Cosmic Rays
R. A. Mewaldt
California Institute of Technology
Cosmic Ray
Cosmic rays are high energy charged particles, originating in outer space, that travel at nearly the speed of light and strike the Earth from all directions. Most cosmic rays are the nuclei of atoms, ranging from the lightest to the heaviest elements in the periodic table. Cosmic rays also include high energy electrons, positrons, and other subatomic particles. The term “cosmic rays” usually refers to galactic cosmic rays, which originate in sources outside the solar system, distributed throughout our Milky Way galaxy. However, this term has also come to include other classes of energetic particles in space, including nuclei and electrons accelerated in association with energetic events on the Sun (called solar energetic particles), and particles accelerated in interplanetary space.
Discovery and Early Research: Cosmic rays were discovered in 1912 by Victor Hess, when he found that an electroscope discharged more rapidly as he ascended in a balloon. He attributed this to a source of radiation entering the atmosphere from above, and in 1936 was awarded the Nobel prize for his discovery. For some time it was believed that the radiation was electromagnetic in nature (hence the name cosmic “rays”), and some textbooks still incorrectly include cosmic rays as part of the electromagnetic spectrum. However, during the 1930′s it was found that cosmic rays must be electrically charged because they are affected by the Earth’s magnetic field.
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