4 April 2017

Impending weak solar activity could expose aircrews to higher radiation levels

Posted by Lauren Lipuma

Pilots fly over Borneo. Aircrews experience some of the highest levels of radiation of all workers due to exposure to cosmic radiation in the atmosphere.
Credit: https://pixabay.com/en/bario-pilots-borneo-1589516/

By Patricia Waldron

Aircrews and frequent fliers may soon experience an uptick in radiation exposure due to the upcoming low point in the solar cycle, when weak solar activity provides less protection against cosmic rays entering the atmosphere.

In a new study published in Space Weather, a journal of the American Geophysical Union, researchers developed a model based on properties of the sun’s magnetic field to predict its ability to slow galactic cosmic rays entering the solar system. They used the model to estimate an aircrew’s risk of radiation exposure when flying at altitude. Pilots and flight attendants have the greatest workplace exposure to radiation, even higher than nuclear power plant workers, according to the National Council on Radiation Protection and Measurements.

The model showed a roughly 20 percent increase in the average annual radiation dose of aircrews around the next coming solar minimum, expected in 2019 or 2020, compared with the annual radiation dose during the previous solar minimum, according to Shoko Miyake, an astrophysicist at Ibaraki College in Ibaraki, Japan, and lead author of the new study.

A worker’s individual exposure will depend on the time spent at altitude, but if a person spent all year flying in a plane, the average radiation dose would be about 65 milliSieverts (mSv) per year in the 5 years around the solar minimum, compared to about 56 mSv during the last minimum and at least 45 mSv during the solar maximum of the previous cycle, according to the new study.

In the U.S., the Federal Aviation Administration does not regulate radiation exposure for aircrews but recommends limits of 20 mSv each year. The Europe Union restricts yearly exposure to 6 mSv and Japan’s National Institute of Radiological Sciences places the limit at 5 mSv, , though the average for the last solar minimum was 2 mSv. For comparison, a CT scan of the chest exposes a patient to 7 mSv.

“I don’t think the obtained results indicate serious concern for health,” said Ryuho Kataoka, a space physicist at the National Institute of Polar Research in Tachikawa, Japan and co-author of the study, noting that because a person does not spend all year flying, the actual radiation dose will be much lower than the numbers estimated by the study.

“However, all of the aircrews and frequent passengers, who have been working at very close to the standard limit every year may want to pay attention to the coming decrease of the solar activity,” he added.

The weak solar cycle

Solar cycles exist in approximately 11-year periods as the strength of the sun’s output and magnetic field increase and weaken in intensity. The current cycle, number 24, began in 2008, peaked in 2014 and will likely hit the next solar minimum in 2019 or 2020.

As the sun’s activity decreases, so does its ability to shield against cosmic radiation – high-energy atomic particles from outside the solar system formed by the explosion of stars and other far-off phenomena. The solar wind, a sheet of charged particles that flows out from the sun, can slow cosmic rays and reduce their effects on planets in our solar system. The radiation increase during a solar minimum stems from the weakening of the solar wind and the sun’s magnetic field, reducing the sun’s ability to block cosmic rays.

Two galactic cosmic ray protons will have different trajectories due to drift motion, depending on whether the sun’s magnetic field has positive or negative polarity. The orange wave represents the heliospheric current sheet, while the sun is orange and the earth is cyan.
Credit: Shoko Miyake.

The researchers used their model to recreate the sun’s ability to modulate cosmic rays from 1980-2015, and then predicted modulation during the next solar cycle to compare aircrew radiation doses.

The model factored in the weakening of the solar wind and the sun’s magnetic field, along with the drift motion of cosmic rays, or a change in the trajectory of the rays in response to the polarity of the sun’s magnetic field, which flips at the peak of each solar cycle. They also considered the tilt angle of the heliospheric current sheet, a surface spiraling out from the sun’s equator where the polarity of the sun’s magnetic field changes from plus to minus.

The researchers found the change in drift effect is responsible for the expected 20 percent radiation increase during the upcoming solar minimum, even though the predicted maximum radiation dose rate is comparable to the dose rate during the previous solar minimum in 2009.

The researchers plan to develop their model to better simulate galactic cosmic drift and predict the dose rate in upcoming solar cycles.

—Patricia Waldron is a freelance writer.