Astroparticle Physics

1.1 Discoveries in the 20th Century 3lead Mairan, in 1733, to the idea that this phenomenon mightbe of solar origin. Northern and southern lights are causedby solar electrons and protons incident in the polar regionstraveling on helical trajectories along the Earth’s magneticfield lines. At high latitudes, the charged particles essentiallyfollow the magnetic field lines. This allows them topenetrate much deeper into the atmosphere, compared toequatorial latitudes where they have to cross the field linesperpendicularly (Fig. 1.2).It is also worth mentioning that the first correct interpretationof nebulae, as accumulations of stars which formgalaxies, was given by a philosopher (Kant 1775) rather thanby an astronomer.Earth´smagnetic fieldparticle trajectoryFig. 1.2Helical trajectory of an electron inthe Earth’s magnetic field1.1 Discoveries in the 20th Century“Astronomy is perhaps the sciencewhose discoveries owe least to chance,in which human understanding appearsin its whole magnitude, and throughwhich man can best learn how small heis.”Georg Christoph LichtenbergThe discovery of X rays (Röntgen 1895, Nobel Prize 1901),radioactivity (Becquerel 1896, Nobel Prize 1903), and theelectron (Thomson 1897, Nobel Prize 1906) already indicateda particle physics aspect of astronomy. At the turn ofthe century Wilson (1900) and Elster & Geitel (1900) wereconcerned with measuring the remnant conductivity of air.Rutherford realized in 1903 that shielding an electroscopereduced the remnant conductivity (Nobel Prize 1908 for investigationson radioactive elements). It was only natural toassume that the radioactivity of certain ores present in theEarth’s crust, as discovered by Becquerel, was responsiblefor this effect.In 1910, Wulf measured a reduced intensity in an electrometerat the top of the Eiffel tower, apparently confirmingthe terrestrial origin of the ionizing radiation. Measurementsby Hess (1911/1912, Nobel Prize 1936) with balloons at altitudesof up to 5 km showed that, in addition to the terrestrialcomponent, there must also be a source of ionizing radiationwhich becomes stronger with increasing altitude (Figs. 1.3and 1.4).astronomyand particle physicscosmic rays