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Direct Observation of Wilsonian Electrons in Thunderstorms

This paper reports the direct observation of Wilsonian runaway electrons in thunderstorms at Aragats, demonstrating that such detection is only possible when the acceleration region is within tens of meters of the detector under specific meteorological conditions, thereby explaining the century-long rarity of these observations.

Original authors: A. Chilingarian, T. Karapetyan, B. Sargsyan

Published 2026-07-15

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CC BY 4.0

Original authors: A. Chilingarian, T. Karapetyan, B. Sargsyan

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a thunderstorm as a giant, invisible particle factory floating in the sky. For over a century, scientists have known this factory exists because they can see the "smoke" it produces: high-energy gamma rays. But for a hundred years, nobody could catch the actual "workers" inside the factory—the super-fast electrons that Wilson (a Nobel Prize winner) predicted were being accelerated by the storm's electric fields. It was like seeing a car zoom by at night and only spotting its headlights, never the car itself.

Why was the car so hard to see? Because the atmosphere is a thick, sticky fog. Electrons are fragile; as soon as they try to zoom down from the storm to the ground, the air eats them up. They lose their energy and vanish long before they reach a detector on the surface. Gamma rays, on the other hand, are tough cookies; they can punch through that thick fog and reach the ground easily. This is why, for decades, we only saw the gamma rays and assumed the electrons were either missing or too rare to find.

But a team of scientists at the Aragats station in Armenia (sitting high up at 3,200 meters) finally caught the electrons. How? They realized the factory didn't always stay high in the sky. Sometimes, the bottom of the storm cloud drops down, getting incredibly close to the ground.

In this study, the researchers looked at six specific moments between 2024 and 2026 where the storm cloud base descended to a shocking distance: just 25 to 87.5 meters above their detector. To put that in perspective, that's like the storm cloud hovering just above the roof of a tall building. Because the cloud was so close, the electrons didn't have to travel far through the "fog" to reach the detector. They survived the trip!

The team used a special instrument called the SEVAN-Light spectrometer, which acts like a bouncer at a club. It can tell the difference between the "charged" party guests (electrons) and the "neutral" ones (gamma rays). During these six rare events, the bouncer saw a massive rush of electrons, with energies reaching up to 45 MeV.

Here is the key takeaway: The paper argues that the electrons were not missing from the storms all along. The factory was always running. The reason we couldn't see the electrons for a century is that the "cloud factory" usually stays too high up. The electrons get eaten by the atmosphere before they can reach us. But when the cloud base drops to within a few tens of meters, the electrons make it through.

The data shows these events happened under very specific conditions:

The air temperature was near 0°C, right in the "mixed-phase" zone where ice and water droplets mix (a great spot for building up electric charge).
The electric field near the ground was strong and disturbed.
Four of the six events ended abruptly when lightning struck, which suggests the electric field that was accelerating the electrons got neutralized or rearranged, shutting down the particle factory instantly.

The scientists measured the energy of these electrons and found they followed a consistent pattern, with a characteristic energy around 7.65 MeV. This consistency suggests that the same physical mechanism was at work in all six events, just varying in how intense the "production line" was.

So, the mystery of the century is solved not by finding a new type of electron, but by realizing that the atmospheric conditions required to see them are incredibly rare. The electrons are there, zooming around in the storm, but they usually get lost in the long journey down. Only when the storm cloud hangs low enough—within a few tens of meters—do they survive the trip to the ground, allowing us to finally say, "There you are!"

Technical Summary: Direct Observation of Wilsonian Electrons in Thunderstorms

Problem Statement
For over a century, a fundamental observational paradox has persisted in high-energy atmospheric physics. While C. T. R. Wilson's hypothesis that thunderstorm electric fields accelerate electrons to relativistic energies is widely accepted, and the resulting gamma-ray emissions (glows, TGFs, TGEs) are routinely detected, the direct observation of the accelerated electrons themselves has remained exceptionally rare. Theoretical models, specifically Relativistic Runaway Electron Avalanches (RREA), predict the production of both relativistic electrons and bremsstrahlung gamma rays. However, a fundamental observational asymmetry exists: electrons lose energy continuously in air and are rapidly attenuated, whereas gamma rays propagate to much greater atmospheric depths. Consequently, most ground-based TGE observations are photon-dominated, even when the source mechanism involves electron acceleration. The central question addressed is identifying the specific atmospheric and geometric conditions that allow TGE electrons to survive the path from the acceleration region to ground-level detectors, thereby explaining the century-long rarity of "Wilsonian" electron detections.

