Although the name would suggest that cosmic rays are some form of electromagnetic radiation, they are actually subatomic particles travelling at significant fractions of the speed of light. Primarily atomic nuclei (hydrogen and helium nuclei are the most common, but nuclei of all naturally occurring elements have been detected), whatever accelerates these nuclei to such high energies, does not appear to have the same effect on electrons which make up less than 1% of all cosmic rays.

Cosmic Ray energies span a truly enormous range, from about 10⁷ eV through to 10²⁰ eV, but at higher energies the numbers of cosmic rays drop off dramatically. Roughly speaking, for every 10% increase in energy, the number of cosmic rays per unit area falls by a factor of 1000.

Since cosmic rays are charged particles whose paths are affected by magnetic fields, determining where they originate is a challenge, and for the most part, an unsolved mystery. For all but the highest-energy cosmic rays (which remain largely unaffected by the magnetic fields), astronomers cannot simply trace the path of the cosmic rays back to their origin, but must infer their place of origin from their energies and composition. For the highest-energy cosmic rays, although in theory it should be possible to trace them back to their origin, their rate of detection is so low that there is no discernable stream of particles coming from any one particular direction.

Detecting cosmic rays is, for the most part, a non-trivial exercise. The Earth’s atmosphere is largely opaque to cosmic rays meaning that spaced-based detectors are required. This works well for the abundant low-energy cosmic rays, but due to the necessarily small size of the detector, the chance of detecting a much rarer, high-energy cosmic ray is very remote.

Fortunately, cosmic rays with energies greater than ~10¹⁴ eV can be indirectly detected from the ground. When they enter the Earth’s atmosphere these cosmic rays interact with atoms to produce secondary particles in a cosmic ray shower. It is then possible to determine the energy and direction of the original cosmic ray by studying the shower of particles.

Cosmic rays were discovered in 1912 by Victor Hess, an Austrian physicist who took an ionisation chamber (a device which detects charged particles) up into the Earth’s atmosphere in a balloon. As the balloon rose he found that the numbers of charged particles initially dropped off, easily explained if these particles came from the Earth. Then, surprisingly, the numbers of charged particles began to rise again. He concluded that these charged particles were coming from outside the Earth’s atmosphere and named them cosmic rays. In 1936 he was awarded the Nobel Prize in physics for this work.

These days, based on their energies and composition, astronomers divide cosmic rays into four main types: solar cosmic rays, anomalous cosmic rays, galactic cosmic rays and ultra-high energy cosmic rays. These divisions are our best guess at classifying different types of cosmic rays given the information we have at the moment, and may be confirmed or revised with the next generation of cosmic ray detectors currently in development.