Solar Activity, the Heliosphere, Cosmic Rays and Their Impact on the Earth’s Atmosphere

During recent years it became evident that the climate of the Earth is not only determined by terrestrial, in particular anthropogenic influences, but also by external parameters. An open question is still whether the solar radiation or the cosmic rays are the main agents regarding the external climate driving. An answer to this question requires quantitative modelling of all related processes. The present chapter concentrates on the modelling of the cosmic ray transport from the interstellar medium into the Earth’s atmosphere and, thus, on interstellar-terrestrial relations. After a discussion of the significance of the local interstellar spectrum of cosmic rays, first their transport in a dynamical heliosphere is considered. Second, the cosmic ray propagation within the terrestrial magnetosphere is studied in order to determine the so-called cut-off rigidities. And, third, the cosmic ray interaction with the Earth’s atmosphere is described, with an emphasis on the ionisation and the production of cosmogenic nuclides. On the basis of the suite of models being discussed in this overview further studies will be possible that should help to quantify the overall effect of cosmic rays on the Earth environment and, particularly, climate.