A new study has found that cosmic radiation at flight altitudes increases when the sun is less active. The research, published in the Journal of Geophysical Research: Atmospheres, was conducted by scientists from Hebrew University of Jerusalem, Reichman University, and Tel Aviv University. Using high-altitude balloon measurements taken over southern Israel between 2014 and 2016, the team analyzed how radiation levels change with altitude and how they are influenced by the sun's activity. The balloons, equipped with radiation sensors, reached up to about 35 kilometers (22 miles) above the Earth, allowing the researchers to study the atmosphere from near ground level to well above typical commercial flight altitudes. The data showed a clear pattern: radiation levels increased as the balloons ascended, peaking at around 17 to 20 kilometers (11 to 12 miles) above Earth. This peak is known as the Regener-Pfotzer maximum, a phenomenon caused by high-energy cosmic rays colliding with atmospheric particles, creating a cascade of secondary particles. At the typical cruising altitude of commercial aircraft—around 10 kilometers (6 miles)—the researchers measured gamma-equivalent radiation dose rates of roughly 0.9 to 1.3 microsieverts per hour. These levels were lower than those observed in higher-latitude regions, likely due to stronger geomagnetic shielding in the Eastern Mediterranean. The study also found that radiation levels vary depending on the solar cycle. During periods of high solar activity, the sun's magnetic field helps shield Earth from galactic cosmic rays. When solar activity is low, more of these energetic particles reach Earth's atmosphere. The researchers observed a negative correlation of r = -0.71 between radiation levels and solar activity, but they caution that this is based on a limited number of observations and should not be considered statistically conclusive. If current trends continue, radiation levels at flight altitudes could rise by 40% to 60% during solar minimum conditions, though the researchers emphasize that this is an extrapolation, not a direct measurement. To validate their findings, the team compared their balloon data with the EXPACS/PARMA atmospheric radiation model. The measured electromagnetic radiation levels closely matched the model's predictions, within about 10% to 15%. However, the model indicated that electromagnetic radiation accounts for only part of the total radiation environment at flight altitudes. Neutrons make up about 40% to 45% of the modeled radiation, while photons, electrons, and positrons contribute about 35% to 40%. The Geiger-Müller detectors on the balloons were mainly sensitive to photons and charged particles, not neutrons. Therefore, the measured radiation levels should not be interpreted as the total dose received by airline passengers or crew, but rather as a reliable indicator of the electromagnetic component of the radiation field. The study has important implications for aviation, where pilots and cabin crews are exposed to higher levels of cosmic radiation than people on the ground. Understanding how radiation varies with altitude, location, and solar conditions is crucial for assessing cumulative occupational exposure. The research also contributes to a broader understanding of how space radiation interacts with Earth's atmosphere. By using relatively inexpensive balloon measurements combined with existing radiation models, the team suggests that similar methods could be used to monitor changes in the atmospheric radiation environment as solar conditions evolve.