OH Zeeman observations of dark clouds

We have made measurements with the Green Bank 43 m telescope of the Zeeman effect in the 1665 and 1667 MHz lines of OH toward dark clouds. The typical 1 σ sensitivity was 3 μG. The only certain detection of a magnetic field was toward B1, for which we measured a line-of-sight component |B|cos θ=−19.1±3.9 μG. Comparison with our earlier measurement of the field toward B1 with the Arecibo telescope provided evidence for a 40% enhancement in field strength between the molecular envelope and core of the B1 cloud, which is consistent with quasi-static contraction of the cloud driven by ambipolar diffusion. Because the Zeeman effect is only sensitive to the line-of-sight component of the magnetic field, a statistical analysis of the detection and upper limits was necessary