Researchers Explore Dark Lightning
April 11, 2013

Researchers Explore Radiation-Rich ‘Dark Lightning’

April Flowers for - Your Universe Online

A group of researchers from Florida Institute of Technology's Department of Physics and Space Science recently asked the somewhat unusual question: "What are the radiation doses to airplane passengers from the intense bursts of gamma-rays that originate from thunderclouds?"

Faculty members Joseph Dwyer, Ningyu Liu and Hamid Rassoul discussed the question and their answers concerning terrestrial gamma ray flashes at a press conference meeting of the European Geosciences Union earlier this week. The team revealed a new physics-based model of radiation dose calculations.

For almost a decade, scientists have known that thunderstorms can generate short-lived but powerful bursts of gamma-rays. These terrestrial gamma-ray flashes (TGFs) are so bright that they are capable of blinding instruments several hundred miles away in outer space. The TFGs originate near the same altitude at which commercial aircraft fly, leading scientists to investigate whether or not they present a radiation hazard for people inside the aircraft.

However, the ability to answer that question was hampered until recently by a poor understanding of how thunderstorms generate these gamma-rays. The initial radiation dose estimates ranged from not-so-safe to seriously scary.

Now, the research team from Florida Tech say they have developed a promising, physics-based model that explains how the thunderstorms produce such high-energy radiation.

Instead of creating normal lightning, thunderstorms are able to produce an exotic kind of electrical breakdown involving high-energy electrons and positrons, the electron´s anti-matter equivalent. According to the model, the interaction between the positrons and the electrons causes an explosive growth in the number of high-energy particles. These particles then emit the TGFs while rapidly discharging the thundercloud.

Discharge caused in this manner can happen even more quickly than with normal lightning. Although large amounts of gamma-rays are emitted by this process, very little visible light is generated. This creates a kind of electrical breakdown within the storms that scientists have dubbed "dark lightning." Dark lightning was first discovered in 1994 by a NASA spacecraft.

Recent modeling demonstrates that dark lightning can explain many of the observed properties of TGFs. The model can also calculate radiation doses absorbed by people inside aircraft that just happen to be in exactly the wrong place at the wrong time. The types of TGFs that can be seen from space are generated near the top of the storms. The radiation dose for these types are equivalent to approximately 10 chest X-rays, or the same amount that someone would receive from natural background sources over the course of a year.

"However, near the middle of the storms, the radiation dose could be about 10 times larger, comparable to some of the largest doses received during medical procedures and roughly equal to a full-body CT scan," said Dwyer.

"Although airline pilots already do their best to avoid thunderstorms, occasionally aircraft do end up inside electrified storms, exposing passengers to terrestrial gamma ray flashes. On rare occasions, according to the model calculation, it may be possible that hundreds of people, without knowing it, may be simultaneously receiving a sizable dose of radiation from dark lightning."

The researchers do not know how frequently, if ever, this actually happens.

“This work is very important because it gets you into the zone where you start to understand how often and how likely they are to happen,” said University of California Santa Cruz physics professor David Smith. Smith said the next step is to determine how often these flashes occur.

Since the bursts of radiation are very brief — approximately 10-100 of microseconds — the majority go undetected. However, Dwyer says it is possible that you might see a diffuse purple light.

“Unless you have gamma ray detectors on board, no one would think anything of it,” Smith said.

According to Smith, the National Science Foundation (NSF) is working on an armored airplane that could fly through thunderstorms. Researchers might begin to understand the frequency of the flashes if such a plane had a gamma ray detector onboard. Current thinking is that the flashes occur less frequently than visible lightning, but that could mean anywhere in a range from 1/100th to 1/1000th as often.

“It´s a very rough number,” Smith said. “The other question is, are there somewhat weaker ones that happen more often?”

The research team does not advise pilots to change course just yet, since avoiding thunderstorms is already part of the job. Future findings, however, may lead frequent flyers to pay more attention to the weather before they fly.

“It´s kind of cool that it´s been 250 years since Benjamin Franklin´s kite experiment, and we´ve realized there´s a different kind of lightning going on that we never knew about,” said Dwyer.