THE RANDOM MAGNETIC-FIELD IN THE GALAXY

We analyse the rotation measures (RMs) of pulsars to study the random magnetic field in the Galaxy without using any large-scale magnetic field models. Since spatial fluctuations of the interstellar magnetic field have scales extending up to a few kiloparsecs, the random magnetic field cannot be evaluated by the analysis of residuals derived from observed RMs and those predicted by a simple large-scale field model which is fitted so as to minimize the variance. We propose the use of pairs of pulsars which are seen in almost the same directions on the sky. We obtain mean strengths of the magnetic field \B(parallel-to)\ in the regions between paired pulsars from differences in their RMs and dispersion measures (DMs). By picking many pairs, we find a correlation between the mean field \B(parallel-to)\ and the difference in DMs, a distance indicator. The correlation is interpreted by means of a Monte Carlo simulation, in which many cells with a constant size L and with fluctuating magnetic and density fields are laid on lines of sight. We find the amplitude of the random field deltaB to be approximately 4-6 muG. This result is almost independent of the cell size L and of the dispersion of the density fluctuation sigma(ne) which are in the ranges L approximately 10-100 pc and sigma(ne) approximately 0.07-0.2 cm-3, respectively.