The Interpretation of Polarization Position Angle Measurements

I review the use of polarization data in the study of interstellar magnetic fields. On large scales in the interstellar medium, the polarization of background starlight is a good tracer of the projected plane-of-the-sky magnetic field structure in the neutral gas, and the field there appears to have roughly equal contributions from uniform and nonuniform components. In cases where a localized region (e.g. a dense, star-forming cloud) is of interest, polarimetry of background starlight may give a misleading impression of the field structure. In these dense regions, polarized thermal emission may be a more reliable tracer of the magnetic field–assuming the grains are aligned by magnetic fields.