Polarization of background starlight and the structure of the interstellar magnetic field

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We discuss the use of polarization maps of background starlight in studying the structure of the interstellar magnetic field. We make the assumption that the polarization observed is due to magnetically aligned dust grains associated with interstellar clouds along the line of sight, and that the position angle (ΘE) of polarization observed gives the direction (modulo 180°) of , the plane-of-the-sky projection of the (line-of-sight-averaged) magnetic field.There are two basic points in this paper. (1.) Out of context, the projected orientation of an elongated dark cloud may appear special (e.g. roughly "parallel" or “perpendicular”) in relation to the local (plane-of-the-sky) field direction given by polarization observations, but, when the view is expanded to include an entire complex of dark clouds, the shape and orientation of clouds within a complex often appears unrelated to the field structure. (2.) the dispersion in the postion angle of polarization observed in a region of the sky contains information about the ratio of the strength of the uniform (straight) component of the local magnetic field as compared to the dispersion (nonuniform component) in the field. Furthermore, in a region where Zeeman measurements covering the same region as polarization observations have been made, the uniform-to-nonuniform ratio deduced for the field from polarization data, can be combined with information about the line-of-sight field and an estimate of the correlation length of the field, to describe the magnetic field in three dimensions. We discuss the results of such an analysis for the dark cloud Lynds 204 (L204).