Cosmic Protons

Unlike electrons, proton interactions with interstellar medium produce a negligible energy loss. The most important physical processes which cosmic protons undergo (beside the interaction with the magnetic field described above) are ionization of the interstellar medium and inelastic scattering with atomic nuclei of the ISM. The latter produce gamma radiation (see par 1.5.1), which is the only observable radiation emission produced by cosmic proton. Observation of galactic gamma diffuse emission are therefore fundamental in order to obtain the proton galactic distribution. In particular, gamma observations of our Galaxy and of the Magellanic Clouds performed by EGRET demonstrate the galactic origin of the cosmic rays, at least for energies up to few hundreds of TeV [Sreekumar, 1993]. The proton spectrum is believed to be less dependent on the position where it has been measured, unlike that of electrons, and therefore local measurements of cosmic rays can be considered more similar to the average galactic spectrum. The measured cosmic proton flux shows a spectrum with a constant index of 2.75 over six orders of magnitude and with an energy ranging between a few GeV (below this energy the effect of the solar magnetic field on the observed fluxes is dominant) and $10^{15}$ eV. At this energy a steeping of the spectrum is observed (known as ``cosmic-ray knee'') and the spectral index is about 3.3 This behavior is maintained up to energy of about $10^{17}$ eV above which the spectral index is again about 2.7 (``cosmic-ray ankle''). The latter value of the spectral index remains constant up to the highest energy measured up to now (3 $10^{20}$ eV) [Biermann & Sigl, 2001]. The ``cosmic-ray knee'' at $10^{15}$ eV is typically understood in terms of disappearing of the galactic cosmic ray component and appearing of an extragalactic contribution. For energy larger than $10^{17}$ eV, protons interact with infrared photons of the cosmic background radiation. This process, known as GKZ effect, is supposed to cause the ``ankle'' in the spectrum and the disappearing of the protons for energy larger than $10^{20}$ eV .

Figure 1.8: Spectrum of the local proton flux.