Cosmic Rays are high energy particles that originate primarily outside Earth and travel through space before interacting with Earth's atmosphere, magnetic field or other matter. Despite the traditional name, Cosmic Rays are mostly atomic nuclei rather than electromagnetic rays. Protons constitute the majority of primary Cosmic Rays, with heavier nuclei also present, along with smaller quantities of electrons and other particles. Their energies span an enormous range, extending far beyond the energies of many terrestrial particle accelerators.
Cosmic Rays originate from several astrophysical environments and mechanisms, including the Sun, supernova remnants and other energetic Galactic or extragalactic sources, although the precise origins of the highest energy particles remain an active research subject. When primary Cosmic Rays enter Earth's atmosphere, they interact with atmospheric nuclei and initiate cascades of secondary particles, producing a radiation environment relevant to aviation, spaceflight, atmospheric physics and particle astrophysics.
Primary Cosmic Rays are commonly classified according to their origin and energy. Solar energetic particles can be accelerated during solar flares and coronal mass ejections and generally have lower energies than the most extreme Galactic and extragalactic cosmic particles. Galactic Cosmic Rays are believed to be accelerated by energetic astrophysical environments such as supernova remnants and related shock processes, although their full acceleration and propagation histories are complex.
Ultra high energy Cosmic Rays have energies above approximately 10 to the power of 18 electronvolts and can exceed 10 to the power of 20 electronvolts in rare observed events. At these energies, individual particles possess extraordinarily large kinetic energies relative to ordinary atomic and nuclear scales. Determining their sources is difficult because electrically charged Cosmic Rays are deflected by magnetic fields during propagation, so their arrival directions do not necessarily point directly back to their astrophysical sources.
Earth's atmosphere acts as a substantial natural Radiation Shield against Cosmic Rays. A primary high energy particle entering the atmosphere can collide with an atmospheric nucleus and produce secondary particles such as pions, kaons, muons, electrons, positrons, photons and neutrons. These secondary particles can undergo further interactions, creating an extensive atmospheric shower. Muons are particularly penetrating and can reach Earth's surface in substantial numbers. Neutrons and other secondary particles also contribute to terrestrial radiation exposure.
The composition and intensity of secondary Cosmic Ray radiation vary with altitude, geomagnetic latitude, solar activity and atmospheric conditions. At greater altitude there is less atmosphere above the observer, so the shielding effect is reduced and radiation levels generally increase. This is why aviation crews and astronauts can receive significantly greater Cosmic Ray exposure than people at typical ground level.
Cosmic Rays are important in particle physics because their natural energies can exceed those readily achieved by human made accelerators. Before the development of modern accelerators, Cosmic Rays were a major source of discoveries in particle physics. Positrons, muons, pions and several other particles were first identified through cosmic ray observations or related experiments. Cosmic ray interactions continue to provide information about high energy particle processes, fundamental interactions and astrophysical environments.
Modern observatories use combinations of surface detectors, fluorescence telescopes, radio detection and other techniques to study extensive air showers. Because a single primary cosmic particle may produce a large cascade of secondary particles, researchers can infer properties of the original particle from the spatial and temporal patterns of the shower observed at or above Earth's surface.
Cosmic Rays are strongly influenced by magnetic fields. Galactic magnetic fields can bend the trajectories of charged cosmic particles, causing them to propagate in ways that obscure their original direction. Earth's magnetic field provides additional protection by deflecting many charged particles, although the effectiveness varies with particle rigidity, geomagnetic latitude and energy. Charged particle rigidity depends on momentum and electric charge and determines how strongly a particle responds to magnetic fields.
Low rigidity particles are more readily deflected, while extremely energetic particles can penetrate magnetic defences more effectively. Solar activity also modifies the heliospheric environment and can influence the flux of Galactic Cosmic Rays reaching Earth. The solar magnetic cycle, solar wind and transient solar events therefore affect the radiation environment encountered by spacecraft and high altitude systems.
Cosmic Rays are particularly important for space exploration because spacecraft and astronauts are exposed to radiation fields substantially different from those at Earth's surface. Galactic Cosmic Rays contain highly energetic heavy ions capable of producing dense ionisation tracks in biological tissue and materials. Solar particle events can also produce intense bursts of energetic particles over relatively short periods.
Spacecraft shielding can reduce some components of the radiation environment, but extremely energetic particles can penetrate substantial material and can generate secondary radiation when they interact with shielding. Consequently, spacecraft radiation protection involves material selection, geometry, mission duration, trajectory, solar weather monitoring, operational procedures and biological risk assessment. Shielding is not simply a matter of adding unlimited thickness because additional material can itself generate secondary particles when struck by high energy Cosmic Rays.
Cosmic Rays also affect technology and the atmosphere. High energy particles can produce single event effects in semiconductor electronics, including transient upsets, latchup or permanent damage in some circumstances. Aviation systems and spacecraft electronics can therefore require radiation tolerant designs, error correction, redundancy and appropriate shielding. Cosmic ray interactions in the atmosphere contribute to the production of cosmogenic radionuclides, which can be used in environmental and geological studies.
Secondary cosmic particles also contribute to naturally occurring background radiation at ground level. Atmospheric ionisation produced by cosmic particles can influence chemical and physical processes, although the magnitude and significance of particular atmospheric effects depend strongly on the process and timescale being considered. Cosmic ray monitoring is consequently relevant to both fundamental science and practical engineering.
The study of Cosmic Rays has developed from early observations of unexplained atmospheric ionisation to modern international observatories capable of detecting extraordinarily energetic events. Balloon experiments, mountain laboratories, underground detectors, satellites and large ground based arrays have all contributed to the field. Modern cosmic ray research combines particle physics with astrophysics because the particles carry information about acceleration processes, magnetic fields and environments far outside the Solar System.
Ultra high energy Cosmic Rays are especially challenging because their flux is extremely low, requiring enormous detector areas and long observation periods. Interactions with background radiation during intergalactic propagation also impose physical limits on how far the highest energy particles can travel while retaining their original energies. These effects provide clues about possible source distances and propagation histories, although source identification remains difficult for charged particles.
