Invisible bursts of radiation that happen inside thunderstorms may hold the key to one of nature’s most spectacular mysteries: how lightning actually begins. Scientists have found that these high-energy flashes, known as terrestrial gamma ray flashes, don’t just accompany lightning strikes—they may help set the stage for them.
For decades, researchers struggled to explain lightning’s first step. Storm clouds build up electrical charge, but the measured electric fields often seemed too weak to spark such enormous discharges. Now, a team led by Victor Pasko at Penn State University suggests the missing piece is a process called photoelectric feedback, in which radiation inside the storm sets off a chain reaction of particles.
Gamma ray flashes were first spotted by satellites in 1994, coming from thunderstorms. They puzzled scientists because they originated from small, dim regions of storms rather than the bright, crackling areas where lightning is usually seen. The flashes lasted just millionths of a second but carried surprising amounts of energy.
The new research, published in Journal of Geophysical Research: Atmospheres, shows these bursts can act like invisible “spark plugs” for lightning. In storm clouds, electric fields accelerate electrons to nearly light speed. As they smash into air molecules, they produce gamma rays. Some of those rays knock loose more electrons when they strike other molecules, multiplying the effect.
This sets off a runaway feedback loop: freed electrons are accelerated again, producing more gamma rays, which free even more electrons. The result is a tiny, fast-moving pocket of electrified air that can seed the larger lightning channel.
Rather than needing impossibly strong electric fields, lightning may actually get started from these small, high-energy sparks that snowball into the dazzling bolts we see from the ground.
From Fleeting Flashes to Mile-Long Bolts
Although these events happen on scales of microseconds and hundreds of meters, about the size of a city block, their impact cascades outward. Within fractions of a second, the conditions they create can blossom into mile-long lightning strikes.
Computer models run by the research team reproduced many lightning-related signals seen in real storms. When electric fields stayed below a critical threshold, the discharges remained faint and invisible—possibly explaining mysterious radio signals detected during storms with no visible lightning. Once the fields grew strong enough, they tipped into streamer discharges that expanded into visible lightning bolts.
The simulations also explained why some lightning produces intense radio signals without obvious flashes. The same underlying process simply looks different depending on the field strength and air density.
Why Altitude Matters
The team found the process unfolds differently depending on height in the atmosphere. Near the ground, it takes only microseconds and stays fairly compact. Higher up, in thinner air, the flashes last longer — hundreds of microseconds — and spread across larger regions. This scaling helps explain why lightning sensors at different altitudes pick up such varied signals, even when observing the same kind of storm activity.
The models also tied together several puzzling phenomena that had been observed separately for years: initial breakdown pulses, narrow bipolar events, energetic in-cloud pulses, and terrestrial gamma ray flashes. They show that all can be seen as different expressions of the same core process.
What It Could Mean for Forecasting
If lightning really does begin with invisible radiation bursts, it could reshape how we predict and protect against strikes. Current forecasts rely mainly on visible lightning counts. Being able to detect these faint gamma ray precursors could one day allow earlier warnings of storm danger.
For now, that’s a distant possibility. The study used simplified one-dimensional models, while real thunderstorms are three-dimensional mazes of ice particles, wind, and shifting temperatures. But the findings give scientists a new framework to build on.
Source: https://studyfinds.org/what-triggers-lightning-mystery/
