ON THE DISPERSION IN DIRECTION OF INTERSTELLAR POLARIZATION

Optical polarization maps of 15 dark clouds, five clusters, and six complexes are analyzed to study the spatial patterns and number distributions of the direction of interstellar optical polarization. Most clouds have a well-defined mean direction over their spatial extent, or two or three spatial zones with distinctly different mean directions. Their number distributions of polarization direction generally have a single local maximum, with dispersion 0.2-0.4 radians. Clouds with embedded star clusters have distributions with greater breadth and different mean direction than those of neighboring clouds with less extinction and fewer young stars. Clouds with embedded clusters have median dispersion 0.4-0.5 radians, while clouds without embedded clusters have median dispersion 0.2-0.3 radians. The enhanced dispersion of embedded clusters may be more closely associated with the dense gas than with the young stars in the cluster. If so, the increased dispersion may result from gas accumulation with infall speed slightly greater than the Alfven speed. The observed distribution of polarization angle is modeled as arising from a magnetic field with uniform and nonuniform parts. The nonuniform part has an isotropic probability distribution of direction, a Gaussian distribution of amplitude, and N correlation lengths along the line of sight through the cloud. The model fits observed distributions of polarization angle in terms of the mean polarization angle and the dispersion about the mean. The model estimates the three-dimensional uniform field, and its inclination to the line of sight, by combining maps of polarization direction and maps of the line-of-sight field component, based on the Zeeman effect. In L204, these quantities are 6-mu-G and 47-degrees. The uniform and nonuniform components of magnetic energy density are comparable if the typical cloud has a few correlation lengths along the line of sight. This requirement on N is consistent with the spatial patterns of several optical polarization maps, and with the estimated upper limit N(max) congruent-to 10, based on the cutoff wavelength of hydromagnetic waves.