The term cosmic rays sounds like a grand term to describe all radiation from the cosmos. In fact, it has taken on a specific and restricted meaning. It refers, for the most part, to high energy charged particles (mostly protons) that travel through the Galaxy, some of which arrive at the earth. It also refers to the progeny (descendants) of these particles that are produced in nuclear collisions in the earth's atmosphere. Many of these "secondary" cosmic rays are charged particles but neutral particles and photons are also present. One component of the "primary" cosmic rays is well known to radio astronomers. It is observed as the diffuse radio background which is synchrotron emission from high energy cosmic-ray electrons spiraling about magnetic field lines in the Galaxy.
Cosmic rays were so named by Millikan in the 1920s because his experiments confirmed earlier results by Hess, in 1912, that mysterious ionizing radiation originated high in the atmosphere, or beyond. The motivation for the Hess experiment was the excess and puzzling radiation observed with electroscopes, e.g., the leaf electroscope. These are devices, often seen in freshman or high school labs, that measure charge deposited on the leaves; the puzzling radiation causes them to discharge more rapidly than was expected.
Early studies of this radiation were carried out with detectors carried to the tops of mountains, taken even higher in balloons, or sunk deep into lakes. Indeed, the radiation studied in this way turned out to be the first signals detected by man from distant regions of the Galaxy, other than meteorites and visible light. Thus, the name "cosmic rays" is indeed appropriate.
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