We describe a method for determining the dispersion of magnetic field vectors about large-scale fields in turbulent molecular clouds. The method is designed to avoid inaccurate estimates of magnetohydrodynamic or turbulent dispersion-and help avoiding inaccurate estimates of field strengths-due to a large-scale, nonturbulent field structure when using the well known method of Chandrasekhar and Fermi. Our method also provides accurate, independent estimates of the turbulent to large-scale magnetic field strength ratio. We discuss applications to the molecular clouds OMC-1, M17, and DR21(Main).

We expand our study on the dispersion of polarization angles in molecular clouds. We show how the effect of signal integration through the thickness of the cloud as well as across the area subtended by the telescope beam inherent to dust continuum measurements can be incorporated in our analysis to correctly account for its effect on the measured angular dispersion and inferred turbulent to large-scale magnetic field strength ratio. We further show how to evaluate the turbulent magnetic field correlation scale from polarization data of sufficient spatial resolution and high enough spatial sampling rate. We apply our results to the molecular cloud OMC-1, where we find a turbulent correlation length of δ ≈ 16 mpc, a turbulent to large-scale magnetic field strength ratio of approximately 0.5, and a plane-of-the-sky large-scale magnetic field strength of approximately 760 μG.

We present a summary of data obtained with the 350 μm polarimeter, Hertz, at the Caltech Submillimeter Observatory. We give tabulated results and maps showing polarization vectors and intensity contours. The summary includes over 4300 individual measurements in 56 Galactic sources and two galaxies. Of these measurements, 2153 have P 3σ p statistical significance. The median polarization of the entire data set is 1.46%.

We have used the SHARP polarimeter at the Caltech Submillimeter Observatory to map the polarization at wavelengths of 350 and 450 µm in a ∼ 2 ′ × 3 ′ region of the Orion Molecular Cloud. The map covers the brightest region of the OMC-1 ridge including the Kleinmann-Low (KL) nebula and the submillimeter source Orion-south. The ratio of 450-to-350 µm polarization is ∼ 1.3 ± 0.3 in the outer parts of the cloud and drops by a factor of 2 towards KL. The outer cloud ratio is consistent with measurements in other clouds at similar wavelengths and confirms previous measurements placing the minimum of the polarization ratio in dusty molecular clouds at λ ∼ 350 µm.

We present a novel statistical analysis aimed at deriving the intrinsic shapes and magnetic field orientations of molecular clouds using dust emission and polarization observations by the Hertz polarimeter. Our observables are the aspect ratio of the projected plane‐of‐the‐sky cloud image and the angle between the mean direction of the plane‐of‐the‐sky component of the magnetic field and the short axis of the cloud image. To overcome projection effects due to the unknown orientation of the line‐of‐sight, we combine observations from 24 clouds, assuming that line‐of‐sight orientations are random and all are equally probable. Through a weighted least‐squares analysis, we find that the best‐fitting intrinsic cloud shape describing our sample is an oblate disc with only small degrees of triaxiality. The best‐fitting intrinsic magnetic field orientation is close to the direction of the shortest cloud axis, with small (∼24°) deviations towards the long/middle cloud axes. However, due to the small number of observed clouds, the power of our analysis to reject alternative configurations is limited.

We have developed a foreoptics module that converts the Submillimeter High Angular Resolution Camera generation II (SHARC-II) camera at the Caltech Submillimeter Observatory into a sensitive imaging polarimeter at wavelengths of 350 and 450 m. We refer to this module as "SHARP." SHARP splits the incident radiation into two orthogonally polarized beams that are then reimaged onto opposite ends of the 32 ϫ 12 pixel detector array in SHARC-II. A rotating half-wave plate is used just upstream from the polarization-splitting optics. The effect of SHARP is to convert SHARC-II into a dual-beam 12 ϫ 12 pixel polarimeter. A novel feature of SHARP's design is the use of a crossed grid in a submillimeter polarimeter. Here we describe the detailed optical design of SHARP and present results of tests carried out during our first few observing runs. At 350 m, the beam size (9 arc sec), throughput (75%), and instrumental polarization ͑Ͻ1%͒ are all very close to our design goals.

The molecular cloud, DR21 Main, is an example of a large-scale gravitational collapse about an axis near the plane of the sky where the collapse is free of major disturbances due to rotation or other effects. Using flux maps, polarimetric maps, and measurements of the field inclination by comparing the line widths of ion and neutral species, we estimate the temperature, mass, magnetic field, and the turbulent kinetic, mean magnetic, and gravitational potential energies, and present a 3D model of the cloud and magnetic field.

Fluctuations in atmospheric emission introduce noise and systematic errors into measurements of polarization at far-infrared and submillimeter wavelengths. We describe a new analysis method that corrects for the bias and reduces the errors caused by the fluctuations. The method exploits repeated observations of a source and is especially effective on faint sources for which the polarized flux is on the same order as the atmospheric fluctuations.