  We estimate the turbulent ambipolar diffusion length scale and magnetic field
strength in the massive dense cores CygX-N03 and CygX-N53, located in the
Cygnus-X star-forming region. The method we use requires comparing the velocity
dispersions in the spectral line profiles of the coexistent ion and neutral
pair H13CN and H13CO+ (J=1-0) at different length scales. We fit
Kolmogorov-type power laws to the lower envelopes of the velocity dispersion
spectra of the two species. This allows to calculate the turbulent ambipolar
diffusion scale, which in turn determines the plane-of-the-sky magnetic field
strength. We find turbulent ambipolar diffusion length scales of 3.8+-0.1 mpc
and 21.2+-0.4 mpc, and magnetic field strengths of 0.33 mG and 0.76 mG for
CygX-N03 and CygX-N53, respectively. These magnetic field values have
uncertainties of a factor of a few. Despite a lower signal-to-noise ratio of
the data in CygX-N53 than in CygX-N03, and the caveat that its stronger field
might stem in part from projection effects, the difference in field strengths
suggests different fragmentation activities of the two cores. Even though the
quality of our data, obtained with the IRAM Plateau de Bure Interferometer
(PdBI), is somewhat inferior to previous single-dish data, we demonstrate that
this method is suited also for observations at high spatial resolution.
