1. Interaction of solar and galactic cosmic rays with Earth’s atmosphere Contributors: Contributors: Rolf Bütikofer 1, Laurent Desorgher 1, Erwin Flückiger 1, Gennady Kovaltsov 5,Marisa Storini 2, Ilya Usoskin 3, Peter Velinov 4 1 University of Bern, Switzerland 2 IFSI-Roma/INAF, Roma, Italy 3 Sodankylä Geophysical Observatory, University of Oulu, Finland 4 Solar-Terrestrial Influences Laboratory, Bulgarian Academy of Sciences, Sofia, Bulgaria 5 Ioffe Physical-Technical Institute, St.Petersburg, Russia

2. CRII models: basic information Bern model (ATMOCOSMIC) Desorgher et al., Internat. J. Modern Phys. A, 20, 6802-6804 (2005). Responsible person: Laurent Desorgher Physikalisches Institut, University of Bern, Switzerland Also: Erwin Flückiger Fan Lei, QinetiQ, ESA/ESTEC Program basis: GEANT 4 application Physics behind: Monte-carlo of the cascade, all included CR particles: protons +  -particles. Energy range: 10 MeV -> Validity: below 100 g/cm 2 (15 km) -10% at 10 g/cm 2 (30 km) – a factor of 2 Oulu CRII model Usoskin and Kovaltsov, J. Geophys. Res., 111, D21206 (2006). Responsible person: Ilya Usoskin Sodankylä Geophysical Observatory University of Oulu, Finland Also: Gennady Kovaltsov Ioffe Phys-Tech. Institute, St. Petersburg, Russia Program basis: CORSIKA + FLUKA Physics behind: Monte-Carlo of the cascade, all included CR particles: protons and  - particles explicitly Energy range: 10 MeV - 5000 GeV/nuc Validity: below 100 g/cm2 (15 km) -10% at 10 g/cm2 (30 km) – a factor of 2 Sofia upper atmosphere model Velinov, and Mateev: C. r. Acad. bulg. Sci., 58, 5, 511-516 (2005). Responsible person: Lachezar Mateev, Marussia Bucharova Peter Velinov Bulgarian Academy of Sciences, Sofia, Bulgaria Program basis: Analytical model, spherical atmosphere Physics behind: Direct ionization (Thin target), no cascade CR particles: protons, , L, M, H, VH Energy range: 1 MeV – 100 GeV Validity: > 16 km (<100 g/cm 2 ) - minimum > 12 km (<180 g/cm 2 ) - maximum

3. Modelling Direct ionization by primaries: Thin target model (analytics) Cascade: Monte-Carlo

4. Cosmic Ray Induced Ionization CRII is defined as an integral product of the ionization yield function Y and the energy spectrum of GCR J. The most effective energy of CRII depends on the atmospheric depth – from ≈1 GeV/nuc in the stratosphere to about 10 GeV/nuc at the sea-level.

5. Comparison with observations Models agree with the observations and with each other within 10% below 15 km (>100 g/cm 2 ). They start underestimating the ionization at higher altitudes and the underestimate becomes a factor of 2 in the upper few g/cm 2 (> 40 km).

6. Upper atmosphere Electron production rate q from different species of CR for the solar minimum conditions (Sofia model).

7. Momentary effect of a severe SEP event CRII in the upper troposphere (300 g/cm 2 ) at 06:55 UT of 20-Jan-2005 (ATMOCOSMIC results)

8. Net effect of a severe SEP event Daily averaged CRII (normalized to the GCR) in a polar region (Oulu results). negative The net effect is negative (suppressed ionization) below 100 g/cm 2.

9. Atmospheric chemistry and SEP events Ozone variability (MLS/AURA data) for several atmospheric levels (after Damiani et al., 2007a).

10. Conclusion An overview is given of the study of cosmic ray terrestrial effects, in the frameworks of COST-724 action. Three models of CRII have been developed: –Bern model is most suitable for precise simulations of CRII in the troposphere-lower stratosphere; –Oulu model is optimized for the study of long-term CRII variability; –Sofia model is specialized in modelling of thin and intermediate target conditions (upper atmosphere and planets with thin atmospheres) An effect of SEP events has been studied in great detail, that is significant in the polar region and in the upper atmosphere.

11. Acknowledgements We are greatful to the COST-724 action for giving us a possibility to work together on this problem. Particularly useful was a STSM (hosts – Desorgher and Flückiger, guests – Usoskin and Velinov) to the University of Bern in Nov. 2005.