Origins of Cosmic Rays

Cosmic rays are very high energetic particles of which 99% are made of the nuclei of atoms. Cosmic rays originate outside of the solar system and when reaching the Earth’s atmosphere produce an array of secondary particles that strike the Earth in all directions. The most energetic cosmic rays have been observed to approach a dozen millions of times the energy reached by particles accelerated by the Large Hadron Collider, as a result, there’s been an increased interest in researching cosmic rays. Even though cosmic rays have been known for a little over one hundred years, their exact origin was not known, but recent discoveries point to the remnants of supernovae explosions as the source of cosmic rays.


Cosmic rays have been known for nearly one hundred years. Their history begins at the beginning of the 20th century, more exactly in 1912, when Victor Hess measured the ionization rate at high altitudes in the atmosphere by using Wulf electrometers on a free balloon flight. The results of this experiment led to the conclusion that very high energetic radiation (cosmic rays) was coming from space. In 1937, Pierre Auger, with the use of two detectors, discovered showers of secondary subatomic particles caused by the interactions between high-energy primary cosmic rays and air molecules. For a long time, it was believed cosmic rays to be electromagnetic, but during the 1930s, it was found that they were electrically charged.

During the 1920s the term “cosmic rays” was coined by Robert Millikan who measured ionization levels and concluded that primary cosmic rays reaching the atmosphere were gamma rays and secondary particles were created in the atmosphere by Compton scattering. In 1927, J. Clay discovered a variation of the intensity of cosmic rays with altitude, suggesting that cosmic rays are deflected by the Earth’s magnetic field; therefore, they should be charged particles. Further studies found out that primary cosmic rays consisted principally of protons, while secondary particles were mainly electrons, photons and muons. Many sources have been proposed as the origin of cosmic rays, but it was not until the second decade of the 21st century that the exact origin of cosmic rays was discovered by NASA Fermi Gamma-Ray Space Telescope.

Origin of cosmic rays

It is believed that the majority of galactic cosmic rays are accelerated by the shock waves of supernova remnants. Confined in the magnetic field of the remnant, high energy particles bounce back and forth randomly. In every bounce, they gain about 1% of their original energy. This energy is accumulated until there is sufficient energy to escape at nearly the speed of light into interstellar space. If the cosmic rays come across a dense molecular cloud, some high energy cosmic rays may interact with the gas and produce gamma rays, which is the emission that the NASA Fermi Gamma-Ray Space Telescope detects. Cosmic rays can only be accelerated up to certain energy in supernova remnants; however, higher energy cosmic rays have been detected, suggesting a different origin.

Primary and secondary cosmic rays

Primary cosmic rays originate outside the solar system. They consist of atomic nuclei that have been stripped out from their surrounding electrons. Almost 90% of cosmic rays that interact with the Earth’s atmosphere are protons (hydrogen nuclei); 9% are alpha particles; 1% is electrons and a small fraction is the nuclei of heavier elements. Collisions between primary cosmic rays and air molecules in the upper atmosphere produce showers of lighter particles (secondary cosmic rays), such as pions and kaons, which eventually decay to produce muons and neutrinos. Primary cosmic rays rarely reach the ground; however, they interact with air molecules to create cascades of lighter cosmic particles.

Although the existence of cosmic rays has been known for over one hundred years, their exact origin was unknown. Cosmic rays are known for the damaging effects they confer to astronauts and electronic instruments in outer space. According to the BBC, scientists had suspected that cosmic rays originated at the shock waves of supernova remnants, but this idea was not really clear due that cosmic rays carry a positive charge that is affected by each magnetic field that they encounter, making it almost impossible to trace their exact origin.