a high-speed particle—either an atomic nucleus or an electron—that travels throughout the Milky Way Galaxy, including the solar system. Some of these particles originate from the Sun, but most come from sources outside the solar system and are known as galactic cosmic rays (GCRs). Cosmic-ray particles that arrive at the top of the Earth's atmosphere are termed primaries; their collisions with atmospheric nuclei give rise to secondaries.

About 85 percent of the GCRs are protons (nuclei of hydrogen atoms), with approximately 12 percent consisting of alpha particles (helium nuclei). The remainder are electrons and nuclei of heavier atoms. Because most cosmic-ray primaries are strongly influenced by the Earth's magnetic field and the interplanetary magnetic field, most of those detected near the Earth have kinetic energies in excess of about 1 GeV (gigaelectron volts, or one billion electron volts). This energy corresponds to speeds greater than about 87 percent the speed of light. The number of particles drops rapidly with increasing energy, but individual particles with energies as high as 10²⁰ eV have been detected.

Because of their deflection by magnetic fields, primary GCRs follow convoluted paths and arrive at the top of the Earth's atmosphere nearly uniformly from all directions. Consequently, identification of cosmic-ray sources cannot be based on direction of arrival but rather must be inferred from their abundances (or charge spectrum). This can be done by comparing cosmic-ray abundances with those deduced spectroscopically for stars and interstellar regions. The relative abundances of different elements among cosmic-ray nuclei have been well studied for particles with energies from roughly 100 MeV (megaelectron volts, or one million electron volts) to several tens of GeV. Isotopic abundances have been measured for the more abundant elements as well. From such data it has been possible to reconstruct much of the history of cosmic-ray particles' journey through the Milky Way Galaxy. The light elements lithium, beryllium, and boron are rare throughout the universe but are surprisingly abundant among the primary GCRs. It is accepted that these light nuclei are produced when heavier primaries (e.g., carbon and oxygen) are fragmented during collisions with the thin interstellar gas composed mostly of hydrogen. The GCRs would have to have been traveling for about 10 million years to produce enough interstellar collisions to yield the observed number of light nuclei. The time scale for this travel is based in part on the observation of such radioactive fragments as beryllium-10. This radionuclide has a half-life of 1.6 million years, and the number of such particles that can survive to be detected on Earth depends on their total travel time.

After correcting for interstellar fragmentation, one finds that the composition of the inferred source is similar in some ways to general solar-system matter; however, too little hydrogen and helium are present, and significant differences exist among certain isotopes. It is thought that the cosmic rays represent a mixture of interstellar material enriched with matter from evolved stars, such as supernovas and perhaps Wolf-Rayet stars.

In collisions between primary cosmic rays and interstellar hydrogen, charged mesons (mostly pions) are produced. These pions have half-lives of about two hundred-millionths of a second and decay through muons to produce electrons and neutrinos. The electrons travel along spiral paths in the galactic magnetic field and so generate synchrotron radiation (q.v.), which is detected by radio telescopes. There is general agreement between the radio observations and calculated intensities. Synchrotron radiation has been detected from supernova remnants such as the Crab Nebula, confirming their identification as potential cosmic-ray sources. Interstellar cosmic-ray collisions also yield neutral pions, which decay quickly to produce high-energy gamma rays (gamma ray). Gamma-ray surveys (conducted by Earth-orbiting satellites) indicate that cosmic rays are strongly concentrated in the disk of the Milky Way Galaxy with a much smaller percentage in the surrounding halo. The measured intensity of the gamma rays is in general agreement with calculated values.

With an average life of 10 million years, GCRs must be replenished at an average power level of about 10⁴¹ ergs per second. Supernova explosions can supply this much power as they occur about every 50 years in the galaxy. Details of the processes involved in cosmic-ray production and acceleration remain unclear, but it appears that particle acceleration can be accomplished by expanding shock waves from supernovas.

A small anisotropy has been detected among the highest energy particles—i.e., those with energies above about 10¹⁸ eV. The galactic magnetic field is not strong enough to confine such energetic primary particles within the Milky Way Galaxy, and in fact the origins of these very high-energy cosmic rays have been correlated with the positions of nearby active galaxies that contain black holes millions of times the mass of the Sun. These extremely high-energy particles are so rare that they can be detected only through the extensive air showers (EAS) that they produce in the atmosphere. An extensive air shower may consist of billions of secondaries (mostly electrons and muons) that arrive at ground level over areas of many square kilometres.

Energetic particles emerge from solar flares where they have been accelerated by strong magnetic fields. Most of these particles are protons, with a decreasing number of helium and heavier nuclei. Observations of the helium-oxygen ratio among energetic solar particles have contributed significantly to solar studies, because the Sun's helium abundance is difficult to estimate by means of conventional spectroscopy. The energy spectrum of solar particles, as compared with that of galactic cosmic rays, generally decreases more rapidly with increasing energy, but there is great variability in the shape of the spectrum from one solar-flare event to another, and the energy spectrum rarely extends above about 10 GeV.

Cosmic-ray studies have been carried out from far below the Earth's surface to outer space. Pioneering studies were conducted atop mountains where only secondary particles were detectable. Some secondary muons have such high energies that they are able to penetrate the Earth to depths of more than 3.2 km (2 miles). To study primary cosmic rays directly, high-altitude balloons (typically reaching altitudes of 37 km 【about 120,000 feet】) have been extensively used. Rockets can reach greater heights but with smaller payloads and for only a few minutes. Cosmic-ray observations also have been made from Earth-orbiting satellites and from long-range probes.

From the early 1930s to the 1950s, cosmic rays played a critical role in the scientific study of the atomic nucleus and its components, for they were the only source of high-energy particles. Short-lived subatomic particles were discovered through cosmic-ray collisions. The field of particle physics was in fact established as a result of such discoveries, beginning with those of the positron and the muon. Even with the advent of powerful particle accelerators in the 1950s, investigators in the field have continued to study cosmic rays, albeit on a more limited scale, because they contain particles with energies far beyond those attainable under laboratory conditions.