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

This study reconstructs and analyzes the first four ground-level enhancements (GLEs) from the 1940s by digitizing forgotten cosmic-ray data, revealing distinct temporal evolution patterns and spectral hardness characteristics for these historically missing events.

The Gist

Unearthing the Sun's Forgotten "Fireworks": A Story of Lost Data and Digital Resurrection

Imagine the Sun as a massive, temperamental fireworks factory. Sometimes, it doesn't just shoot off a few sparks; it launches a massive, high-speed barrage of particles toward Earth. When these particles hit our atmosphere, they create a phenomenon scientists call a Ground-Level Enhancement (GLE). Think of a GLE as a sudden, intense "radiation storm" that we can actually feel on the ground, detected by special instruments.

For decades, scientists have kept a detailed logbook of these storms. They know of 77 major events, starting from 1956. But there was a mysterious gap in the history books: the first four storms happened in the 1940s, and their records were missing.

Why were they missing? Because back then, the "cameras" we use today (standard neutron monitors) didn't exist yet. Scientists were using older, clunkier tools—like giant ionization chambers and early Geiger counters. The data existed, but it was trapped in dusty archives, handwritten in tables, or sketched on paper charts that no one had ever turned into digital numbers.

This paper is the story of a team of scientists (led by Hayakawa and Poluianov) who went on a digital treasure hunt to find, scan, and analyze these lost records.

The Digital Archaeology

Imagine trying to read a recipe written in a faded, hand-drawn notebook from 1942. You can't just copy-paste it into a computer; you have to look at every squiggle and number and type it in yourself.

The team did exactly this. They scoured libraries, university archives, and old scientific journals across the globe (from the US and UK to Japan and Russia). They found:

• Old Photos: Pictures of giant metal chambers (ionization chambers) sitting in observatories.
• Hand-Drawn Graphs: Curves showing how cosmic rays spiked during the storms.
• Secret Letters: A hidden table found in a box at the University of Chicago, written by a scientist named Braddick to another named Simpson.

They used a tool called WebPlotDigitizer (think of it as a high-tech scanner that turns pictures of graphs into Excel spreadsheets) to convert these old, blurry images into clean, modern data.

The Four "Forgotten" Storms

Once they had the data, they could finally see what happened during these four events in the 1940s. Here is what they discovered, using some simple analogies:

1. The Speed of the Storm (Rise Time)

Imagine a storm cloud rolling in. Does it arrive slowly, like a fog, or does it hit you like a sudden wall of wind?

• GLE #1 and #3 (The Slow Rollers): These storms took their time. It took about 45 to 105 minutes for the radiation to climb to its peak. It was like a slow, steady rain that gradually got heavier.
• GLE #2 and #4 (The Sudden Blasts): These were shockingly fast. The radiation spiked in just 15 minutes. It was like a lightning strike—sudden, sharp, and intense.

2. The "Hardness" of the Particles

Think of the solar particles as different types of bullets. Some are "soft" (low energy) and get stopped by a thin wall. Others are "hard" (high energy) and can punch right through thick armor.

• GLE #2 and #4: These were "extremely hard." The particles were so energetic they could punch through the Earth's magnetic shield and reach even the low-latitude stations (like Tokyo and Huancayo). It was like a tank breaking through a fence.
• GLE #1 and #3: These were "mildly hard." They were strong, but not quite as aggressive as the others. They mostly hit the polar regions where the magnetic shield is weaker.

3. The Biggest Show

Among all four, GLE #3 (from July 1946) was the heavyweight champion. It delivered the most total "ionization" (the amount of radiation hitting the detectors). If the other storms were a loud shout, GLE #3 was a full-blown scream.

Why Does This Matter?

You might ask, "Why dig up 80-year-old data?"

