New clues to the magnetic field structure of M31
  We observed 21 polarized background radio sources in the field of M31 at 1.365 GHz and 1.652 GHz, and determined their rotation measures (RMs). The RM data show that the regular magnetic field of the disk probably extends from about 5 kpc to 25 kpc from the center with similar structure. The RMs obtained from the polarized emission from M31 at $\lambda$6 cm and $\lambda$11 cm indicate that M31 might have a weak poloidal field in its tenuous halo. Observational features of the odd and even dynamo modes in a galaxy are discussed. An even mode (S0) dynamo may operate in M31. 
J.L. Han(1,2), R. Beck(1), & E.M. Berkhuijsen(1) ((1)   Max-Planck-Institut f\"ur Radioastronomie, Bonn, Germany (2)Beijing ...