Radiation inside airliners climbs by up to 60 per cent when the Sun goes quiet

A new atmospheric physics study out of Israel has put fresh numbers on a long-known aviation puzzle: how much cosmic radiation passengers absorb at cruising altitude shifts with the roughly eleven-year rhythm of the Sun. Researchers who spent years hauling radiation sensors to the edge of the stratosphere on weather balloons found that dose rates at commercial flight levels can climb by roughly 40 to 60 per cent when the Sun is quiet compared with when it is at its most active. The finding does not describe a new hazard; it describes a familiar one becoming measurably larger during a specific phase of a cycle the Sun has repeated for centuries.

The dose itself remains small by any medical standard, and the researchers are explicit that the headline percentage is drawn from extrapolation rather than direct measurement of the most extreme solar-minimum conditions. What the data does settle is the shape of the relationship: less solar activity means less shielding from the Sun’s own magnetic field, and less shielding means more galactic cosmic radiation reaching the altitudes where airliners fly.

Six Balloon Flights Over Southern Israel Mapped the Dose

The measurements behind the new estimate came from six weather-balloon launches carried out from southern Israel between 2014 and 2016. Each balloon carried a radiation sensor package as it climbed to roughly 35 kilometres, well above the altitude of any commercial flight, recording how ionising radiation changed from the ground to the edge of the stratosphere. The project was led by Roy Yaniv, now affiliated with the Hebrew University of Jerusalem and Sheba Medical Center, working with Yoav Yair of Reichman University and Colin Price of Tel Aviv University. The team’s results are published as a paper in the Journal of Geophysical Research: Atmospheres, which combines the balloon data with established atmospheric radiation models to estimate dose rates at any altitude and at any point in the solar cycle, not only the specific days the balloons happened to fly.

The Regener-Pfotzer Peak, and Why Cruising Altitude Matters

As the balloons climbed, radiation did not increase steadily. It rose sharply, peaked at around 17 to 20 kilometres, then declined again. That peak is known as the Regener-Pfotzer maximum, produced when high-energy particles arriving from outside the solar system strike the top of the atmosphere and shatter into cascades of secondary particles. Commercial jets cruise well below that peak, typically between 9 and 12 kilometres, on the rising side of the curve. That is precisely why the dose airline passengers receive is sensitive to anything that changes how many primary particles reach the atmosphere in the first place, including the state of the solar cycle.

Why a Quiet Sun Means More Radiation, Not Less

The relationship runs opposite to intuition. During solar maximum, the Sun’s more turbulent magnetic field and stronger solar wind push outward through the solar system, deflecting a share of the galactic cosmic rays that would otherwise reach Earth. During solar minimum, that shielding weakens, and more of those particles get through. The effect has been documented for decades by ground-based neutron monitors and by dosimeters carried on commercial flights; what the Israeli balloon data adds is a detailed vertical profile specific to the Middle East, a region with less existing high-altitude radiation data than North America or Europe.

From 0.9 to 1.3 Microsieverts an Hour at Cruising Height

At around 10 kilometres, the altitude band where most long-haul jets spend the bulk of a flight, the balloon-derived measurements put the gamma-equivalent dose rate at roughly 0.9 to 1.3 microsieverts an hour, broadly consistent with values reported by other aviation dosimetry programmes. A microsievert is a millionth of a sievert, the standard unit for radiation dose that accounts for biological effect; a single chest X-ray delivers on the order of 100 microsieverts, so even a full hour at cruising altitude is a small fraction of one routine medical scan. Over a long flight that adds up, which is why frequent flyers and flight crew, rather than occasional travellers, are the group whose annual totals get tracked most closely.

The 40-to-60 Per Cent Estimate Comes With a Caveat

The researchers describe the 40-to-60 per cent swing between solar maximum and solar minimum as a modelled extrapolation, built on their balloon measurements and compared against existing dose-rate models, rather than a figure drawn from direct readings taken during the deepest part of a solar minimum. That distinction matters for how the number should be read: it is a well-grounded estimate of how far dose rates can move across the solar cycle, not a measured spike already recorded on a specific flight. The researchers frame the work as an argument for better real-time monitoring of aviation radiation, particularly outside the regions where such monitoring is already dense.

How This Compares With Aircrew Dose Limits

Regulatory bodies including the International Commission on Radiological Protection already treat cosmic radiation as an occupational exposure for flight crew, who are expected to stay within roughly 20 millisieverts a year, the same limit applied to other radiation workers. Published dosimetry studies of commercial flight routes, compiled by groups such as the Health Physics Society, put typical annual crew doses in the low single-digit millisieverts, with long polar or transoceanic routes running higher than short domestic hops. A 40-to-60 per cent increase applied to those baseline numbers still leaves crew exposure well inside existing limits; it shifts the number, not the safety conclusion. For occasional passengers, whose total flying hours are a fraction of a crew member’s, the practical difference between a solar-maximum year and a solar-minimum year is smaller still.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

More from Morning Overview