Astroparticle Physics

10 1 Historical Introductionsolar windenergy source of starscompressedmagnetic fieldnuclear fusionsolar windtail of Earth´smagnetic fieldFig. 1.13Influence of the solar wind on theEarth’s magnetic fieldFermiacceleration mechanismchemical compositionof primary cosmic raysdiscovery of antiparticlesof electromagnetic cascades, which was successfully used todescribe the extensive air showers.In 1938, Bethe together with Weizsäcker, solved thelong-standing mystery of the energy generation in stars. Thefusion of protons leads to the production of helium nuclei,in which the binding energy of 6.6 MeV per nucleon is released,making the stars shine (Nobel Prize 1967).In 1937, Forbush realised that a significant decrease ofthe cosmic-ray intensity correlated with an increased solaractivity. The active Sun appears to create some sort of solarwind which consists of charged particles whose flux generatesa magnetic field in addition to the geomagnetic field.The solar activity thereby modulates the galactic componentof cosmic rays (Fig. 1.13).The observation that the tails of comets always pointaway from the Sun led Biermann to conclude in 1951, thatsome kind of solar wind must exist. This more or less continuousparticle flux was first directly observed by the Mariner2 space probe in 1962. The solar wind consists predominantlyof electrons and protons, with a small admixture of αparticles. The particle intensities at a distance of one astronomicalunit (the distance from Sun to Earth) are 2 × 10 8ions/(cm 2 s). This propagating solar plasma carries part ofthe solar magnetic field with it, thereby preventing some primarycosmic-ray particles to reach the Earth.In 1949 it became clear that primary cosmic rays consistedmainly of protons. Schein, Jesse, and Wollan used balloonexperiments to identify protons as the carriers of cosmicradiation.Fermi (Nobel Prize in 1938 for experiments on radioactivityand the theory of nuclear beta decay) investigated theinteractions of cosmic-ray particles with atmospheric atomicnuclei and with the solar and terrestrial magnetic fields. Byas early as 1949, he also had considered possible mechanismsthat accelerated cosmic-ray particles to very high energies.Meanwhile, it had been discovered that in addition toelectrons, protons, and α particles, the whole spectrum ofheavy nuclei existed in cosmic radiation (Freier, Bradt, Peters,1948). In 1950, ter Haar discussed supernova explosionsas the possible origin of cosmic rays, an idea that waslater confirmed by simulations and measurements.After discovering the positron in 1932, the antiproton,the second known antiparticle, was found in an accelerator