Annu. Rev. Astron. Astrophys. 1990. 28: 37-70
Copyright © 1990 by . All rights reserved

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Interstellar polarization arises from the propagation of radiation through the ISM containing aligned elongated interstellar grains. The galactic magnetic field is responsible for aligning the grains (71), which spin with their long axes perpendicular to the field. Under these conditions, radiation is subjected to extinction, to linear dichroism (differential linear extinction for the two waves polarized along and perpendicular to the direction of alignment), and to linear birefringence (differential phase shift between the two waves). The birefrigence produces circular polarization from the linearly polarized light, but to date the quantitative interpretation of the circular polarization has not been very fruitful because it depends upon the unknown geometry of the change of the direction of grain alignment along the line of sight.

In principle, polarization is a diagnostic which provides another integral of a grain property over the size distribution, similar to extinction and scattering. However, it involves an additional function which is poorly understood: the alignment of grains of various sizes (reviewed in ref. 71). Even so, polarization is important because it provides information regarding the optical properties of grains, and the conditions under which grains can be aligned.