20025-07-28 ペンシルベニア州立大学（Penn State）
＜関連情報＞
- https://www.psu.edu/news/engineering/story/bolt-born-atmospheric-events-underpinning-lightning-strikes-explained
- https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2025JD043897
空気中の光電効果は、雷の発生と地表ガンマ線フラッシュを説明する Photoelectric Effect in Air Explains Lightning Initiation and Terrestrial Gamma Ray Flashes
Victor P. Pasko, Sebastien Celestin, Anne Bourdon, Reza Janalizadeh, Zaid Pervez, Jaroslav Jansky, Pierre Gourbin
Journal of Geophysical Research: Atmospheres Published: 28 July 2025
DOI:https://doi.org/10.1029/2025JD043897
Abstract
Terrestrial gamma ray flashes (TGFs) are high-energy photon bursts that have been linked to short bursts of electromagnetic radiation associated with lightning activity. The most puzzling unexplained aspect of these events is that gamma rays originate from very compact regions of space while the source regions often seem to be optically dim and radio silent when compared to processes in ordinary lightning discharges. In this work, we report a mechanism that allows precise quantitative explanation of these peculiar features of TGFs and their relationships to the observed waveform characteristics of associated radio emissions. The mechanism represents an extension of earlier ideas on feedback processes in growth of relativistic runaway electron avalanches (Dwyer, 2003, https://doi.org/10.1029/2003GL017781), and is based on a recent demonstration of the dominant role of the photoelectric feedback on compact spatial scales (Pasko, Celestin, et al., 2023, https://doi.org/10.1029/2022GL102710). Since discussed events often occur in isolation or precede formation of lightning discharges, the reported findings propose a straightforward solution for the long-standing problem of lightning initiation.
Key Points
- Photoelectric feedback discharges initiate lightning with no constrains on densities or physical origin of seed runaway electrons
- Photoelectric feedback discharges are a unified mechanism for production of initial breakdown pulses (IBPs), narrow bipolar events (NBEs), energetic in cloud pulses (EIPs), and terrestrial gamma ray flashes (TGFs)
- Similarity laws explain observed variability of IBPs, NBEs, EIPs, and TGFs produced at different altitudes in the Earth’s atmosphere
Plain Language Summary
We provide explanation of spectacular naturally occurring bursts of X-rays and accompanying radio emissions that are observed in association with lightning activity in the Earth’s atmosphere. These burst events are commonly referred to as terrestrial gamma ray flashes (TGFs). The core physical process leading to these events is avalanche multiplication of relativistic electrons accelerated by force exerted by electric field. In geophysical context, the electric field of sufficient magnitude and spatial extent for supporting these avalanches is generated either as an ambient field due to thundercloud electrification or produced in streamer zones of lightning leaders. These electrons radiate energetic photons (X-rays) as they scatter by nuclei of nitrogen and oxygen atoms in air. These X-rays are radiated in all directions and some fraction is radiated in opposite direction of electron motion. These particular X-rays lead to seeding of new relativistic seed electrons due to the photoelectric effect and thus strong amplification of the original avalanche. It has been established recently that these feedback effects occur on very small spatial scales, and in this work, we report the first fully time-dependent simulations of related dynamics, similarity analysis allowing better understanding of characteristics of events observed at different altitudes, and quantitative comparisons with observations.
