Introduction

Cosmic rays have a remarkably uniform distribution over the sky. The deviation from isotropy (the anisotropy) is typically below 1% and can be as low as 0.03% (e.g. Clay and Smith 1996, Smith and Clay 1997). Recent compilations of data have shown that measurements of this anisotropy give a coherent picture in the energy range from about eV up to almost eV (Clay et al. 1997a,b). That is, in the lower half of that range the relatively plentiful northern data show a peak of cosmic ray intensity from the spiral arm inward direction. At about eV, this changes dramatically to a more complex picture which nevertheless is consistent in phase over the next decade and a half of energy.

Cosmic rays in the lower energy range have gyro radii of about one parsec or less in typical galactic magnetic fields (a proton with an energy of eV would have a gyro radius of 1pc in a one microgauss field). In order to have a low anisotropy such as is observed, we must assume that their propagation is diffusive in some way (see e.g. Allan 1972). It is likely that this diffusion is broadly along magnetic field lines which are in tubes of dimensions greater than the gyro radii. The direction of the peak of the anisotropy would then indicate the direction back towards the cosmic ray source and the amplitude of the anisotropy would give information on the scattering process involved in the diffusion. In particular, an estimate of the mean free path might be obtained.

The exact nature of the flow of the cosmic rays has been unclear but can be greatly clarified if observations can be made in both the northern and southern hemispheres at the same energies. We will examine the limited southern data at energies in the half decade above eV and confirm that the northern anisotropies do indeed correspond to a general diffusive flow past the solar system. We can then estimate the power requirement for the galaxy to produce its cosmic rays.

© Copyright Astronomical Society of Australia 1997