Over the past three years, cosmic radiation at aviation altitudes has increased by 12%.
In January 2020, students of Earth to Sky Calculus and Spaceweather.com travelled to Abisko, Sweden, to launch a pair of cosmic ray balloons. They were also in Abisko in March 2017, launching three identical balloons. Putting all the data together, 2017 and 2020, they found that radiation had increased by 12% over the past three years.
The graph shows radiation dose rate (uGy/hr) vs. altitude (feet) from ground level to the stratosphere. Radiation appears to be increasing at nearly all altitudes – even in the range 25,000 ft to 40,000 ft where passenger planes fly. Polar flight crews and passengers are therefore absorbing ~12% more cosmic radiation than they did only a few years ago.
The increase in cosmic rays has been linked to the current solar minimum. The 11-year solar cycle is currently at the bottom, with the sun’s magnetic field weakened, allowing extra cosmic rays from deep space to penetrate the solar system. These cosmic rays are hitting Earth’s atmosphere, creating a spray of secondary cosmic rays that shower toward the ground below.
Secondary cosmic rays are measured by the radiation sensors onboard the balloons, detecting X-rays and gamma-rays in the energy range 10 keV to 20 MeV, similar to what one would get from medical X-ray machines and airport security scanners.
The group has been launching radiation sensors almost weekly for five years–mainly from California, the “home base” of Spaceweather.com. Cosmic rays in the stratosphere have been increasing the entire time, a sign of a deepening Solar Minimum.
The new data from Abisko, Sweden, shows that the increase is not limited to the stratosphere. It is also happening at aviation altitudes with a three year increase of ~12% even below 40,000 ft. The group is planning another ballooning trip to Sweden in August 2020 to confirm these results.
How were cosmic rays discovered?
In August 1912, Austrian physicist Victor Hess made a historic balloon flight that opened a new window on matter in the universe. As he ascended to 5300 metres, he measured the rate of ionisation in the atmosphere and found that it increased to some three times that at sea level. He concluded that penetrating radiation was entering the atmosphere from above. He had discovered cosmic rays.
These high energy particles arriving from outer space are mainly (89%) protons – nuclei of hydrogen, the lightest and most common element in the universe – but they also include nuclei of helium (10%) and heavier nuclei (1%), all the way up to uranium. When they arrive on Earth, they collide with the nuclei of atoms in the upper atmosphere, creating more particles, mainly pions. The charged pions can swiftly decay, emitting particles called muons. Unlike pions, these do not interact strongly with matter and can travel through the atmosphere to penetrate below ground. The rate of muons arriving at the surface of the Earth is such that about one per second passes through a volume the size of a person’s head.