On October 3, 2015, a National Science Foundation Gulfstream V research jet recorded a massive increase in atmospheric radiation as it flew over the South Atlantic between Antarctica and the tip of South America. For 11 minutes, the onboard radiation detector watched the readings double, as if the plane had flown through a cloud of radiation.
That wasn’t the only incident. Since 2013, airborne detectors have recorded 57 similar radiation bursts, each lasting between 10 minutes and an hour.
The observations have raised significant questions, not least about the risks to flight crew and the safe operation of avionics. Above all, scientists want to know what causes these radiation bursts.
Today we have an answer thanks to the work of Kent Tobiska of Space Environment Technologies in California and 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 fliers like aircrew and how to protect them from events “analogous to planes flying through clouds of radiation,” Tobiska and co.
Space scientists have long known that the Earth is bombarded from space with a steady flow of high-energy radiation from outside the solar system called galactic cosmic rays, and by more sporadic bursts from the sun called solar energy particles. This radiation generally 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 high above the atmosphere, which directs charged particles toward the poles.
However, high-energy particles still enter the upper atmosphere about 100 km away, where they collide with oxygen and nitrogen atoms, producing lower-energy electrons and photons that cascade into the lower atmosphere in chain reactions. These cascades reach a maximum intensity at altitudes of about 20 km, but are steadily absorbed by the thicker atmosphere at lower altitudes.
Commercial aircraft operate at altitudes of up to around 10 km and are therefore exposed to a higher dose of this radiation than on the ground. The concern is that this type of radiation can ionize atoms and tear apart DNA, potentially causing health problems like cancer. It can also interfere with electronic instruments.
Therefore, any new source of ionizing radiation is a significant problem.
Although high-altitude radiation has been a known problem for decades, there has been no attempt to continuously monitor it on a global scale. Therefore, in 2012, various US agencies began developing a real-time global monitoring system to measure concentrations. The system was called the Automated Radiation Measurements for Aerospace Safety (ARMAS) program, and Tobiska’s company Space Environment Technologies played a key role in it.
Since then, the program has taken hundreds of thousands of radiation readings during 599 flights around the world.
It quickly became clear that the background radiation is by no means constant. Instead, various surveillance aircraft experienced a sudden, rapid increase in radiation that decayed relatively quickly.
Tobiska and Co focus their analysis on 57 events of this type.
First, they compared the flares to background measurements elsewhere on the planet to rule out the possibility that a solar storm or an increase in galactic cosmic rays were to blame. They found no increase in activity elsewhere.
This radiation was clearly generated closer to home and only in small areas. Judging by the speed of the aircraft, these radiation spots cover areas no more than 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 and creates an ocean of high-altitude plasma. Like any other ocean, this plasma is rocked by environmental conditions — space weather in the form of changes in the Sun’s magnetic field and from solar storms.
This creates turbulence and builds up powerful plasma waves. It is these plasma waves, called electromagnetic ion cyclotron waves, that Tobiska and co. believe are the source of the bursts of radiation.
This is because charged particles can surf ion cyclotron electromagnetic waves and are accelerated enormously. Any particles that escape do so as powerful bursts of high-energy radiation. In fact, plasma wave accelerators are an emerging technology for next-generation particle accelerators.
But the bursts only go downward, towards the Earth’s surface, where the Earth’s magnetic field begins to point back toward the surface. And that only happens at high latitudes. As it turns out, one of the characteristics of the observed radiation spikes is that they only occur at high latitudes.
The team also found that the radiation spikes tended to coincide with periods of turbulent space weather that promote conditions in which ion cyclotron electromagnetic waves can form. It is also possible to see evidence of these waves from the ground.
“Evidence suggests so [radiation] Beam produced at higher altitudes by incident relativistic electrons originating from the Van Allen radiation belts generated by ion cyclotron electromagnetic waves,” say Tobiska and co.
The consequences are significant. For many years scientists have calculated background levels of cosmic radiation and devised safe limits for the time that flight crews should spend in these conditions. This allows them to work safely.
But this new source of radiation threatens to upend those calculations, particularly for those flying more northerly routes.
“The net effect for flight crews and frequent flyers on these routes will be an increase in monthly and annual exposures, which can have career-limiting health consequences,” say Tobiska and co.
This conclusion is likely to send shockwaves through the airline industry. The significance of this additional exposure has yet to be calculated, but this work must be given top priority. This was primarily to provide clarity to the many men and women who are concerned about the effects of this radiation on their health.
Ref: Elevated radiation events detected at high altitudes in commercial aviation: https://arxiv.org/abs/2209.05599