Cosmic radiation at flight altitudes rises when the sun is less active

Radiation from space is constantly entering Earth’s atmosphere, but how much of it reaches the altitudes used by commercial aircraft depends not only on height and location, but also on the activity of the sun. A new study based on high-altitude balloon measurements over southern Israel has mapped these changes through the atmosphere and found that radiation levels at aviation altitudes rise as solar activity falls.

The study, published in the Journal of Geophysical Research: Atmospheres, was conducted by Dr. Roy Yaniv of Hebrew University of Jerusalem and Sheba Medical Center; Professor Yoav Yair of Reichman University; and Professor Colin Price of Tel Aviv University.

The researchers analyzed measurements collected during six balloon launches from southern Israel between 2014 and 2016. The balloons, equipped with radiation sensors, climbed as high as approximately 35 kilometers (22 miles), allowing the team to understand how atmospheric ionization and radiation changed from near the ground to well above the altitudes used by passenger aircraft.

Where radiation peaks aloft

The measurements revealed a clear vertical pattern. Radiation increased as the balloons climbed, reaching its highest levels at around 17 to 20 km (11 to 12 miles) above Earth before declining again. This peak, known as the Regener-Pfotzer maximum, is created as high-energy cosmic rays entering the atmosphere collide with atmospheric particles and produce cascades of secondary particles.

At approximately 10 km (6 miles), around the altitude at which commercial aircraft typically cruise, the researchers measured gamma-equivalent radiation dose rates of roughly 0.9 to 1.3 microsieverts per hour. The relatively low levels measured over Israel compared with some higher-latitude regions are consistent with the stronger geomagnetic shielding found in the Eastern Mediterranean.

Solar lulls bring higher exposure

But altitude was only part of the story. The team also found that radiation levels changed during the solar cycle.

When solar activity is high, the sun’s magnetic influence helps shield the solar system from harmful galactic cosmic rays. During periods of lower solar activity, more of these energetic particles can reach Earth’s atmosphere.

Accordingly, the researchers observed higher radiation levels when solar activity was lower. The measured relationship showed a negative correlation of r = -0.71, although the researchers stress that the limited number of observations means that this correlation should be considered indicative rather than statistically definitive.

Extrapolating the relationship to solar-minimum conditions suggested that radiation levels at commercial aviation altitudes could rise by approximately 40%–60% compared with solar-maximum conditions, although the authors caution that this estimate is based on extrapolation rather than direct measurements under those extreme conditions.

“We tend to think of the atmosphere above us as relatively stable, but the radiation environment at flight altitude is continuously shaped by processes taking place far beyond Earth,” the researchers said.

“Our measurements show that the solar cycle has a measurable effect on the radiation environment experienced at aviation altitudes. By combining direct balloon observations with atmospheric radiation models, we can better understand when and where these levels change and improve the tools used to assess long-term exposure for aircrew and frequent flyers.”

What the balloons did not capture

The researchers also compared their balloon measurements with the EXPACS/PARMA atmospheric radiation model. The measured values closely matched the model’s predicted electromagnetic radiation component, generally within about 10% to 15%. The model showed, however, that electromagnetic radiation represents only part of the total radiation environment at flight altitude. Neutrons account for roughly 40% to 45% of the modeled total ambient dose, while photons, electrons and positrons account for approximately 35% to 40%.

That distinction is important. The Geiger-Müller detectors carried by the balloons were primarily sensitive to photons and charged particles and had little direct sensitivity to neutrons. The reported measurements therefore should not be interpreted as the total radiation dose received by an airline passenger or crew member, but rather as a reliable indicator of the electromagnetic component of the atmospheric radiation field.

Tracking risk above the clouds

The findings have particular relevance for aviation, where pilots, cabin crews and frequent flyers spend extended periods at altitudes where atmospheric protection from cosmic radiation is considerably weaker than at ground level. The researchers note that continued monitoring is important for understanding cumulative occupational exposure, particularly as radiation levels vary according to altitude, geographic location and solar conditions.

Beyond aviation, the work provides a clearer picture of how radiation from space interacts with Earth’s atmosphere. By combining relatively inexpensive balloon-borne measurements with established radiation models, the researchers say similar approaches could help track changes in the atmospheric radiation environment as solar conditions evolve.

Who’s behind this story?

Robert Egan

Bachelor’s in mathematical biology, Master’s in creative writing. Well-traveled with unique perspectives on science and language.

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