digitised, and verified the source records for NM measurements during GLE #5 based on contemporaneous publications and unpublished materials in the University of Chicago Archives. Using the revised datasets and full modelling, we critically revised the reconstruction of the energy spectra and angular characteristics of the SEPs and the event-integrated omnidirectional SEP flux (fluence) for GLE #5.

Results. The energy spectrum of the SEPs during the initial and main phases of GLE #5 was revised based on the new dataset, resulting in a slightly softer, but still agreeing within the uncertainties of the recent studies, SEP spectral estimate. The SEP flux was found to be highly anisotropic in the early phase of the event. This provides a revised reference basis for further analyses and modelling of strong and extreme SEP events and their terrestrial impacts.