An Introduction to Transient Luminous Events

By Owais AliReviewed by Louis CastelSep 12 2025

Transient Luminous Events (TLEs) are short-lived optical phenomena generated above active thunderstorms in the upper atmosphere. Unlike traditional lightning confined to the troposphere, TLEs originate at higher altitudes and manifest as colorful, rapidly evolving flashes that extend into the stratosphere, mesosphere, or lower ionosphere. 

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Since their first photographic documentation in 1989, several classes of TLEs have been identified.

Sprites appear as red, filamentary or jellyfish-shaped discharges between 50 and 90 km, triggered by powerful lightning strokes. Elves form around 90–100 km as expanding rings of light up to 300 km wide, lasting less than a millisecond. Halos occur near 80 km as diffuse disks spanning 40–70 km and often precede sprites. Blue jets propagate from thundercloud tops to about 40 km in the stratosphere, while gigantic jets extend to 70–90 km, linking storms directly to the ionosphere.

Other forms include trolls, downward-moving trails following sprites, and gnomes, compact upward flashes from cloud tops.

All TLEs are brief, lasting only milliseconds, and occur solely above active thunderstorms, making their observation and study particularly challenging.^1,2

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The Physics Behind TLEs

The formation of TLEs results from complex electrodynamic coupling between thunderstorms and the mesosphere-ionosphere system. When powerful positive cloud-to-ground lightning discharges remove substantial charge from storm tops, they produce significant charge moment changes that modify the vertical electric field structure. This quasi-electrostatic field propagates upward through the atmosphere, and when it exceeds the breakdown threshold in the mesosphere, localized ionization occurs, giving rise to sprites, halos, or jets.

These upward-propagating electric fields interact with the lower ionosphere, where reduced air density lowers the breakdown threshold, allowing ionization and the rapid formation of luminous structures even under relatively weak field conditions.

The resulting ionization develops through electron avalanches and streamer formation, producing plasma instabilities that drive the characteristic optical emissions observed in different TLEs. For instance, sprites primarily radiate red light from excited molecular nitrogen's first positive band system, while elves and jets emit blue and ultraviolet light from the second positive system and related electronic transitions.

The specific morphology and duration of each TLE are shaped by the interplay of electron attachment, dielectric relaxation, and the transition from avalanches to streamers, resulting in distinct diffuse, transition, and streamer regions across different altitudes. The ionosphere further influences TLE behavior by modulating field propagation and reflecting electromagnetic pulses, which accounts for the vertical structuring of sprites and the extensive lateral expansion of elves.^2,3

How Are TLEs Observed and Studied?

The study of TLEs employs diverse observational platforms, each providing complementary insights into their occurrence patterns, morphology, and underlying physical mechanisms.

Space-Based Observations

Space-based instruments monitor TLEs from orbit, capturing optical, radio, and high-energy signals to determine their occurrence, classification, and interaction with lightning. These platforms integrate cameras, photometers, spectrometers, and particle detectors to record emissions across ultraviolet, visible, and gamma-ray wavelengths.

Long-term missions such as ISUAL on FORMOSAT–2 (2004–2016) documents sprites, halos, and elves with photometric and spectral imaging, while JEM-GLIMS (ISS, 2012–2015) combined optical sensors with VLF/VHF radio receivers to classify TLEs and track their electromagnetic signatures.

Short-duration projects, including LSO on the ISS and MEIDEX on the Space Shuttle, supplements high-resolution imagery, and instruments like ASIM (ISS, launched 2018) now provide simultaneous optical and high-energy observations of TLEs and Terrestrial Gamma-Ray Flashes.

Balloon and Aircraft-Based Observations

High-altitude balloons and aircraft provide direct measurements of local atmospheric perturbations caused by TLEs, capturing spectral and optical data that are not accessible from the ground. For instance, the HALESIS project utilizes infrared imaging to monitor variations in CO2, NO, O3, and H2O at altitudes ranging from 20 to 40 km.

Additionally, the first aircraft color images of sprites at approximately 12 km, recorded in 1995 with a high-sensitivity color camera, revealed red and blue emissions from excited N2. Subsequent airborne campaigns have employed high-speed spectroscopy and near-ultraviolet imaging to investigate sprite streamer dynamics and excitation mechanisms.

Ground-Based Observations

Ground-based instruments deliver high temporal and spectral resolution for TLE observations. For example, PIPER (2006–present) captures 185–900 nm emissions at up to 25,000 fps using multichannel photomultiplier arrays, facilitating detailed chemical and optical analyses of TLEs. Its predecessor, Fly’s Eye, verified the radial expansion of elves and contributed to understanding the electromagnetic propagation of TLE-associated pulses.

Emerging detection technologies increasingly leverage commercial sensing systems, high-speed optical cameras, photometers, and spectrometers with enhanced temporal resolution capabilities. These systems target the capture of millisecond-scale events while probing potential chemical effects, including the excitation of metastable nitrogen and oxygen states and local production of radical species such as HO2, as recently confirmed by the SMILES limb-viewing spectrometer aboard the ISS.⁴

Why Do TLEs Matter?

TLEs offer critical insights into the coupling between thunderstorms, the mesosphere, and the lower ionosphere, serving as natural laboratories for plasma processes under low-pressure conditions between 50 and 100 km altitude.

