  Within four nearby (d < 160 pc) molecular clouds, we statistically evaluate
the structure of the interstellar magnetic field, projected on the plane of the
sky and integrated along the line of sight, as inferred from the polarized
thermal emission of Galactic dust observed by Planck at 353 GHz and from the
optical and NIR polarization of background starlight. We compare the dispersion
of the field orientation directly in vicinities with an area equivalent to that
subtended by the Planck effective beam at 353 GHz (10') and using the
second-order structure functions of the field orientation angles. We find that
the average dispersion of the starlight-inferred field orientations within
10'-diameter vicinities is less than 20 deg, and that at these scales the mean
field orientation is on average within 5 deg of that inferred from the
submillimetre polarization observations in the considered regions. We also find
that the dispersion of starlight polarization orientations and the polarization
fractions within these vicinities are well reproduced by a Gaussian model of
the turbulent structure of the magnetic field, in agreement with the findings
reported by the Planck collaboration at scales greater than 10' and for
comparable column densities. At scales greater than 10', we find differences of
up to 14.7 deg between the second-order structure functions obtained from
starlight and submillimetre polarization observations in the same positions in
the plane of the sky, but comparison with a Gaussian model of the turbulent
structure of the magnetic field indicates that these differences are small and
are consistent with the difference in angular resolution between both
techniques.