1. Filling the Gap: Before this, our history of solar storms started in 1956. Now, we have a complete timeline going back to 1942. It's like finally finding the missing first chapter of a mystery novel.
2. Understanding the Worst-Case Scenarios: We need to know how bad these storms can get to protect our satellites, airplanes, and power grids. By studying these early, intense events, scientists can better predict what a "once-in-a-millennium" super-storm might look like.
3. Calibrating the Past: The old instruments were different from today's. By comparing the old data with new models, scientists can learn how to translate "old language" measurements into modern terms.

The Takeaway

This paper is a victory for curiosity and persistence. The scientists didn't just accept that the data was lost. They treated the archives like a time capsule, carefully cleaning off the dust, digitizing the forgotten numbers, and bringing the first four solar storms of the space age back to life.

Thanks to their work, we now know that the Sun has been throwing massive, high-speed parties at Earth since at least 1942, and we finally have the guest list to prove it.

Technical Summary

Here is a detailed technical summary of the paper "The First Four Ground-Level Enhancements in the 1940s: Investigation, Digitisation, and Analysis of Forgotten Data" by Hayakawa et al. (2026).

1. Problem Statement

Ground-Level Enhancements (GLEs) are intense solar energetic particle (SEP) events detectable by ground-based instruments. While 77 GLEs have been registered since 1942, the first four events (GLEs #1–4, occurring in the 1940s) suffer from a significant data gap.

• Missing Data: These events predated the standard global network of Neutron Monitors (NMs). Consequently, they are absent from the International GLE Database (IGLED).
• Quantification Challenges: Existing data for these events were limited to sparse, low-resolution (bi-hourly) measurements from Carnegie ionization chambers (ICs) and lacked global coverage.
• Instrumental Limitations: Early detectors (ICs, Geiger-Müller tubes, electroscopes, and prototype NMs) had broad energy responses, lower sensitivity, higher noise, and stability issues (drifts) compared to modern NMs, making cross-calibration and quantitative analysis difficult.
• Scientific Gap: The lack of high-resolution data for these early events hinders the understanding of extreme solar particle events (ESPEs) and their comparison with the benchmark GLE #5 (1956) and cosmogenic isotope records.

2. Methodology

The authors employed a rigorous archival research and data reconstruction approach:

• Bibliographic Survey & Archival Search: A comprehensive search of early reviews, contemporary scientific journals, and archives (e.g., University of Chicago, Carnegie Science Collections, RIKEN) was conducted to locate original diagrams, tables, and correspondence related to GLEs #1–4.
• Digitization:
  • Published diagrams and tables were digitized using WebPlotDigitizer.
  • To avoid interpolation errors, data points were extracted directly rather than digitizing curves.
  • New Discoveries: Crucially, the team found previously unused source tables, including Manchester Neutron Monitor (NM) data from Henry Braddick's correspondence to John Simpson (University of Chicago Archives) and high-resolution Japanese ionization chamber records.
• Data Standardization:
  • Baseline Adjustment: Relative increases ($\delta I$) were standardized against a common baseline (average of 2 hours pre-onset) to ensure comparability across different instruments and reporting styles.
  • Time Correction: Timestamps were converted from local time to Coordinated Universal Time (UTC) using historical navigation manuals.
  • Error Estimation: Digitization errors were conservatively estimated (±0.1–0.3% for relative increases; timing errors ranging from ±1 min to ±15 min depending on resolution).
• Geomagnetic Modeling:
  • Geomagnetic cutoff rigidities ($P_c$) for each station were calculated using the OTSO tool, the IGRF-14 model (internal field), and the Tsyganenko-89 model (external field) with specific Kp indices for each event.
• Analysis: The study focused on reconstructing temporal evolution (rise times), calculating integral ionization increases, and estimating spectral hardness based on the distribution of detections across different cutoff rigidities.

