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Cosmic rays are energetic particles originating from outer space that impact the Earth’s atmosphere. About 89% of these particles are simple protons, 10% are alpha particles(helium), and 1% are beta particles(electrons). These particles arrive individually and are not actually rays. Cosmic rays can have energies of over 1020 eV.
Cosmic rays may broadly be divided into two categories, primary and secondary. The cosmic rays that arise in extrasolar astrophysical sources are primary cosmic rays. They can interact with interstellar matter to create secondary cosmic rays. The composition of primary cosmic rays is dependent on which part of the energy spectrum is being observed, but they are made of the abundant end products of nuclear synthesis. Secondary cosmic rays consist primarily of lithium, beryllium, and boron are much less abundant that primary rays.
Cosmic rays are able to travel great distances to Earth because of the low density of matter in space. Nuclei interact strongly with other matter, so when the cosmic rays approach Earth they begin to collide with atmospheric gases. These collisions result in the production of pions, kaons, and mesons that decay into muons. Muons do not interact strongly with the atmosphere, this coupled with the relativistic effect of time dilation in the Earth’s reference frame, allows many of these muons to reach the surface of the Earth and penetrate for some distance.
Cosmic rays impacting other celestial bodies in the Solar System which are made of elements heavier than hydrogen and helium, can be detected indirectly by observing high energy gamma ray emissions coming from these bodies by using a gamma-ray telescope. When such gammas are of energy too high to result from radioactive decay processes they must be secondary to cosmic ray bombardment.
When cosmic ray particles enter the Earth’s atmosphere they collide with oxygen and nitrogen, to produce a cascade of lighter particles(air shower). All of the produced particles stay within about one degree of the primary particle’s path. Particles produced in such collisions are charged mesons. Cosmic rays are also responsible for the production of some unstable isotopes in the Earth’s atmosphere.
Cosmic rays have helped to shape the world we live in and the planet that we live on. Doesn’t it make you curious how they affect other planets in our Solar System.
We’ve also recorded an entire episode of Astronomy Cast all about Cosmic Rays. Listen here, Episode 72: Cosmic Rays.