Airline passengers flying through storms might have more to worry about than a little turbulence.
A new study suggests that if jets pass near lightning discharges or related phenomena known as terrestrial gamma-ray flashes, passengers and crew members could be exposed to harmful levels of radiation, a dose equal to that of 400 chest X-rays.
However, the likelihood of encountering these lightning events is very small, the researchers say. In addition, airline passengers are always exposed to slightly elevated radiation levels due to cosmic rays, which bombard Earth's upper atmosphere constantly but typically don't make it to the surface.
Airplane passengers would only be exposed to this high radiation dose if their airplane happens to be near the point of origin of a lightning discharge or a gamma-ray flash, and scientists aren't sure how often, if ever, such exposure occurs. The radiation bursts are extremely brief and extend over just a few hundred feet in the clouds.
"We know that commercial airplanes are typically struck by lightning once or twice a year," said Joe Dwyer, professor of physics and space sciences at Florida Tech. "What we don't know is how often planes happen to be in just the right place or right time to receive a high radiation dose. We believe it is very rare, but more research is needed to answer the question definitively."
Lightning and other mysterious flashes
Scientists admit lightning is still mysterious. They don't really know why it produces X-rays or gamma rays (which are more intense than X-rays), or even how it gets from there to here.
The researchers did not measure high radiation doses directly with airplanes. Instead, they estimated radiation based on satellite and ground observations of X-rays and gamma rays.
With orbiting satellite data, they were able to study terrestrial gamma-ray flashes, or TGFs, mysterious phenomena that appear to originate at the same altitudes used by jet airliners and occur along with lightning. While scientists don't know what causes TGFs, they believe they are produced by electric fields above the thunderstorms.
The research team also included measurements of X-rays and gamma rays from natural lightning on the ground, as well as artificial lightning triggered with wire-trailing rockets fired into storm clouds.
They then used computer models to estimate the amount of radiation that could be produced within, or very near, thunderclouds during lightning storms.
They concluded the radiation in a football field-sized space around these lightning events could reach "biologically significant levels," up to 10 rem (roentgen equivalent man), which is the dosage considered the maximum safe radiation exposure over a person's lifetime.
While the research raises obvious concerns, recent in-flight experiments suggest the incidents are rare, according to study scientist David Smith, an associate professor of physics at UC-Santa Cruz. Flying aboard an aircraft this past summer in Florida, Smith and several of the other researchers used a highly sophisticated instrument to measure gamma-ray flashes from thunderstorms. Over the course of several flights, they only detected one such flash, at a safe distance from the plane.
"These observations show that although thunderstorms do occasionally create intense gamma-ray flashes, the chance of accidentally being directly hit by one is small," Smith said.
More inquiry needed
Martin Uman, another author and a professor of electrical and computer engineering at UF, noted that airline pilots typically seek to avoid flying through storms.
However, he said, the fact that commercial planes are struck once or twice a year suggests more inquiry is needed. He said he would recommend to the Federal Aviation Administration that it place detectors aboard planes capable of measuring the radiation bursts to determine how often they occur.
"We also need to spend more time looking at gamma and x-ray radiation from lightning and thunderstorms and trying to understand how it works," Uman said.
The research will be detailed in an upcoming issue of the Journal of Geophysical Research — Atmospheres.