Danger zones in the air where radiation levels surge could pose an unrecognised health hazard. Airliners may have to avoid these in future, just as they do with volcanic ash clouds, to minimise any risk to travellers and crew.
We have long known that high-altitude flight exposes us to cosmic rays. The radiation dose on a flight from London to Tokyo is roughly equivalent to a chest X-ray.
Now research flights have revealed the existence of 鈥渃louds鈥� where radiation levels can be at least double the usual level. They were discovered as a result of the NASA-funded programme, which aims to develop new methods of measuring and monitoring high-altitude radiation.
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In 265 flights, radiation levels detected generally followed the expected pattern, but in at least six instances they surged, as though the aircraft was flying through a radiation cloud.
鈥淲e have seen several cases where the exposure is doubled while flying through the cloud,鈥� says ARMAS principal investigator W. Kent Tobiska, of Los Angeles firm . 鈥淚t is quite variable and can easily be more or less than that.鈥�
Even higher levels have been recorded in some cases, but those results remain unpublished while the team considers alternative explanations for the data.
Tobiska says the two main sources of radiation, cosmic rays and the solar wind, can鈥檛 account for the surges. 鈥淥ur new measurements show a third component.鈥�
Freed electrons
The surges coincided with geomagnetic storms. This points the finger at energetic electrons being lost from the outer Van Allen radiation belts, where charged particles mostly from the solar wind are trapped by Earth鈥檚 magnetic field.
Tobiska believes that such a storm can liberate electrons trapped in the Van Allen belts. 鈥淭hose electrons are driven into the upper atmosphere, collide with nitrogen and oxygen atoms and molecules, and then create a spray of secondary and tertiary radiation, likely in the form of gamma rays.鈥� This radiation, he thinks, is what the ARMAS flights are detecting across a wide area.
Daniel Baker of the University of Colorado鈥檚 Laboratory for Atmospheric and Space Physics says this mechanism seems feasible. 鈥淚t is plausible that the ARMAS results are related to enhanced loss of radiation belt particles from the magnetosphere into the middle and lower atmosphere.鈥�
There are no set standards for radiation safety in US aviation at present, but Tobiska says that regulations are likely in the next few years.
The absolute risk may be low, as a chest X-ray only increases the risk of a fatal cancer by 1 in 200,000, but these must be balanced against the large number of flights and whether risk is avoidable.
鈥淭his is mainly for crew members,鈥� says Tobiska, 鈥渂ut would certainly benefit frequent flyers and even fetuses in their first trimester.鈥�
ARMAS work using satellite data and airborne sensors may allow the radiation 鈥渃louds鈥� to be tracked. Tobiska says that in future, flights may be diverted or directed to a lower altitude to avoid them.
Space Weather
