The Earth is surrounded by a spherical magnetic field known as the magnetosphere, which protects it from cosmic radiation. However, at a certain location over the South Atlantic Ocean – off the coast of Brazil – this shielding effect is weaker. Spacecraft passing through this so-called “South Atlantic Anomaly” (SAA) are exposed to higher fluxes of cosmic rays – in particular protons.

The Mir astronauts recorded the number of flashes they observed while wearing helmets containing particle detectors as Mir’s orbit passed through the SAA. Marco Casolino from the University of Tor Vergata in Rome and an international team of colleagues plotted the light-flash rate against the number of particles – mostly protons. They also plotted the flash rate against the number of relativistic nuclei, to completely remove the proton component, inside and outside the SAA.

The researchers found that the light-flash rate was not related to the number of protons detected, which means that that the light flashes cannot be caused by protons alone. Moreover, light flashes inside the SAA are more frequent than outside it: 0.15 flashes per minute were counted inside compared to only 0.06 per minute outside. This implies that there must be another factor contributing to flashes inside the anomaly.

The team proposes two complementary mechanisms to explain these findings. The first is the direct interaction of heavy nuclei with the retina that causes ionization or excitation and the second, proton-induced nuclear interactions in the eye that produce knock-on particles.