Astroparticle Physics

80 6 Primary Cosmic RaysThe all-particle spectrum of charged primary cosmicrays (Fig. 6.4) is relatively steep so that practically no detailsare observable. Only after multiplication of the intensitywith a power of the primary energy, structures in theprimary spectrum become visible (Fig. 6.5). The bulk ofcosmic rays up to at least an energy of 10 15 eV is believed tooriginate from within our galaxy. Above that energy which isassociated with the so-called ‘knee’ the spectrum steepens.Above the so-called ‘ankle’ at energies around 5 × 10 18 eVthe spectrum flattens again. This latter feature is often interpretedas a crossover from a steeper galactic component to aharder component of extragalactic origin.Fig. 6.4Energy spectrum of all particles ofprimary cosmic raysFig. 6.5Energy spectrum of primarycosmic rays scaled by a factor E 3 .The data from the Japaneseair-shower experiment AGASAagree well – except at very highenergies – with the air-scintillationresults of the Utah High Resolutionexperiment as far as the spectralshape is concerned, but theydisagree in absolute intensity(AGASA – Akeno Giant AirShower Array, HiRes – HighResolution Fly’s Eye)10 5Fig. 6.6Artist’s impression of the differentstructures in the primarycosmic-ray spectrum[m s sr GeV ]2.5 –2 –1 –1 1.5N(E) · E10 3 10 12 all particle spectrum10 4 = 2.7knee = 3.010 210ankle toe10 14 10 1610 18 10 20energy per nucleus [eV]