Cosmic rays can loosely be defined as energetic particles originating outside of the Earth. The composition includes electrons, protons, gamma rays, and atomic nuclei from a large region of the periodic table. The kinetic energies of these particles span over fourteen orders of magnitude, with the flux of cosmic rays on the Earth's surface falling approximately as the inverse cube of the energy. The wide variety of particle energies is reflected in the wide variety of sources. Cosmic rays originate from energetic processes on the Sun all the way to the farthest reaches of the visible universe.
Cosmic rays can be conceptually broken into different kinds:
Cosmic rays were initially believed to originate in radioactive isotopes in the ground. This theory was disproven in 1911 by Victor Hess, who in 1936 received the Nobel prize in physics for his work. Hess used electroscope measurements taken at different altitudes from a hot air balloon to conclude that the radiation was cosmic in origin. Hess further showed that the sun could not be the primary source of cosmic rays by taking balloon measurements during a 1912 solar eclipse.
In 1938, Pierre Auger observed near-simultaneous cosmic ray events at widely separated locations. He concluded that they were due to incident particles whose energy was too high to penetrate the atmosphere. Such particles instead collide with nuclei in the atmosphere, initiating a particle cascade known as a cosmic ray air shower. The events Auger had observed were found to have energies of 1015eV, 10 million times higher than had previously been known.
The measurement of high-energy cosmic rays via sampling of extended air showers was first implemented in 1954 at the Harvard College Observatory. From their work, and from the many ground-array experiments that followed it, the cosmic ray spectrum is now known to extend up to at least 1020 eV.
Cosmicrays were first found to be of extraterrestrial origin by Victor F. Hess (c.1912) when he recorded them with electrometers carried to high altitudes in balloons, an achievement for which he won the Nobel Prize in 1936.
Cosmicrays are composed mainly of bare nuclei, roughly 87% protons, 12% alpha particles (heliumnuclei) and most of the rest being made up of heavier atomic nuclei with relative abundances comparable to those found in the Sun.
Then, during the decade from 1927 to 1937 a wide variety of experimental investigations demonstrated that the primary cosmicrays are mostly positive charged particles, and the secondary radiation observed at ground level is composed primarily of a "soft component" of electrons and photons and a "hard component" of penetrating particles, muons.
Cosmicrays also have an extreme energy range of incident particles, which have allowed physicists to study aspects of their field that can not be studied in any other way.
The portion of the cosmicray spectrum that reaches the Earth's atmosphere is controlled by the geomagnetic cutoff which varies from a minimum (theoretically zero) at the magnetic poles to a vertical cosmicray cutoff of about 15 GV (ranging from 13 to 17) in the equatorial regions.
The cosmicrays show an inverse relationship to the sunspot cycle because Sun's magnetic field is stronger during sunspot maximum and shields the Earth from cosmicrays.
Feeds |
Contact