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Our analysis of a VLBA 12-hour synthesis observation of the OH masers in awell-known star-forming region W49N has yielded valuable data that enables usto probe distributions of magnetic fields in both the maser columns and theintervening interstellar medium (ISM). The data consisting of detailed highangular-resolution images (with beam-width ~20 milli-arc-seconds) of severaldozen OH maser sources or "spots", at 1612, 1665 and 1667 MHz, revealanisotropic scatter broadening, with typical sizes of a few tens ofmilli-arc-seconds and axial ratios between 1.5 to 3. Such anisotropies havebeen reported earlier by Desai, Gwinn & Diamond (1994) and interpreted asinduced by the local magnetic field parallel to the Galactic plane. However, wefind a) the apparent angular sizes on the average a factor of ~2.5 less thanthose reported by Desai et al. (1994), indicating significantly less scatteringthan inferred earlier, and b) a significant deviation in the averageorientation of the scatter-broadened images (by ~10 degrees) from that impliedby the magnetic field in the Galactic plane. More intriguingly, for a fewZeeman pairs in our set, significant differences (up to 6 sigma) are apparentin the scatter broadened images for the two hands of circular polarization,even when apparent velocity separation is less than 0.1 km/s. This may possiblybe the first example of a Faraday rotation contribution to the diffractiveeffects in the ISM. Using the Zeeman pairs, we also study the distribution ofmagnetic field in the W49N complex, finding no significant trend in the spatialstructure function. In this paper, we present the details of our observationsand analysis leading to these findings, discuss implications of our results forthe intervening anisotropic magneto-ionic medium, and suggest the possibleimplications for the structure of magnetic fields within this star-formingregion.

OH Maser Sources in W49N: Probing Magnetic Field and Differential Anisotropic Scattering with Zeeman pairs using the VLBA