2.1.2. Infrared Dust Emission
The presence of magnetic fields in molecular clouds can be revealed by dust grains aligned by the magnetic fields. Elongated dust grains are aligned with their long axis perpendicular to the magnetic field lines. These needle-like grains rotate primarily around one of the short axes (i.e., end-over-end). The rotation axis is parallel to the magnetic field lines. As such, original unpolarized background stellar light at optical wavelengths is mostly transmitted (not absorbed) if the photon's electric vector is in the plane of the grain's short axis, so we observe at optical wavelengths the transmitted photons to be polarized at a position angle parallel to the ambient magnetic field lines. The dust's own emission in the far infrared, mm and submm wavelengths is primarily in the plane of rotation of the long axis of the needle, so we observe at far infrared, mm and submm wavelengths the emitted photons to be polarized at a position angle perpendicular to the ambient magnetic field lines. The first successful polarimetry at submillimeter wavelengths was done by Flett and Murray (1991), using the James Clerk Maxwell Telescope (JCMT). The first successful polarimetry at millimeter wavelengths was done by Barvainis et al (1987). For an early review, see Heiles et al. (1991)
Dust grains are in an active, dynamic environment, so to keep them aligned with a magnetic field would require a sufficient magnetic field strength (e.g., Hildebrand 1988a, 1988b, 1989). Looking at dust emission at far infrared and extreme infrared wavelengths, one finds that the ratio of the polarization amplitude at 2 different wavelengths varies with wavelength according to grain composition, i.e., silicate grains give a nearly constant ratio of polarization percentage, while graphite grains give an increasing polarization percentage at longer wavelengths (e.g., Hildebrand 1988; Minchin & Murray 1994; Leach et al. 1991; Clayton 1996).