Galactic Cosmic Rays

The overall cosmic ray beam is largely isotropic and the cosmic ray energy spectrum is a rather featureless power law. It is believed that cosmic rays at the lower energies (of the order of 10¹⁶eV and below) originate in sources within our galaxy and have their directions made isotropic in the galactic magnetic fields. However, there are data at these energies which show deviations from isotropy. Some of these are significant and fit an overall pattern which is somewhat clouded by limited observational statistics (Clay and Smith 1996a). One of those deviations shows a form of unidirectional anisotropy. This is an outward flow along our galactic spiral arm at energies of about 10¹⁵eV (Clay et al. 1998). There is also a change in phase of the anisotropy at about the energy of a spectral feature (the knee) in the vicinity of 10¹⁶eV. This may be associated with the beginning of the effect of an intergalactic cosmic ray component which is thought to dominate at the highest energies. It is thought that the extragalactic component completely dominates at energies above 10¹⁹eV (see the spectrum in Takeda et al. 1998) where there is a second spectral feature (the ankle). The energy range between the knee and the ankle is probably associated (at least in part) with a progressive reduction in the ability of our galaxy to contain particles and allow their flux to build. It is noteworthy that any interpretation similar to this model implies that our galaxy is capable of accelerating particles at least up to the vicinity of 10¹⁹eV, a point not often emphasised.

б© Copyright Astronomical Society of Australia 1997