cosmic rays

rays of extremely high penetrating power originating beyond the earth's atmosphere

Cosmic rays are energetic particles originating from space that impinge on Earth's atmosphere. Almost 90% of all the incoming cosmic ray particles are protons, about 9 re helium nuclei (alpha particles) and about 1 re electrons. The term "ray" is a misnomer, as cosmic particles arrive individually, not in the form of a ray or beam of particles.
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Cosmic rays ionizing irradiation from outer space bombarding the earth and its atmosphere. They contribute to the background radiation that is [more]Cosmic rays - Community and Resources

Notionally, the cause of bit rot. However, this is a semi-independent usage that may be invoked as a humorous way to handwave away any minor randomness that doesn't seem worth the bother of investigating. "Hey, Eric - I just got a burst of garbage on my tube, where did that come from?" "Cosmic rays, I guess." Compare sunspots, phase of the moon. The British seem to prefer the usage "cosmic showers"; "alpha particles" is also heard, because stray alpha particles passing through a memory chip can cause single bit errors (this becomes increasingly more likely as memory sizes and densities increase).
Factual note: Alpha particles cause bit rot, cosmic rays do not (except occasionally in spaceborne computers). Intel could not explain random bit drops in their early chips, and one hypothesis was cosmic rays. So they created the World's Largest Lead Safe, using 25 tons of the stuff, and used two identical boards for testing. One was placed in the safe, one outside. The hypothesis was that if cosmic rays were causing the bit drops, they should see a statistically significant difference between the error rates on the two boards. They did not observe such a difference. Further investigation demonstrated conclusively that the bit drops were due to alpha particle emissions from thorium (and to a much lesser degree uranium) in the encapsulation material. Since it is impossible to eliminate these radioactives (they are uniformly distributed through the earth's crust, with the statistically insignificant exception of uranium lodes) it became obvious that one has to design memories to withstand these hits.
[Jargon File]

In space, away from the earth, there is a very small flux of electrons and high speed nuclei of light atoms moving in all directions. Their origin and how they are accelerated is not always certain but they are probably formed in our galaxy in supernova explosions, or in collisions or outbursts of stars. Those nuclei which enter the region of the earth's atmosphere are found to have energies which are anything up to millions of GeV. Their spectrum is such that the higher the energy the fewer there are (see graph). Those with energies less than a certain value are deflected away again into space by the earth's magnetic field. These primary cosmic rays, which impinge on the earth's atmosphere, are chiefly protons (approx 90%), helium 4 nuclei (approx. 9%) and the nuclei of atoms up to zirconium (Z=40). The predominant flux is from the solar wind of particles boiled off the sun. There are no high energy electrons in primary cosmic rays probably because in their long journey through space they have lost most of their energy in collisions with interstellar matter (whose density could be of the order of 1 particle/cm3). Some low energy electrons and protons get trapped by the earth's magnetic field into Van Allan radiation belts around the world and eventually leak to earth, giving rise to the spectacular aurora . Some primary cosmic ray particles in coming down through the atmosphere dissipate their energy entirely by ionisation, while others terminate in collision with atomic nuclei and induce a large variety of nuclear reactions. In such cases a primary proton or alpha particle has sufficient energy to knock many nucleons out of a nucleus (a process termed spallation) and also materialise its kinetic energy into entirely new short lived particles such as mesons . Particles in this jet induce further nuclear reactions or decay individually into lighter particles. A large number of pi-zero mesons are produced that decay into 2 gamma photons and result in the large background gamma ray flux experienced by ourselves. In this way a shower of secondary cosmic rays of many thousands is spread over square kilometres penetrating metres into the earth.