Mainly atomic nuclei.z ~2% electrons and positronsz Origin: Solar flares Outside solar system (low-energy particlesdeflected by the geomagnetic field)z Elements generated by: Primordial and stellar nucleosynthesis Spallation in ISMObserved properties of cosmic rays 3z 79% of nuclei are protons; 14% He.z Above 10 GeV, 10-4 antiprotons/protonz Slightly less p, He than in Solar System(acceleration process?)z Li, Be, B and Sc, Ti, V, Cr, Mn overabundant, dueto spallation of C, O and Fe, respectively. Thisallows us to estimate the typical path length forcosmic rays (50 kgm-2), and hence to infer that theyare confined in the Galaxy