After the discovery of radioactivity, a source of ionising radiations, it was found that radiation detection instruments showed the presence of radiations even when not exposed to radioactive sources. This background effect was first attributed to natural radioactive substance in the earth. The shielding of the detection device oven with thick walls of lead could never eliminate the effect. Moreover, by removing the detection device several thousand meters from the ground, the effect increased manifold instead of decreasing. The conclusion was that a radiation of very high penetrating power is falling from outer space. This radiation was therefore, named cosmic rays.
Early investigations were confined to the earth's surface or to low altitudes within earth's atmosphere where the radiations are observed, infact, not the primary particles but the secondary radiations produced by the interaction of the primary particles wit the atoms or molecules of the gases at the top of earth's atmosphere.
These secondry radiations were found to consist of very high energy radiations comprising nearly all known elementary particles and the gamma rays. They are capable of penetrating lead more than a meter thick and are observed up to appreciable depths below water and ground.
It has been found that the primary cosmic radiations reaching the top of the earth's atmosphere consist mostly of protons . A small percentage of heavier nuclei is also present such as those of of helium and carbon,nitrogen and oxygen ( less than 1%).
Most of the primary cosmic rays are believed to originate from the galaxies, shooting out constantly as showers in all directions and filling the space. They are accelerated to high energies by interstellar magnetic fields.
During the solar flares the sun contributes significantly to the low energy cosmic rays arriving the earth. During intense sun-spot activity the cosmic rays flux reaching the earth decreases a bit, which probably, is a magnetic effect.
Reference:
https://en.m.wikipedia.org/wiki/Cosmic_ray