Cosmicray

Cosmic ray refers to high-energy particles that originate in outer space and strike the Earth. Primary cosmic rays are composed mainly of protons, with a substantial fraction of helium nuclei and a smaller number of heavier elements; electrons make up a minor component. Their sources include supernova remnants within the Milky Way and extragalactic objects such as active galactic nuclei and gamma-ray bursts. Solar energetic particles contribute at lower energies during solar flares and coronal mass ejections. The energies span roughly from 1 MeV to beyond 10^20 eV.

When primary cosmic rays enter the atmosphere, they collide with atmospheric nuclei and generate extensive air showers consisting of secondary particles such as pions, muons, electrons, photons, and neutrinos. These cascades are detected by ground-based arrays, air fluorescence telescopes, and space- or balloon-borne instruments. Methods include scintillator and water Cherenkov detectors, as well as fluorescence and radio-detection techniques.

The energy spectrum of cosmic rays follows a power law with notable features. The knee, around 3×10^15 eV, marks a change in the slope, while the ankle near 3×10^18 eV indicates a transition to a possibly different population of sources. At energies above roughly 5×10^19 eV, a suppression is expected due to interactions with the cosmic microwave background, known as the GZK cutoff. The flux of lower-energy cosmic rays is modulated by the solar cycle, leading to variations in the near-Earth intensity.

Cosmic rays contribute to natural background radiation and pose radiation risks for high-altitude aviation and spaceflight. They can induce radiation damage in electronics and cause single-event effects. Research continues to clarify their origins, especially for the highest-energy particles, and to improve measurements of their composition and arrival directions.