  Recent Planck results have shown that radiation from the cosmic microwave
background passes through foregrounds in which aligned dust grains produce
polarized dust emission, even in regions of the sky with the lowest level of
dust emission. One of the most commonly used ways to remove the dust foreground
is to extrapolate the polarized dust emission signal from frequencies where it
dominates (e.g., ~ 350 GHz) to frequencies commonly targeted by cosmic
microwave background experiments (e.g., ~150 GHz). In this paper, we describe
an interstellar medium effect that can lead to decorrelation of the dust
emission polarization pattern between different frequencies due to multiple
contributions along the line of sight. Using a simple 2-cloud model we show
that there are two conditions under which this decorrelation can be large: (a)
the ratio of polarized intensities between the two clouds changes between the
two frequencies; (b) the magnetic fields between the two clouds contributing
along a line of sight are significantly misaligned. In such cases, the 350 GHz
polarized sky map is not predictive of that at 150 GHz. We propose a possible
correction for this effect, using information from optopolarimetric surveys of
dichroicly absorbed starlight.
