NEW CLUES TO THE MAGNETIC-FIELD STRUCTURE OF M-31

We observed 21 polarized background radio sources in the field of M 31 at 1.365 GHz and 1.652 GHz, acid determined their rotation measures ( RMs). The RM data show that the regular magnetic field of the disk pro bably extends from about 5 kpc to 25 kpc from the center with similar structure. The RMs obtained from the polarized emission from M 31 at l ambda 6 cm and lambda 11 cm indicate that M 31 might have a weak poloi dal field in its tenuous halo. Observational features of the odd and e ven dynamo modes in a galaxy are discussed. An even mode (S0) dynamo m ay operate in M 31.