Cosmic rays are high-energy protons and atomic nuclei which move through space at nearly the speed of light. They originate from the sun, from outside of the solar system, and from distant galaxies. They were discovered by Victor Hess in 1912 in balloon experiments. Direct measurement of cosmic rays, especially at lower energies, has become possible since the launch of the first satellites in the late 1950s.
Of primary cosmic rays, which originate outside of Earth’s atmosphere, about 99% are the nuclei of well-known atoms (stripped of their electron shells), and about 1% are solitary electrons (similar to beta particles). Of the nuclei, about 90% are simple protons (i.e., hydrogen nuclei); 9% are alpha particles, identical to helium nuclei; and 1% are the nuclei of heavier elements, called HZE ions. These fractions vary highly over the energy range of cosmic rays. A very small fraction are stable particles of antimatter, such as positrons or antiprotons. The precise nature of this remaining fraction is an area of active research. An active search from Earth orbit for anti-alpha particles has failed to detect them.
Particle detectors similar to those used in nuclear and high-energy physics are used on satellites and space probes for research into cosmic rays. Upon impact with the Earth’s atmosphere, cosmic rays can produce showers of secondary particles that sometimes reach the surface.
Data from the Fermi Space Telescope (2013) have been interpreted as evidence that a significant fraction of primary cosmic rays originate from the supernova explosions of stars. Active galactic nuclei also appear to produce cosmic rays, based on observations of neutrinos and gamma rays from blazar TXS 0506+056 in 2018.
Cosmic rays can be divided into two types:
- galactic cosmic rays (GCR) and extragalactic cosmic rays, i.e., high-energy particles originating outside the solar system, and
- solar energetic particles, high-energy particles (predominantly protons) emitted by the sun, primarily in solar eruptions.
Cosmic rays originate as primary cosmic rays, which are those originally produced in various astrophysical processes.
Primary cosmic rays are composed mainly of protons and alpha particles (99%), with a small amount of heavier nuclei (≈1%) and an extremely minute proportion of positrons and antiprotons. Primary cosmic rays mostly originate from outside the Solar System and sometimes even the Milky Way. The mass ratio of helium to hydrogen nuclei, 28%, is similar to the primordial elemental abundance ratio of these elements, 24%. The remaining fraction is made up of the other heavier nuclei, primarily lithium, beryllium, and boron.
Secondary cosmic rays, caused by a decay of primary cosmic rays as they impact an atmosphere, include photons, leptons, and hadrons, such as electrons, positrons, muons, and pions. The latter three of these were first detected in cosmic rays. When cosmic rays enter the Earth’s atmosphere they collide with atoms and molecules, mainly oxygen and nitrogen. The interaction produces a cascade of lighter particles, a so-called air shower secondary radiation that rains down, including x-rays, protons, alpha particles, pions, muons, electrons, neutrinos, and neutrons. All of the secondary particles produced by the collision continue onward on paths within about one degree of the primary particle’s original path.