Astroparticle Physics

1.1 Discoveries in the 20th Century 7from sea level to discover where the Earth’s magnetic fieldwould guide them. In these studies, he observed that particleswith certain momenta could be trapped by the magneticfield, which caused them to propagate back and forth fromone magnetic pole to the other in a process called ‘magneticmirroring’. The accumulated particles form radiation belts,which were discovered in 1958 by Van Allen with experimentson board the Explorer I satellite (Fig. 1.9, see alsoFig. 7.2).Van Allen beltsFig. 1.9Van Allen beltsThe final proof that primary cosmic rays consist predominantlyof positively charged particles was establishedby the observation of the east–west effect (Johnson and Alvarez& Compton, Nobel Prize Alvarez 1968, Nobel PrizeCompton 1927). Considering the direction of incidence ofcosmic-ray particles at the north pole, one finds a higher intensityfrom the west compared to the east. The origin of thisasymmetry relates to the fact that some possible trajectoriesof positively charged particles from easterly directions donot reach out into space (dashed tracks in Fig. 1.10). Therefore,the intensity from these directions is reduced.In 1933, Rossi showed in a coincidence experiment thatsecondary cosmic rays at sea level initiate cascades in a leadabsorber of variable thickness (‘Rossi curve’). The absorptionmeasurements in his apparatus also indicated that cosmicrays at sea level consist of a soft and a penetrating component.observation pointFig. 1.10East–west effect