How to survive a solar proton flare

Solar proton flares appear to be fairly rare. The last ones to cause severe injury to life on Earth seem to have occurred 12,900 tears ago, producing what some call the Rancholabrean (or Labrean for short, for the La Brea tar pits) extinction, that wiped out most megafauna, such as mastodons, mammoths, and giant ground sloths (megatherium) from North America, Northern Europe, and northern Asia. It was not a complete, worldwide, extinction event, It is discussed in a companion article, Earth Changes .
The focus in this article is how to survive another such event.
The threat is from a solar flare that delivers intense proton radiation that breaks through the Earth’s magnetosphere. Most of the radiation would come almost straight down, and consist not only of protons but of secondary radiation such as neutrons and gamma radiation, both of which can penetrate several feet of shielding, with up to 3-10 sieverts of radiation. 3 are usually fatal. This is not like the radiation that would be produced in a nuclear war, which would include radiation from dust (fallout).
There may be little or no warning, and such warning as might be issued might be only about an hour in advance. Flares can be seen by solar observatories in time to warn astronauts of a proton storm, but it does not appear that the Emergency Warning System is prepared for an event of this kind for the entire nation. If it came in the middle of the night, most people would probably just never wake up.

Most modern homes will not provide enough shelter. You need to get under at least three feet of concrete, stone, or soil. The basement of a three-story office building might work, if the floors are concrete. A concrete bridge or drain pipe might work.

Be on the lookout for shelters.

Try to identify anything that might provide shelter near where you live, work, or travel regularly. You may not find much. If you get warning in time, you may have some time to look, nut it is best to have already spotted some.

Carry a radiation detector.

There are some nice ones available, but the most practical is likely a radiation badge, that needs no power, can be carried around at all times. One I like is the RAD Triage 50. It can be worn every day for two years, and one can keep a backup in a freezer for another ten years. One would use it to determine which locations have the most shelter.