On October 3, 2015, a Gulfstream V research jet belonging to the National Science Foundation recorded a massive spike in atmospheric radiation while flying over the South Atlantic between Antarctica and the tip of South America. For 11 minutes, its onboard radiation detector observed levels doubling, as if the plane had flown over a cloud of radiation.
This was not the only incident. Since 2013, airborne detectors have recorded 57 similar bursts of radiation, each lasting between ten minutes and an hour.
The observations raised important questions, including the risks to aircrew and the safe operation of on-board electronics. But above all, scientists want to know what causes these bursts of radiation.
Radiation dose
Today we have an answer thanks to the work of Kent Tobiska of Space Environment Technologies in California and his colleagues, who have investigated the incidents and believe they know the source of the radiation.
Their work raises important questions about the safety of frequent flyers such as aircrew and how to protect them from events that “are analogous to airplanes flying through clouds of radiation”, say Tobiska and co. .
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 more sporadic bursts from the Sun. called solar energetic particles. This radiation usually takes the form of high energy electrons, protons and alpha particles.
Earth is shielded from these particles by the Sun’s magnetic field, which slows much of the incoming galactic radiation, and by its own magnetic field above the atmosphere, which funnels charged particles towards the poles.
However, the high-energy particles still make it to 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 around 20 km but are regularly absorbed at lower altitudes by the thicker atmosphere.
Commercial aircraft operate at altitudes up to about 10 km and therefore experience a higher dose of this radiation than on the ground. The problem is that this type of radiation can ionize atoms and tear apart DNA, which could cause health problems such as cancer. It can also interfere with electronic instruments.
Thus, any new source of ionizing radiation is a significant concern.
Although radiation has been a well-known problem at high altitudes for decades, there have been no attempts to continuously monitor it on a global scale. So, in 2012, various US agencies began developing a global real-time monitoring system to measure levels. The system was called the ARMAS (Automated Radiation Measurements for Aerospace Safety) program, 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 level of background radiation is not constant at all. Instead, various surveillance aircraft experienced rapid, sudden increases in radiation that would fade relatively quickly.
Tobiska and co focus their analysis on 57 such events.
They first compared the radiation bursts to background readings elsewhere on the planet to rule out the possibility that a solar storm or increased galactic cosmic rays were to blame. They found no increase in activity elsewhere.
Obviously, this radiation was being generated closer to home and only over small areas. Judging by the speed of the aircraft, these radiation spots cover areas not exceeding 1000 km in diameter.
Catch 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 shaken by environmental conditions — space weather in the form of changes in the solar magnetic field and solar storms.
This generates turbulence and creates powerful plasma waves. It is these plasma waves, called electromagnetic ion cyclotron waves, that Tobiska and co believe are the source of the radiation bursts.
This is because charged particles can ride the electromagnetic waves of the ion cyclotron, becoming greatly accelerated. Any escaping particles do so as powerful bursts of high-energy radiation. Indeed, plasma wave accelerators are an emerging technology for next-generation particle colliders.
But the bursts only point downward, toward the Earth’s surface, where Earth’s magnetic field begins to point toward the surface. And this only happens at high latitudes. It turns out that one of the characteristics of the observed radiation peaks is that they only occur at high latitudes.
The team also found that the radiation peaks tend to coincide with periods of turbulent space weather, which favor the conditions in which electromagnetic ion cyclotron waves can form. It is also possible to see evidence of these waves from the ground.
“The evidence shows that [radiation] beam produced at higher altitudes by incident relativistic electrons from Van Allen radiation belts that were generated by an ion cyclotron electromagnetic wave,” state Tobiska and co.
The consequences are significant. For many years, scientists calculated background radiation rates at high altitudes and set safe limits on how long crews would have to spend in those conditions. This allows them to operate safely.
But this new source of radiation threatens to upend those calculations, especially for those taking routes further north.
“The net effect on aircrews and frequent flyers for these routes will be increased monthly and annual exposures, which can have career-limiting health consequences,” state Tobiska and co.
This finding is likely to send shockwaves through the aviation industry. The magnitude of this additional exposure has not yet been calculated, but this work should be given the highest priority. At most, it should be designed to give clarity to the many men and women who will be concerned about the effect of this radiation on their health.
Ref: Increase in radiation events discovered at commercial aviation altitudes: https://arxiv.org/abs/2209.05599