Danish Space Research Institute,Juliane Maries Vej 30, 2200 Copenhagen Oe, Denmark.

"Indirect mechanisms of solar-terrestrial relations"
Satellite data have revealed a striking correlation between the intensity of galactic cosmic rays (GCR) and low liquid clouds ( < 3 km). GCR is responsible for nearly all ionisation in the atmosphere below 35 km. However, a direct link between GCR and cloud formation is yet to be unambiguously established and, moreover, the microphysical mechanism is not understood. One mechanism could involve aerosol particles (0.001- 1 mu in diameter) and the formation of cloud condensation nuclei (CCN). A systematic variation in the properties of CCN will affect the cloud droplet distribution and thereby influence the radiative properties of clouds. If the GCR/Cloud link is confirmed, variations in galactic cosmic ray flux, caused by changes in the solar activity could influence Earth's radiation energy budget. The possible link between solar activity and clouds will be discussed. Finally plans to test the link under controlled laboratory conditions in a particle beam at CERN with a cloud chamber, will be presented.