Acta Geophysica vol. 57, no. 1, pp. 88-101 DOI: 10.2478/s11600-008-0019-9 ________________________________________________ © 2009 Institute of Geophysics, Polish Academy of Sciences Ionization of the Earth’s Atmosphere by Solar and Galactic Cosmic Rays Ilya G. USOSKIN 1 , Laurent DESORGHER 2 , Peter VELINOV 3 , Marisa STORINI 4 , Erwin O. FLÜCKIGER 2 , Rolf BÜTIKOFER 2 , and Gennady A. KOVALTSOV 5 1 Sodankylä Geophysical Observatory, University of Oulu, Finland e-mail: Ilya.Usoskin@oulu.fi (corresponding author) 2 Physikalisches Institut, University of Bern, Bern, Switzerland e-mails: desorgher@space.unibe.ch, erwin.flueckiger@space.unibe.ch, rolf.buetikofer@space.unibe.ch 3 Solar-Terrestrial Influences Laboratory, Bulgarian Academy of Sciences, Sofia, Bulgaria; e-mail: pvelinov@bas.bg 4 IFSI-Roma/INAF, Roma, Italy; e-mail: marisa.storini@ifsi-roma.inaf.it 5 Ioffe Physical-Technical Institute, St. Petersburg, Russia Abstract A brief review of the research of atmospheric effects of cosmic rays is presented. Numerical models are discussed, that are capable to com- pute the cosmic ray induced ionization at a given location and time. Intercomparison of the models, as well as comparison with fragmentary direct measurements of the atmospheric ionization, validates their appli- cability for the entire atmosphere and the whole range of the solar activ- ity level variations. The effect of sporadic solar energetic particle events is shown to be limited on the global scale, even for the most severe event, but can be very strong locally in polar regions, affecting the physi- cal-chemical properties of the upper atmosphere, especially at high alti- tudes. Thus, a new methodology is presented to study cosmic ray induced ionization of the atmosphere in full detail using realistic numerical mod- els calibrated to direct observations. Key words: cosmic rays, atmosphere, ionization, solar activity, solar- terrestrial relations.