Abstract
THE origin and propagation of cosmic rays have been intriguing physicists ever since their discovery in 1912. The main obstacle to identifying the sources of charged particles is the galactic magnetic field. Since the Larmor radii for protons in a typical interstellar field of 3 µgauss are below 30 pc (the typical scale for field irregularities) for E<1017 eV, there should be no direct connection between the arrival directions and the source directions at those energies. Above a few times 1017 eV, however, protons coming from galactic sources should be increasingly anisotropic1. Comparison of the calculated and observed distributions2–4 shows that if the primaries are protons then they should be predominantly extragalactic between a few times 1017 and 1019 eV (ref. 5). More recent data in this energy range6,7 do not change this conclusion. At 1019 eV the Larmor radius of a proton in the Galaxy is about 3 kpc, much larger than the scale height of the magnetic field in most conventional models1,8 and an isotropic distribution above that energy would almost certainly imply a predominantly extragalactic origin if the primaries of the extensive air showers detected are mainly protons. What evidence there is about the mass composition is suggestive of at least some of the very energetic primaries being protons9,10.
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KIRALY, P., OSBORNE, J., WHITE, M. et al. Origin of ultra high energy cosmic rays. Nature 255, 619–620 (1975). https://doi.org/10.1038/255619a0
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DOI: https://doi.org/10.1038/255619a0
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