Cosmic Ray
High-energy particles or clusters of particles — primarily protons and atomic nuclei — that travel through space at nearly the speed of light. They originate from outside the Solar System in the Milky Way, from distant galaxies, and from the Sun. Discovered by Victor Hess in 1912 via balloon experiments; he was awarded the 1936 Nobel Prize in Physics.
Referenced throughout the vault: deflected by the heliosphere and magnetosphere, drive chemistry in the interstellar-medium, produced in bulk by supernovae, and pose radiation hazards to spacecraft and astronauts.
Composition
Primary cosmic rays (arriving at Earth's atmosphere) are ~99% bare atomic nuclei stripped of their electrons:
- ~90% protons (hydrogen nuclei)
- ~9% alpha particles (helium nuclei)
- ~1% HZE ions (heavier nuclei — lithium through iron and beyond)
- <1% solitary electrons
- Trace antimatter: positrons and antiprotons (active area of research)
The He/H mass ratio (~28%) is close to the primordial Big Bang nucleosynthesis ratio (~24%), suggesting primary cosmic rays sample ancient galactic material. Lighter elements — lithium, beryllium, boron — are *over-represented* compared to stellar abundances, because they are produced by cosmic ray spallation: carbon and oxygen nuclei striking interstellar matter and fragmenting.
Types
Galactic and extragalactic cosmic rays — originate outside the Solar System; the dominant meaning of the term.
Solar energetic particles (SEPs) — high-energy protons emitted by the Sun during solar eruptions. Arrive minutes to hours after an eruption. Part of space-weather hazards.
Sources
Sources of galactic cosmic rays are not fully resolved, but the leading confirmed and candidate sources are:
Supernovae (confirmed, 2013) — Fermi Space Telescope data showing neutral pion decay confirmed supernovae as cosmic ray sources. Each supernova explosion produces ~3×10⁴² – 3×10⁴³ J of cosmic rays. The acceleration mechanism is shock front acceleration: stripped atoms are bounced between shock fronts and gain energy with each crossing.
Active galactic nuclei (AGN) — 2018 observations of neutrinos and gamma rays from blazar TXS 0506+056 confirmed AGN produce cosmic rays. Also: quasars, gamma-ray bursts.
Ultra-high energy origin — Above 10²⁰ eV, the 2017 Pierre Auger Collaboration detected a weak anisotropy pointing *away* from the Galactic Center, suggesting extragalactic origin for the highest-energy particles. There is likely a transition from galactic to extragalactic sources at some intermediate energy.
Energy Range
Cosmic rays span an enormous energy range: from ~10⁶ eV to >10²⁰ eV. Peak flux occurs at ~10⁹ eV. The GZK limit (~10²⁰ eV) is a theoretical maximum for cosmic rays from distant sources — above this energy, particles interact with CMB photons and lose energy, limiting their range to ~160 million light years.
Atmospheric Effects — Air Showers
When a primary cosmic ray strikes Earth's atmosphere it collides with oxygen and nitrogen atoms, producing a cascade of secondary particles — an air shower — including:
- X-rays, protons, alpha particles, pions, kaons
- Muons — from pion decay; do not interact strongly with matter; penetrate to ground level and even into shallow mines. Approximately one muon per second passes through a volume the size of a person's head.
- Electrons, positrons, photons (electromagnetic cascades from neutral pion decay)
- Neutrinos (traverse Earth without interaction)
The bulk of incoming cosmic rays are deflected harmlessly by the heliosphere and magnetosphere before reaching the atmosphere.
Hazards
Outside Earth's protective shields, cosmic rays are a significant radiation hazard:
- Satellites: HZE ions damage electronics even at low flux due to their high charge and mass
- Astronauts: dose accumulates during long missions; critical concern for Moon and Mars transit outside the magnetosphere
- Van Allen belts: the inner belt protons originate partly from cosmic ray collisions with the upper atmosphere producing neutrons, which then beta-decay
Connection to Dark Matter
Some positrons in the cosmic ray spectrum arrive with no preferred direction and at energies suggesting production in annihilation events of massive particles — a potential signature of dark-matter annihilation, though not yet confirmed.