Cosmic ray

Composition and energy

Primary cosmic rays consist largely of protons (hydrogen nuclei) and helium nuclei (alpha particles), with smaller fractions of heavier nuclei and electrons. A minority of the incoming flux can include high-energy photons or neutrinos produced at astrophysical sources or in interactions. Their energies span many orders of magnitude, from about 10^6 electronvolts (eV) to beyond 10^20 eV, making them the highest-energy particles observed in nature.

Sources and acceleration

Cosmic rays arise from several kinds of sources. The Sun produces lower-energy solar energetic particles during flares and coronal mass ejections. A large galactic component is believed to be accelerated by shock waves in supernova remnants and other energetic environments. The most energetic particles likely originate outside the Milky Way, from active galactic nuclei, gamma-ray bursts or other powerful extragalactic accelerators. Many details of origin and acceleration mechanisms remain under active investigation.

Propagation and modulation

On their way to Earth, charged cosmic rays are deflected by magnetic fields in the galaxy and the heliosphere. This randomizes arrival directions for most energies and produces a composition and energy-dependent modulation by the solar wind and the geomagnetic field. Low-energy particles are strongly affected by these fields, while the highest-energy particles travel in nearly straight lines over cosmological distances.

Interaction with atmosphere and detection

When a primary cosmic ray collides with an atmospheric nucleus it creates an extensive air shower: a cascade of secondary particles such as pions, muons, electrons, neutrons and photons. Ground-based detectors measure these showers directly with arrays of particle counters or indirectly by observing fluorescence or Cherenkov light produced as the shower develops. Spaceborne instruments and high-altitude balloons can detect primary particles above most of the atmosphere and measure their charge and energy.

• Direct detection: instruments on satellites and balloons determine elemental composition and energy spectra.
• Indirect detection: ground arrays and optical telescopes record air showers and infer primary properties.

Scientific importance and effects

Studies of cosmic rays have influenced both astrophysics and particle physics, contributing to the discovery of new particles and to our understanding of high-energy acceleration and interstellar matter. Cosmic rays also affect space weather, contribute to background radiation at aircraft altitudes, and can pose a hazard to satellite electronics and human explorers beyond low Earth orbit.

History and continuing research

Research into cosmic rays began in the early 20th century with measurements of ionizing radiation at altitude that established an extraterrestrial origin. Modern experiments and observatories continue to map the energy spectrum, composition and arrival directions, and to search for the sources of the highest-energy particles. For background on radiation see radiation overview, and for information about cosmic origins see origin beyond the Solar System. The historical confusion with electromagnetic rays is discussed in resources on electromagnetic radiation. Typical particle constituents are summarized in discussions of protons and alpha particles and of helium nuclei.