The students of Earth to Sky Calculus recently flew across the U.S. to conduct a transcontinental launch of space weather balloons. They took radiation sensors on board the plane to find out how many cosmic rays they would absorb during the flight. As expected, radiation levels in the cabin of the Airbus 319 (Spirit Airlines FL640) tripled within ten minutes after takeoff, and were nearly 30 times ground level by the time the plane reached cruising altitude at 39,000 feet. Summing over the entire 4-hour flight, the sensors measured about 1 mrem of radiation - similar to that of a dental x-ray.
There was no solar storm in progress. Radiation comes from cosmic rays. This radiation is ever-present and comes from supernovas, black holes, and other sources across the galaxy. Different-colored lines in the data plot represent different locations inside the cabin. On this particular flight, dose rates were highest in First Class and lowest near the toilets in the rear. The gradient is not understood; presumably, it has something to do with the way cosmic rays interact with the fuselage and fuel tanks. The altitude of the plane matters as well. When the cruising altitude increased about two hours into the flight, dose rates increased accordingly. The closer you get to space, the stronger cosmic radiation becomes. These variables make it difficult to predict how much radiation you will absorb during any given flight.
The radiation sensors are the same ones that Earth to Sky Calculus routinely flies on-board helium balloons to measure cosmic rays in the stratosphere. They detect X-rays and gamma-rays in the energy range 10 keV to 20 MeV. These energies span the range of medical X-ray machines and airport security scanners.
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