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On October 3, 2015, a Gulfstream V research aircraft belonging to the National Science Foundation recorded a huge spike in atmospheric radiation while flying over the South Atlantic between Antarctica and the tip of South America. For 11 minutes, his onboard radiation detector watched the levels double, as if the plane had flown through a cloud of radiation.

This was not the only incident. Since 2013, airborne detectors have recorded 57 such radiation bursts, each lasting between ten minutes and an hour.

The sightings raised significant questions, not least about the risks to the crew and the safe operation of on-board electronics. But most of all, scientists want to know what causes these radiation bursts.

Radiation dose

Today, we have an answer thanks to the work of Kent Tobiska of Space Environment Technologies in California and colleagues who have studied the incidents and believe they know the source of the radiation.

Their work raises important questions about the safety of frequent flyers such as crew and how to protect them from events that “are analogous to airplanes flying through radiation clouds,” Tobiska and company say.

Space scientists have long known that Earth is bombarded from space by a steady stream of high-energy radiation from beyond the Solar System called Galactic Cosmic Rays and by more sporadic bursts from the Sun called Solar Energetic Particles. This radiation is usually in the form of high-energy electrons, protons, and alpha particles.

Earth is shielded from these particles by the Sun’s magnetic field, which slows down much of the incoming galactic radiation, and by its own magnetic field high above the atmosphere, which directs the charged particles toward the poles.

However, the high-energy particles still reach the upper atmosphere about 100 km away, where they collide with oxygen and nitrogen atoms, creating lower-energy electrons and photons that cascade in chain reactions in the lower atmosphere. These cascades reach maximum intensity at altitudes of about 20 km, but are constantly absorbed at lower altitudes by the thicker atmosphere.

Commercial aircraft operate at altitudes of up to about 10 km and thus experience a higher dose of this radiation than on the ground. The concern is that this type of radiation can ionize atoms and tear DNA, potentially causing health problems such as cancer. It can also affect electronic instruments.

So any new source of ionizing radiation is a serious concern.

Although radiation has been a well-known problem at high altitudes for decades, no continuous worldwide monitoring has been attempted. Thus, in 2012, various US agencies began developing a global real-time monitoring system to measure levels. The system was called the Program for Automated Radiation Measurements for Aerospace Safety (ARMAS), and Tobiska’s company, Space Environment Technologies, played a key role in it.

Since then, the program has made hundreds of thousands of radiation measurements during 599 flights around the world.

It soon became clear that the background level of radiation was not constant at all. Instead, various surveillance aircraft experienced sudden, rapid increases in radiation that would disappear relatively quickly.

Tobiska and colleagues focused their analysis on 57 such events.

They first compared the radiation bursts with background readings elsewhere on the planet to rule out the possibility that a solar storm or an increase in galactic cosmic rays was to blame. They found no increase in activity elsewhere.

It is clear that this radiation was generated closer to home and only over small areas. Judging by the speed of the plane, these radiation spots cover areas with a diameter of no more than 1000 km.

Catching the culprit

The obvious culprit was Earth’s Van Allen radiation belts, the part of the magnetosphere that traps high-energy charged particles, creating a high-altitude ocean of plasma. Like any other ocean, this plasma is buffeted by environmental conditions space weather in the form of changes in the sun’s magnetic field and through solar storms.

This generates turbulence and creates powerful plasma waves. It is these plasma waves, called electromagnetic ion cyclotron waves, that Tobiska and colleagues believe are the source of the radiation bursts.

This is because charged particles can surf on electromagnetic ion cyclotron waves, speeding up significantly. Any particles that escape do so as powerful bursts of high-energy radiation. Indeed, plasma wave accelerators are an emerging technology for next-generation particle accelerators.

But the bursts are directed only downward, toward the surface of the Earth, where the Earth’s magnetic field begins to point back toward the surface. And this only happens at high latitudes. As it turns out, one of the characteristics of the observed radiation peaks is that they only occur at high latitudes.

The team also found that radiation peaks tend to coincide with periods of turbulent space weather, which promotes conditions where electromagnetic ion cyclotron waves can form. It is also possible to see evidence of these waves from the ground.

“The evidence points to [radiation] beam produced at higher altitudes by infalling relativistic electrons coming from the Van Allen radiation belts and generated by electromagnetic ion cyclotron waves,” say Tobiska et al.

The consequences are significant. For many years, scientists have calculated background radiation levels at high altitudes and worked out safe limits for the amount of time a crew should spend in those conditions. This allows them to work safely.

But this new source of radiation threatens to change those calculations, especially for those flying more northerly routes.

“The net effect on aircrew and frequent flyers on these routes will be an increase in monthly and annual exposures that may have career-limiting health consequences,” say Tobiska and co.

This conclusion is likely to send shockwaves through the aviation industry. The significance of this additional exposure has not yet been calculated, but this work should be given the highest priority. Overall, this should be intended to provide clarity to the many men and women who will be concerned about the effect of this radiation on their health.

Reference: Elevated radiation events detected at commercial aviation altitudes:

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