Most measurements of Zeeman splitting of thermal OH in cloud cores have been done with single dishes. These measurements average over regions of a source where physical conditions probably vary considerably. As highly clumpy substructure is likely, it is not clear what densities are involved. The E configuration would be the ideal instrument to achieve greatly increased angular resolution, compared with most single dishes. (Compared to Arecibo, the VLA offers greatly increased sky coverage, integration time, side-lobe rejection, and spectral baseline stability.)
The Milky Way is known to possess some sort of large scale magnetic field configuration, based largely on analysis of the rotation and emission measures of pulsars. However, recently Zeeman-effect measurements of interstellar OH masers has suggested an additional source of information about the magnetic field of the Milky Way. Magnetic field directions determined from OH masers show large scale structures, largely consistent with those inferred from pulsars. Since magnetic fields measured in regions of OH masers, provide in situ measurements of the field, the information obtained from OH masers is different from that of pulsars, where an integral (of the electron density weighted) magnetic field along the line of sight is obtained. Thus, OH masers provide information complementary to that obtained from pulsars about the magnetic field of the Milky Way.
Interferometric mapping of large numbers of OH masers is needed to make significant progress in this field of study. These observations offer the opportunity to measure magnetic fields in many regions across the Galaxy, and thus to strongly constrain the structure of the magnetic field of the Galaxy. Currently the VLA suffers from limited angular resolution of the A-configuration. Extending baseline coverage to about 500 km would be a major advance, providing the highest sensitivity and angular resolution that can be applied to a large number of (usually weak) sources, many of which are heavily resolved by VLBI observations as the maser spots are often broadened by interstellar scattering.