1999ApJ...516..834H

We present data on the polarization of the thermal emission from Galactic Clouds at 60 µm, 100 µm, and 350 µm. There are examples of rising polarization spectra in dense cloud cores [P(350 µm/P(100 µm)~2], and falling spectra in cloud envelopes [P(350)/P(100 µm)~0.6]. We also present data showing that the relationship, P(τ), between polarization and optical depth in cloud cores is different from that in cloud envelopes. We review the principles governing the far-infrared polarization spectrum and discuss applications to the data on P(λ) and P(τ). We conclude that the cloud envelopes we have observed must contain two populations of grains that differ in their polarization efficiencies and in their emission spectra. We propose a model for cloud envelopes in which the contrasting populations reside in domains of different mean temperatures where the warmer domains contain the aligned grains.