  Observations of the effect of the magnetic field on its environment are
usually achieved with techniques which rely on the interaction with the spin of
the particles under study. Because of the relative weakness of this effect,
extraction of the field characteristics proves to be a most challenging task.
We take a totally different approach to the problem and show that the
manifestation of the magnetic field can be directly observed by means of a
comparison of the spectra of molecular ions with those of neutral molecules.
This takes advantage of the strong cyclotron interaction between the ions and
the field, but requires the presence of flows or turbulent motion in the gas.
We compare our theory to data obtained on the OMC-1, OMC-2, OMC-3 and DR21(OH)
molecular clouds.
