From “How do you go to the Bathroom in Space?” by William R. Pogue, Astronaut

173. Could you tell when you were receiving the radiation?

Normally, you would not be aware of receiving moderate doses of radiation any more than you feel the effects of an X-ray at the dentist or one taken by a doctor to check for broken bones. However, the radiation does cause damage to body tissue, and exposure should be kept as low as possible.

Sometimes the presence of radiation can be detected. When we were sleeping or when our eyes were dark adapted, we could tell when we passed through zones of high levels of radiation. You begin to see light flashes even though your eyes are closed. As you first enter such an area, the light flashes are infrequent and are "seen" as streaks, point flashes, and occasionally as bursts. It's like watching a miniature fireworks display. Then, as you approach the more intense region of this zone, the flashes become more frequent and varied. When passing through the "hottest" part, the effect is almost dazzling, and it looks like your entire field of vision is filled with sparkling points, streaks, and bomb bursts.

The area where this occurred was called the South Atlantic Anomaly [SAA]; it is a depression or low spot in the radiation belts that are created by the Earth's magnetic field. The magnetic field traps and holds certain ionized or charged particles, mostly electrons and protons. The Earth's magnetic poles do not coincide with the Earth's axis of rotation or geographic poles. This mismatch is thought to be the cause of this irregularity or low spot in the belts.

Light flashes were also noted on the Apollo missions to the moon and were caused by the same particles mentioned above. Many of the earlier orbital flights [Mercury and Gemini] were too low to encounter the South Atlantic Anomaly, so they didn't see the flashes. Once the Apollo flights were on their way to the moon, they were beyond the protective shield of the radiation belts around the Earth and were exposed to the direct emission of solar particles. Many astronauts noticed the light flashes, but they were very reluctant to report them. They were probably afraid the doctors would think they had a serious neurological problem, since no one had even suggested such a peculiar effect might occur. It wasn't until after several Apollo astronauts had left NASA that the stories about the light flashes began to filter back to the rest of us. For Skylab we were well briefed on them and were very grateful. If we had noticed them without being told ahead of time, we probably would have kept our mouths shut too.