Gamma rays from solar flares do not significantly produce radioactive isotopes in Earth’s atmosphere. While solar flares can emit gamma rays, the processes that generate radioactive isotopes in the atmosphere primarily involve cosmic rays—high-energy particles from space—rather than gamma rays from the Sun itself.
To understand why, it helps to look at what solar flares and gamma rays are, and how radioactive isotopes form in the atmosphere. Solar flares are sudden, intense bursts of radiation caused by magnetic energy release on the Sun’s surface. These flares emit a broad spectrum of electromagnetic radiation, including X-rays and gamma rays, which are the highest-energy forms of light. However, the Sun’s gamma-ray emission is generally weak compared to cosmic sources, and the gamma rays from solar flares do not penetrate deeply into Earth’s atmosphere.
Radioactive isotopes in the atmosphere, such as carbon-14 and beryllium-10, are mainly produced by interactions between cosmic rays and atmospheric atoms. Cosmic rays are mostly high-energy protons and atomic nuclei originating from outside the solar system. When these cosmic rays collide with nitrogen and oxygen nuclei in the atmosphere, they cause nuclear reactions that create radioactive isotopes. This process is called spallation, where the incoming particle knocks out several nucleons (protons or neutrons) from the atmospheric nucleus, resulting in a new, often radioactive, isotope.
Gamma rays themselves are photons—packets of electromagnetic energy—and do not have mass or charge. Because of this, gamma rays are less effective at inducing nuclear reactions that produce radioactive isotopes compared to charged particles like protons and neutrons. Gamma rays can cause ionization and excite atoms, but they rarely have enough energy or the right interaction mechanism to knock out nucleons and create new isotopes in significant amounts.
Moreover, Earth’s atmosphere acts as a shield that absorbs most high-energy gamma rays before they reach the lower layers. The atmosphere’s composition and density cause gamma rays to lose energy or be absorbed through processes like Compton scattering and pair production. This attenuation means that even if solar flares emit gamma rays, very few reach the atmospheric depths where isotope production would be efficient.
In contrast, cosmic rays, being charged particles with high energies, penetrate the atmosphere more deeply and directly collide with atomic nuclei, triggering nuclear reactions. These cosmic ray interactions are the dominant source of cosmogenic radioactive isotopes in the atmosphere. Solar flares can increase the flux of energetic particles, including protons, during solar energetic particle (SEP) events, which can enhance isotope production temporarily, but this is due to the particles themselves, not the gamma rays.
Interestingly, the Sun’s gamma-ray emission is mostly a secondary effect. Some gamma rays detected near the Sun come from cosmic rays interacting with heavier elements in the solar atmosphere or the Moon’s surface, which contains heavier elements that can be excited to emit gamma rays. However, the Sun’s photosphere is mostly hydrogen and helium, with very few heavier elements, limiting gamma-ray production from solar material itself.
In summary, while solar flares do produce gamma rays, these gamma rays are not a significant source of radioactive isotopes in Earth’s atmosphere. Instead, the creation of such isotopes is primarily driven by cosmic ray particles colliding with atmospheric nuclei. The gamma rays from solar flares are mostly absorbed or scattered before they can induce nuclear reactions, and their energy and interaction type are not conducive to producing radioactive isotopes in meaningful quantities.
