Properties of the interstellar medium and the propagation of cosmic rays in the Galaxy

The problem of the origin of cosmic rays in the shocks produced by supernova explosions at energies below the so-called ‘knee’ (at ∼3×106 GeV) in the energy spectrum is addressed, with special attention to the propagation of the particles through the inhomogeneous interstellar medium (ISM) and the need to explain recent anisotropy results [Proc. 27th Int. Conf. Cosmic Rays, Hamburg 1 (2001) 10]. It is shown that the fractal character of the matter density and magnetic field distribution leads to the likelihood of a substantial increase of spatial fluctuations in the cosmic ray energy spectra. While the spatial distribution of cosmic rays in the vicinity of their sources (e.g. inside the Galactic disk) does not depend much on the character of propagation and is largely determined by the distribution of their sources, the distribution at large distances from the Galactic disk depends strongly on the character of the propagation. In particular, the fractal character of the ISM leads to what is known as ‘anomalous diffusion’ and such diffusion helps us to understand the formation of the cosmic ray halo. Anomalous diffusion allows an explanation of the recent important result from the Chacaltaya extensive air shower experiment [Proc. 27th Int. Conf. Cosmic Rays, Hamburg 1 (2001) 10], viz. a Galactic plane enhancement of cosmic ray intensity in the outer Galaxy, which is otherwise absent for the case of the so-called ‘normal’ diffusion. All these effects are for just one reason: anomalous diffusion emphasizes the role of local phenomena in the formation of cosmic ray characteristics in our Galaxy and elsewhere.