The galactic cosmic radiation (GCR), which originates from stellar development processes, creates a complex radiation field in the Earth atmosphere by interaction processes with atoms of the atmosphere. The GCR consists mainly of protons (85 %), helium (12 %), electrons (2 %) and heavy nuclei (1 %).
The intensity of the GCR depends on the Solar cycle. During a Solar cycle, which lasts 22 years, there are two different phases of Solar activity. The phase during which the sun is not active is called Solar minimum, the other, active phase is called Solar maximum.The number of sunspots can be seen as a measure for Solar activity. In figure 1 you can see the number of sunspots from 1960 to 2010 in red. The blue curve shows count rates from the Kiel neutron monitor, which are an indicator for the intensity of the galactic cosmic radiation
It can be clearly seen that the intensity of the GCR is anti-correlated with the Solar activity, which means that the intensity is highest during Solar minimum. When a galactic cosmic particle enters the Earth atmosphere, it interacts with the atoms of the atmosphere. This leads to a so-called particle cascade which continues down to the ground. One galactic cosmic particle can trigger the creation of millions of secondary particles. The resulting complex radiation field consists of different secondary particles, which can be classified in three main groups. For one, there is the hadronic component, which consists of protons, neutrons, and pions. Electrons and photons belong to the second group, while the third group consists of muons. Therefore, the radiation field is made up of charged and neutral particles. The composition ratio is dependent on the atmospheric altitude and hence the density of the atmosphere.