  Full-polarization MERLIN and VLBA observations of the 1720 MHz maser emission
from the OH molecule in the supernova remnant W28 are presented. Zeeman
splitting (|B| \approx 0.75 mG) has been directly resolved between right- and
left-circularly polarized spectra for the first time. Linear-polarization
position angles and circular-polarization Zeeman splitting observed in the
maser emission permit interpretation of a comprehensive picture of the magnetic
field at the supernova/molecular cloud interface marked by the masers. We find
the post-SNR-shock magnetic field to be well-ordered over the ~1 pc covered by
the maser region (compared with the approximately 30-pc diameter of the entire
SNR) and well-aligned with the shock front that is traced by synchrotron
radiation and molecular emission. Based on MERLIN data having a resolution of
200 mas and VLBA data with a resolution of 15 mas, the masers are measured to
have deconvolved angular sizes of 90 to 350 mas (225 to 875 AU) with compact
cores 20 mas (50 AU) in size, consistent with theoretical expectation and
previous observations.