3. Key Contributions

• First Comprehensive Dataset: The paper produces the first machine-readable, globally distributed datasets for GLEs #1–4, integrating data from 8, 9, 7, and 17 stations respectively.
• High Temporal Resolution: The study significantly improves temporal resolution from the previously available bi-hourly data to:
  • 1–15 min for GLEs #1 and #3.
  • ~5 min for GLE #2.
  • ~1 min for GLE #4 (using Tokyo and Manchester data).
• Discovery of New Records: Uncovered and digitized critical data sources, including the Manchester prototype NM (GLE #4), Tokyo counter telescope data, and various European and Soviet ionization chamber records.
• Integration into IGLED: The reconstructed datasets are formatted to be integrated into the International GLE Database, effectively filling the historical gap for the entire observational history of GLEs.

4. Key Results

A. Temporal Evolution and Rise Times

The high-resolution data revealed distinct rise-time characteristics, contrasting gradual vs. abrupt events:

• GLE #1 (Feb 28, 1942): Gradual rise of 45 ± 15 min.
• GLE #2 (Mar 7, 1942): Abrupt rise of 15 ± 15 min (refined to 32 ± 4 min with high-res data).
• GLE #3 (Jul 25, 1946): Very gradual rise of 105 ± 15 min.
• GLE #4 (Nov 19, 1949): Abrupt rise of 15 ± 15 min (refined to 35 ± 1 min).
• Observation: GLEs #2 and #4 were "prompt" events, while #1 and #3 were "gradual."

B. Spectral Hardness and Energy Caps

By analyzing detections at stations with varying geomagnetic cutoff rigidities ($P_c$), the authors estimated the energy caps ($E_{cap}$) and spectral hardness:

• GLE #1: Detected up to Friedrichshafen ($P_c \approx 4.6$ GV) but not Tokyo ($11.5$GV).$E_{cap} \approx 3.8 - 10.6$ GeV. Mildly hard spectrum.
• GLE #2: Detected in Tokyo ($12.2$GV) but marginal in Huancayo ($14.4$GV).$E_{cap} \approx 11.3 - 13.5$ GeV. Extremely hard spectrum.
• GLE #3: Detected at Mount Wilson ($5.5$GV) but not Huancayo.$E_{cap} \approx 4.7 - 13.4$ GeV. Mildly hard spectrum.
• GLE #4: Detected in Tokyo ($12.3$GV) but marginal in Huancayo.$E_{cap} \approx 11.4 - 13.4$ GeV. Extremely hard spectrum.

C. Integral Ionization and Fluence

• GLE #3 exhibited the greatest integral ionization increases, particularly at polar detectors (Godhavn), suggesting it had the largest fluence among the four.
• GLE #4 had the second-largest fluence.
• GLEs #1 and #2 were comparable in fluence but lower than #3 and #4.
• Saturation Issues: Several instruments (e.g., Cheltenham IC during GLE #4) saturated, creating data gaps in high-resolution plots that were not visible in bi-hourly summaries, highlighting the importance of high-resolution data for accurate peak intensity assessment.

5. Significance

• Historical Context: This work bridges the gap between modern GLE observations and pre-1950s records, allowing for a continuous 80+ year timeline of extreme solar events.
• Extreme Event Modeling: The reconstructed data provides a more robust basis for modeling worst-case radiation scenarios for aviation and spaceflight, as GLE #5 (1956) is no longer the sole reference for "hard spectrum" events; GLE #2 and #4 are now identified as having similarly hard spectra.
• Methodological Benchmark: The study demonstrates the feasibility of digitizing and standardizing heterogeneous, low-quality historical data from non-standard instruments (ICs, GM tubes) to create scientifically usable datasets.
• Future Calibration: The findings highlight the need for future studies to calibrate these early non-standard detectors against modern Neutron Monitors to derive absolute fluence values, which is essential for linking ground-based observations with cosmogenic isotope records (e.g., $^{10}$Be, $^{14}$C) used to study millennial-scale solar activity.

In summary, Hayakawa et al. have successfully recovered, digitized, and analyzed the "forgotten" data of the first four GLEs, transforming them from qualitative historical footnotes into quantitatively analyzed events with defined temporal structures and spectral characteristics.