Tropical thunderstorms, a breathtaking display of nature’s power, harbor a surprising secret: radioactivity. Recent research, utilizing a modified spy plane, has revealed intriguing new insights into the gamma radiation emitted by these powerful storms. Two papers published in Nature detail the findings, shedding light on long-duration gamma-ray glows, a newly identified phenomenon called flickering gamma-ray flashes (FGFs), and their potential relationship with lightning.
The research team employed a NASA ER-2, a modified U-2 spy plane, to fly above tropical thunderstorms and observe the gamma radiation. This unique vantage point allowed them to detect gamma-ray glows lasting for hours and spanning an area of nearly 3,500 square miles. FGFs are typically much weaker than the well-known Terrestrial Gamma-ray Flashes (TGFs) observed from space since the 90s,” explains Martino Marisaldi, a physicist at the University of Bergen and lead author of one of the Nature papers. “This is why you need to get close to the thundercloud to detect them.”
Tendrils of lightning from a tropical thunderstorm in ColombiaTendrils of lightning illuminate the Colombian sky during a tropical thunderstorm. Photo: Oscar van der Velde
While gamma-ray glows and terrestrial flashes were already known to emanate from thunderstorms, FGFs represent a “goldilocks” radiation, lasting longer than terrestrial bursts but much shorter than the glows. “FGFs are remarkably different from both TGFs and glows but carry some features of both,” Marisaldi elaborates. “This is why we figure them as the missing link between the two phenomena.” Intriguingly, FGFs, while radio silent themselves, are often followed by radio pulses called Narrow Bipolar Events (NBEs), which are frequently associated with lightning initiation. This suggests a potential causal link between FGFs and lightning.
Rethinking the Lightning-Gamma Ray Connection
Previous research relying on space-based observations established a relationship between TGFs and lightning, suggesting that one TGF occurred for every 10,000 lightning strikes. However, the spy plane data revealed over 100 TGFs too faint to detect from space. “This means that the TGF-lightning ratio is not 1-to-10,000 as established from space observations but 1-to-100 or even higher,” states Nikolai Østgaard, a physicist at the University of Bergen and co-author of both Nature papers. “All the transient events (TGFs, FGFs, and glow bursts) were followed by intense lightning activity, and might play an important role in lightning initiation.”
A Closer Look at Lightning
Lightning, an electrical spark in the Earth’s atmosphere, can remain aloft or strike the ground in dramatic displays of energy. Despite the frequent occurrence of thunderstorms – approximately 40,000 daily – the precise mechanisms of lightning initiation remain a mystery. Joseph Dwyer, a physicist at the University of New Hampshire, notes in a related News & Views article that the observed glows, lasting several seconds, were dynamic and “bubbled up,” challenging the previous understanding of glows as slow and uniform. He adds that flickering gamma-ray flashes resemble multi-pulsed TGFs observed from satellites, suggesting a common underlying mechanism for these energetic phenomena.
According to Dwyer, the new research “effectively rules out” alternative lightning initiation theories, such as cosmic rays and lightning leaders, leaving positron feedback as the most likely explanation.
The Evolving Picture of Thunderstorms
This research provides a foundation for further investigating the relationship between gamma outbursts, thunderclouds, and lightning. The observation of gamma flashes occurring without lightning suggests that electrified storms themselves can generate gamma rays. While the presence of gamma radiation in tropical storms isn’t entirely new, this study contributes to a more nuanced understanding of thunderclouds, enriching our appreciation for these awe-inspiring natural phenomena.
