Discussion

A unidirectional anisotropy appears to be a straightforward way of interpreting data from the AGASA cosmic ray array at energies of a little above 10¹⁸eV. However, whilst the data are compatible with our understanding of the properties of the magnetic fields and spiral arm structure within the galactic plane, the lack of a deficit out of the plane is disconcerting. As has been noted before (e.g. Berezinsky et al. 1991, Lee and Clay 1995), cosmic ray anisotropies are best fitted if there is a strong (several microgauss), turbulent, halo magnetic field which extends several (Lee and Clay (1995) suggest 15) kiloparsecs out of the galactic plane. Those previous suggestions have had the weakness that they were based on unsubstantiated assumptions of a broad distribution of cosmic ray sources which, whilst plausible at lower energies, is probably not the case at 10¹⁸eV. We now have a particular source to deal with, and we still find the same effect. These data seem to argue that there is the need to assume the existence of a strong halo field out to at least several kiloparsecs. The actual extent of such a field beyond those distances is not determined by the cosmic ray data.

We note in this context that we have only discussed one galactic source component. The flux associated with the AGASA source alone is not well defined, due to the analysis procedures, but a comparison with the AGASA energy spectrum suggests that there are significant remaining galactic sources at 10¹⁸eV. The remaining directions show no clear anisotropy and we are left with the problem addressed by Lee and Clay (1995) in a rather stronger form. This is that, despite the likelihood that there are only a few active cosmic ray sources at 10¹⁸eV, the galactic cosmic ray anisotropy is very low. It is not clear how that remaining anisotropy can be explained by galactic plane magnetic fields alone.

© Copyright Astronomical Society of Australia 1997