  Linearly polarized, non-masing, rotational lines have been detected for the
first time in the interstellar medium. This effect occurs in molecular clouds
with a magnetic field, and traces the field direction, offering an alternative
technique to dust emission polarimetry. The line polarization mechanism is
similar to that in masers, but with lower degrees of polarization (~1%). We
have detected the effect towards the Galactic Centre and its surrounding `2 pc
ring', and in the molecular clouds S140 and DR21 (tentatively), in the lines CO
J=2-1 and J=3-2, and 13CO J=2-1. The deduced magnetic field directions agree
well with previous dust polarimetry results, confirming that the line
polarization is a real effect. This new technique will be useful in sources
that are too faint for dust polarimetry, and can also be used to investigate
3-dimensional morphology of magnetic fields, where the velocity structure of
the clouds is known.