Sprites, for instance, generate radicals such as H, OH, and HO2 through streamer discharges at 70–80 km, with satellite observations, including the SMILES instrument, confirming localized HO2 enhancements above sprite-producing storms. These chemical perturbations, though modest, influence mesospheric oxidation capacity and provide diagnostics of ionospheric conductivity. High-speed video and electromagnetic measurements further indicate that sprites preferentially form at pre-existing plasma irregularities, revealing structural features of the ionosphere.^5,6

TLE-associated electromagnetic emissions span extremely low frequency (ELF), ultra-low frequency (ULF), and very low frequency (VLF) bands, affecting satellite operations and radio wave propagation through localized perturbations of lower ionospheric conductivity. Research indicates that sprite-producing lightning can modify ionospheric electric fields by up to approximately 20 percent of the breakdown threshold. While TLEs do not pose direct threats to satellites, the associated changes in electron density can cause short-term propagation delays and degrade the performance of remote sensing, communication, and Global Navigation Satellite System (GNSS) applications.⁷

Additionally, TLEs serve as indicators of intense storm activity and vertical coupling between the troposphere and the upper atmosphere. Their occurrence patterns provide essential data for improving understanding of upper atmospheric chemistry, electrodynamics, and storm intensity forecasting. The electromagnetic pulses associated with TLEs can influence communication and navigation systems despite their high-altitude origins, making their study relevant to both scientific understanding and practical applications.⁸

Current Research and Future Directions

Research on TLEs is actively pursued worldwide through laboratory experiments, field campaigns, and space-based observations.

In the United States, the University of Alaska Fairbanks conducts field observations and high-speed imaging of sprites and jets, complementing computational studies of initiation conditions, while the Pennsylvania State University investigates sprite chemistry and streamer breakdown using combined modeling and observational data.

European research efforts include the University of Oulu and Sodankylä Geophysical Observatory in Finland, which study the chemical effects of sprites and elves using the Sodankylä Ion Chemistry (SIC) model.

Japan contributes through the JEM-GLIMS mission aboard the ISS, operated by Hokkaido University and JAXA, providing statistical datasets of thousands of lightning events.

These space-based and field efforts are complemented by several global missions and instruments. ESA’s ASIM on the ISS delivers high-resolution optical and high-energy measurements of TLEs and Terrestrial Gamma-Ray Flashes (TGFs), capturing phenomena such as ELVES and ultra-brief corona discharges at cloud tops.

The ILAN-ES experiment on the Axiom Space AX-1 mission recorded blue corona discharges from cumulonimbus cloud tops, establishing links to lightning activity. TARANIS (CNES, 2020) was designed to investigate TLEs and associated electromagnetic emissions, but its launch failure prompted plans for a follow-up TARANIS-2 mission.^9,10,11

Despite these advances, key questions remain regarding TLEs, including the energy scales of different events, their triggering mechanisms and predictability, and their interactions with atmospheric chemistry and climate. Future research seeks to integrate high-resolution space- and ground-based observations with modeling to quantify these processes, improve forecasts of upper-atmospheric electrical activity, and assess the broader environmental and climatic impacts of TLEs.

References and Further Reading

1. Kaiser, C. (2023). Spritacular. NASA Science. https://science.nasa.gov/citizen-science/spritacular/
2. Pasko, V. P. (2010). Recent advances in theory of transient luminous events. Journal of Geophysical Research: Space Physics, 115(A6). https://doi.org/10.1029/2009JA014860
3. Surkov, V. V., & Hayakawa, M. (2012). Underlying mechanisms of transient luminous events: a review. Annales Geophysicae, 30(8), 1185–1212. https://doi.org/10.5194/angeo-30-1185-2012
4. Gordillo-Vázquez, F., & Pérez-Invernón, F. (2021). A review of the impact of transient luminous events on the atmospheric chemistry: Past, present, and future. Atmospheric Research, 252, 105432. https://doi.org/10.1016/j.atmosres.2020.105432
5. Winkler, H., Yamada, T., Kasai, Y., Berger, U., & Justus Notholt. (2021). Model simulations of chemical effects of sprites in relation with observed HO₂ enhancements over sprite-producing thunderstorms. Atmospheric Chemistry and Physics, 21(10), 7579–7596. https://doi.org/10.5194/acp-21-7579-2021
6. Qin, J., Pasko, V. P., McHarg, M. G., & C., H. (2014). Plasma irregularities in the D-region ionosphere in association with sprite streamer initiation. Nature Communications, 5(1), 1-6. https://doi.org/10.1038/ncomms4740
7. Pérez-Invernón, F. J., Luque, A., & Gordillo-Vázquez, F. J. (2018). Modeling the Chemical Impact and the Optical Emissions Produced by Lightning-Induced Electromagnetic Fields in the Upper Atmosphere: The case of Halos and Elves Triggered by Different Lightning Discharges. Journal of Geophysical Research: Atmospheres, 123(14), 7615-7641. https://doi.org/10.1029/2017JD028235
8. Garnung, M. B., Celestin, S., & Farges, T. (2021). HF-VHF Electromagnetic Emissions From Collisions of Sprite Streamers. Journal of Geophysical Research: Space Physics, 126(6), e2020JA028824. https://doi.org/10.1029/2020JA028824
9. Hanks, M., & Hanks, M. (2025, June 18). Scientists Are Tracking Mysterious “Transient Luminous Events” from Space Using This Innovative Tech. The Debrief. https://thedebrief.org/scientists-are-tracking-mysterious-transient-luminous-events-from-space-using-this-innovative-tech/
10. NASA. (2025). Studying Storms from Space Station. https://www.nasa.gov/missions/station/iss-research/studying-storms-from-space-station/
11. Yair, Y., Korzets, M., Devir, A., Korman, M., & Stibbe, E. (2024). Space-based optical imaging of blue corona discharges on a cumulonimbus cloud top. Atmospheric Research, 305, 107445. https://doi.org/10.1016/j.atmosres.2024.107445

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