At left we show a histogram of the inferred magnetic field directions, for all polarization measurements obtained in the four molecular clouds. The angles are given in Galactic coordinates, so that zero degrees is Galactic North-South, and 90 degrees is parallel to the Galactic plane. Our observations have revealed a tendency for the magnetic fields to be parallel to the Galactic plane. This has not been seen in previous studies of Giant Molecular Clouds, but past submillimeter polarimetric studies were not very sensitive to faint extended emission. SPARO's excellent sensitivity results from the superb observing site.
On the largest scales, the magnetic field in the Milky Way's disk is known to be azimuthal. Thus, the simplest explanation for the above correlation is that when a molecular cloud is formed from diffuse interstellar matter, the mean field direction is preserved. This places a constraint on theories that aim to explain how star-forming molecular clouds are themselves formed out of the diffuse interstellar Galactic gas.