Magnetic Cut-off of Extragalactic Protons in the Galactic Disk

Abstract

ACCORDING to the results of extensive air shower experiments, as inferred from observed electron densities, the integral energy spectrum of cosmic particle radiation , where f is the combined distribution function of energy E, atomic number A and direction Ω, is of the form s^−γ, with γ≅1.6 for E<∼10¹⁶ eV, γ≅1.8 for ∼10¹⁶ eV <E < ∼ 10¹⁸ eV and γ≅l.6 for E>∼10¹⁸ eV. There is still some uncertainty about the exact position of the first knee and about the existence of the second one. If they really exist, they may be consequences of the transition from galactic to extragalactic contributions caused by the presence of a large scale ordered magnetic field in the galactic disk. The chemical composition of cosmic particle radiation, ∫dΩ f(E,A,Ω), which is well known in the low energy range, has also been the object of intensive, though still inconclusive, investigations in the high energy region E> ∼10¹⁴ eV, by the measurement of the muon content¹ or from the core structure of extensive air showers² in connexion with Monte Carlo simulations of cascade development³. The amplitude of possible anisotropic contributions to the angular distribution has been found to be within the limits of statistical error, which are typically of the order of 0.1 per cent for E<∼10¹⁶ eV (refs. 4 and 5), of the order of 1 per cent for ∼10¹⁶ eV <E< ∼10¹⁸ eV (ref. 6) and of the order of 10 per cent for E> ∼10¹⁸ eV. When studying the magnetic cut-off, the combined distribution function f′(ρ,Ω) of magnetic rigidity ρ= E/Ze and direction Ω may be considered instead of f(E,A,Ω), where Z is the charge number corresponding to A.