An overview is presented of basic results and recent developments in the field of cosmic ray induced ionisation in the atmosphere, including a general introduction to the mechanism of cosmic ray induced ion production. We summarize the results of direct and indirect measurements of the atmospheric ionisation with special emphasis to long-term variations. Models describing the ion production in the atmosphere are also overviewed together with detailed results of the full Monte-Carlo simulation of a cosmic ray induced atmospheric cascade. Finally, conclusions are drawn on the present state and further perspectives of measuring and modeling cosmic ray induced ionisation in the terrestrial atmosphere.

For given parameters of the interplanetary plasma near Earth the magnetic field in the magnetosheath can bc described by fluctuations superimposed upon an average static field configuration. Using potential theory, we derived analytical expressions describing the steady state magnetic field in the transition region between the bow shock and the magnetopause. The model contains five parameters, namely, the stand-off distance of the magnetopause, the stand-off distance of the bow shock, and the three components of the interplanetary magnetic field. The shape of the magnetopause and of the bow shock is modeled by a paraboloid with rotational symmetry around the Sun-Earth axis. Compared to other models, our procedure leads to simple analytical formulae which make it most suitable for studies of the effects of the magnetosheath on charged particle propagation.

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The physical processes affecting the dynamics of the Earth's particle radiation environment are reviewed along with scientific and engineering models developed for its description. The emphasis is on models that are either operational engineering models or R. Vainio ( ) R. Vainio et al. models presently under development for this purpose. Three components of the radiation environment, i.e., galactic cosmic rays (GCRs), solar energetic particles (SEPs) and trapped radiation, are considered separately. In the case of SEP models, we make a distinction between statistical flux/fluence models and those aimed at forecasting events. Models of the effects of particle radiation on the atmosphere are also reviewed. Further, we summarize the main features of the models and discuss the main outstanding issues concerning the models and their possible use in operational space weather forecasting. We emphasize the need for continuing the development of physics-based models of the Earth's particle radiation environment, and their validation with observational data, until the models are ready to be used for nowcasting and/or forecasting the dynamics of the environment.

We have developed a new Monte Carlo code called ATMOCOSMICS based on GEANT 4 that simulates the interaction of cosmic rays with the Earth's atmosphere. The code allows to compute the flux of secondaries (e-, e+, protons, neutrons, gammas, muons, pions, …) at user-defined atmospheric depths and/or altitudes. Profiles of the energy deposited by atmospheric shower particles into the atmosphere vs depth and/or altitude can be calculated. We present examples of simulation results obtained with the code, and compare them with experimental data.