Methodology
The study analyzes six electron-rich Thunderstorm Ground Enhancements (TGEs) detected at the Aragats station (3200 m altitude) between 2024 and 2026. The primary instrument is the SEVAN-Light spectrometer, a compact, energy-deposit-resolving particle spectrometer capable of separating neutral components (neutrons, gamma rays) from charged components (muons, electrons).

Detection Logic: The detector utilizes a 0.25 m² scintillator (20 cm thick) and a 1 m² veto scintillator (1 cm thick). Events are classified via coincidence logic: "11" coincidence (signal in both layers) is enriched in charged particles (predominantly TGE electrons), while "01" coincidence (signal only in the lower layer) is enriched in neutral particles. Electron registration efficiency ranges from 60% to 99% for energies between 10 and 50 MeV, with gamma-ray contamination kept below 1.8%.
Event Selection: TGE candidates were identified from a continuous 1-minute time series archive as sustained positive enhancements (several standard deviations above background). Events were confirmed only if they showed coherent responses across independent channels and were accompanied by a disturbed near-surface electric field (|NSEF| > 5 kV m⁻¹).
Meteorological Correlation: For each event, the study correlated particle data with meteorological parameters, including cloud-base height (estimated via temperature and dew point spread), near-surface electric field (NSEF), and lightning flash distances. These specific events were compared against a broader archive of TGEs from 2012–2025 to identify distinct atmospheric regimes.
Spectral Reconstruction: Electron energy spectra were reconstructed using a GEANT4-based response matrix and an inverse-problem method, fitting the data to exponential functions.

Key Results
The analysis of the six selected electron-rich TGEs reveals a distinct and compact set of atmospheric conditions:

Proximity of Acceleration Region: All six events occurred with cloud-base heights between 25 and 87.5 meters above the detector (mean: 44.7 ± 19.6 m). This is significantly lower than the central population of the broader TGE archive.
Atmospheric Regime: The events occurred in a narrow temperature range near 0°C (mean: 0.0 ± 1.3°C), placing them within the mixed-phase region of thunderclouds where charge separation is efficient.
Particle Flux and Energy: The events exhibited large electron fluxes with reconstructed energies extending up to 45 MeV. The differential energy spectra for all six events followed a remarkably stable exponential shape with characteristic energies ( $E_0$ ) ranging from 6.33 to 8.57 MeV (mean: 7.65 ± 0.73 MeV).
Lightning Correlation: Four of the six events were abruptly terminated by nearby lightning flashes, suggesting the particle flux is controlled by the persistence of the pre-discharge thundercloud electric field.
Spectral Stability: Despite large variations in event intensity (normalization parameter $A$ varied by a factor of ~4), the spectral slope ( $E_0$ ) remained stable, indicating a consistent acceleration mechanism across different events.

Key Contributions

Experimental Explanation for Rarity: The study provides an experimental explanation for the historical scarcity of direct Wilsonian electron observations. It demonstrates that the rarity is not due to the absence of runaway electrons in thunderstorms, but rather the infrequency of atmospheric conditions where the acceleration region approaches within tens of meters of the detector.
Identification of a Near-Ground RREA Regime: The paper establishes the existence of a distinct "near-ground" RREA regime where relativistic electrons can survive atmospheric attenuation to reach ground level, a condition previously difficult to isolate.
Validation of Detection Criteria: The results confirm that direct electron detection requires not only high TGE intensity but also specific geometric constraints (low cloud base) and charged-neutral separation capabilities to distinguish electrons from the dominant gamma-ray background.

Significance and Claims
The authors claim that these findings resolve a long-standing observational paradox in atmospheric physics. By identifying the specific atmospheric geometry required for electron survival, the study establishes the physical conditions necessary for the direct observation of runaway electrons. The results confirm that thunderstorms act as natural particle accelerators capable of producing relativistic electrons that reach the ground, provided the acceleration region is sufficiently close to the detector. This work provides an experimental anchor for models of RREA, thunderstorm gamma-ray glows, and lightning initiation, clarifying that the dominance of gamma-ray observations in previous decades was a consequence of atmospheric attenuation rather than a lack of electron production. The study does not claim to have discovered new physics but rather to have experimentally verified the conditions under which existing theoretical predictions (RREA) manifest as detectable ground-level electron fluxes.

Technical Summary: Direct Observation of Wilsonian Electrons in Thunderstorms

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