“Lightning is the most spectacular manifestation of atmospheric electricity”, yet its precise trigger has long eluded science. Here’s the voice of Victor Pasko, professor of electrical engineering at Penn State, expressing the heart of a three-decade-long enigma that has just begun to be deciphered with the help of both sophisticated modeling and high-altitude observation.
At the center of this achievement is the Photoelectric Feedback Discharge model, a complex simulation tool created to describe how lightning arises from the frenzied interaction of fields and particles inside thunderclouds. The model shows that strong electric fields in thunderclouds can accelerate electrons to relativistic speeds, paving the way for a spectacular chain reaction. These electrons, frequently seeded by cosmic rays entering the atmosphere from space, strike air molecules mainly nitrogen and oxygen generating X-rays and releasing a cascade of further electrons and high-energy photons. As Pasko describes, “Our findings provide the first precise, quantitative explanation for how lightning initiates in nature. It connects the dots between X-rays, electric fields and the physics of electron avalanches.”
This avalanche process is more than an abstract concept. The model was tested rigorously against observational data in the field, which were collected using a combination of ground-based sensors, satellites, and even specially equipped high-flying aircraft. Zaid Pervez, one of the team’s doctoral students, said, “We explained how photoelectric events occur, what conditions need to be in thunderclouds to initiate the cascade of electrons, and what is causing the wide variety of radio signals that we observe in clouds all prior to a lightning strike.” These observations were further supported by comparing the model’s predictions to measurements from compact intercloud discharges, a form of lightning that normally appears in very localized areas in thunderclouds.
Cosmic ray’s role is especially significant. When entering the atmosphere, the high-energy particles create relativistic seed electrons that are collected by thundercloud electric fields. If the field is stronger than a critical value, these electrons will experience runaway acceleration, triggering what is referred to as a relativistic runaway electron avalanche (RREA). It is not only responsible for the initiation of lightning but also for the generation of terrestrial gamma-ray flashes (TGFs) bright, short pulses of X-rays and gamma rays, which have been observed with both ground-based and space-borne detectors. The event is so powerful that TGFs are the most powerful naturally occurring radiation on Earth, with energies reaching up to 40 MeV.
One of the model’s more interesting conclusions solves an old mystery: how and why TGFs sometimes occur without visible lightning or intense radio emissions. Pasko explains, In our modeling, the high-energy X-rays from relativistic electron avalanches create new seed electrons due to the photoelectric effect in air, which quickly grow these avalanches. Apart from being generated in extremely small volumes, this uncontrolled chain reaction can take place with extremely variable intensity, frequently resulting in measurable amounts of X-rays, but with very faint optical and radio emissions accompanying them. This is why such gamma-ray flashes may issue from source areas that are otherwise optically faint and radio quiet. “Such silent” TGFs have been validated by airborne sensors such as ADELE, which detected sudden, bright gamma-ray glows at cruise levels, often with no associated coincident lightning signals in both the optical and the radio frequency bands.
The quality of the modeling is matched by the range of detection techniques. Detectors like the Modular X- and Gamma-ray Sensor (MXGS) onboard the International Space Station and aerial platforms like ADELE utilize plastic scintillators, photomultiplier tubes, and advanced spectral analysis to detect and measure TGFs and corresponding high-energy events. The data are decoded and the spatial and energetic composition of these events is reconstructed with very high accuracy using Monte Carlo simulations that include realistic atmospheric profiles and models of detector response.
These implications extend further than lightning initiation, as they offer a quantitative framework for understanding charge dynamics and energy transfer in thunderstorms and the role of cosmic rays in atmospheric electricity, complicated to describe due to a complex relationship between high-energy particle physics processes and meteorological phenomena. Since the equations that form the basis of the Photoelectric Feedback Discharge model are now accessible to the research community, this research establishes a new benchmark for the synthesis of theoretical, computational, and observational methods in atmospheric science.
