3.2. Cosmic rays
The total nonthermal continuum intensity (the signature of Btot2) at 6 cm shows a much weaker variation with viewing angle than the polarized intensity (slopes of 0.3 - 0.5 compared with 2). In case of equipartition between cosmic rays and magnetic fields, Ip also depends on Btot2 (see above) which should flatten the variation of Ip with viewing angle, in contrast to observations. Cosmic-ray energy density is not in equipartition with the total magnetic field, but almost constant along the ring. Urbanik et al. (1994) and Hoernes et al. (1998a) came to a similar conclusion, based on the comparison between the radio and FIR intensities. The regular field dominates in the ring. Cosmic rays can propagate along the (almost) toroidal field and fill the torus smoothly, without being scattered by field irregularities. This allows diffusion speeds larger than the Alfvén speed.
As the regular fields extend to at least 25 kpc radius (Han et al., 1998), the concentration of the radio continuum emission to the ring is a result of the cosmic-ray distribution. Star formation in M31 is mainly occuring in the ring, and the limitation of cosmic rays to the same region is an impressive confirmation that these are accelerated in Pop I objects, e.g. shock fronts of supernova remnants (Duric, this volume). The energy density of cosmic rays drops outside of the ring (ie. at smaller and larger radii), while the field strength is almost radially constant (Han et al., 1998), so that the energy density of the field is larger than that of the cosmic rays outside of the ring: energy equipartition is also invalid outside the ring.