With extremely small wavelengths (as low as 10-12cm), Cosmic rays are the highest energy radiations in the electromagnetic spectrum, and contain highly energetic atomic and nuclear particles. They come from all directions in space, the origin of many of these still being unknown. They were discovered because of the ionozation produced by them in our atmosphere.
Cosmic rays can cause changes in the earth's weather, like cloud formation in the upper atmosphere, on colliding with particles in the atmosphere and disintegrating them into smaller pions, muons, etc. Such a shower of cosmic rays is called a "cosmic ray shower".
Atomic nuclei in cosmic rays traveling at nearly the speed of light, while smashing into nuclei of atmosphere atoms, high above the earth, lead to the conversion of a significant fraction of the incoming energy into matter in the form of subatomic particles, which in turn collide violently with other atoms in the atmosphere to create what is known as an "air shower".
Particles in the initial stages of the cascade of collisions, which are traveling at very fast speeds can exceed the speed of light in the tenuous upper atmosphere, which is a little denser than pure vacuum, and can then emit an optical analogue of the sonic boom, Cerenkov radiation. As the particles created in these collisions strike atmospheric nuclei, more particles and high-energy radiations are emitted, with photons being emitted in all directions.
Interaction of Cosmic-ray debris remaining after an air shower with the atmosphere gives rise to ultraviolet light, which can be detected by sensitive photomultipliers.
In the past, cosmic rays were often referred to as "Galactic cosmic rays", but now scientists have discovered a significant amount of cosmic rays that originate in the Sun's chromosphere, known as "Solar cosmic rays". The former, Galactic cosmic rays (GCRs), are atomic nuclei from which all the surrounding electrons have been removed and come from outside the solar system after being accelarated in the last few million years to nearly the speed of light.
Another type of cosmic rays, "Anomalous cosmic rays" are those that consist of accelarated ions picked up by the Sun's magnetic field from interstellar winds, and photo-ionised by its UV radiation.
Cosmic rays are, however not very intense when they strike the atmosphere (with energy equivalent to that of starlight of a distant star) due to their overall small density. But, whereas starlight moves in straight lines, and is gone forever if it enters a void in space, cosmic particles may be trapped and then released by weak magnetic fields, with a period of the order of 107 years. Hence cosmic ray sources only have a fraction of the energy output of stars, which is still quite high to destroy any life on Earth if produced by a nearby source, such as the Sun.
Observing cosmic rays requires sensitive photographic plates lifted to the top of the atmosphere, since only the few particles with very high energies reach the Earth's surface. This method has revealed that cosmic rays mostly consist of particles of Hydrogen, Helium and Carbon dioxide, and few other atoms of heavier elements, which might have undergone some extraordinary process to gain huge energies.
Our atmosphere shields us from cosmic rays about as effectively as a 13-foot concrete layer. Some energetic cosmic fragments have even been registered underground. But, with such high energies, equivalent to those of huge celestial bodies and such small size, they will always be of interest to the scientists.