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If a single line of sight (LOS) intercepts multiple dust clouds of differentspectral energy distributions and magnetic field orientations, the frequencyscaling of each of the Stokes $Q$ and $U$ parameters of thermal dust emissionmay be different ("LOS frequency decorrelation"). We present first evidence forLOS frequency decorrelation in $Planck$ data. We use independent,neutral-hydrogen--measurements of the number of clouds per LOS and the magneticfield orientation in each cloud to select two sets of sightlines: (i) a targetsample (pixels likely to exhibit LOS frequency decorrelation); (ii) a controlsample (pixels lacking complex LOS structure). We test the null hypothesis thatLOS frequency decorrelation is not detectable in $Planck$ 353 and 217~GHzpolarization data at high Galactic latitudes. The data reject this hypothesisat high significance. The detection is robust against choice of CMB map andmap-making pipeline. The observed change in polarization angle due to LOSfrequency decorrelation is detectable above the $Planck$ noise level. Theprobability that the detected effect is due to noise alone ranges from $5\times10^{-2}$ to $4\times 10^{-7}$, depending on the CMB subtraction algorithm andtreatment of residual systematics; correcting for residual systematicsincreases the significance of the effect. The LOS decorrelation effect isstronger for sightlines with more misaligned magnetic fields, as expected. Weestimate that an intrinsic variation of $\sim15\%$ in the ratio of 353 to217~GHz polarized emission between clouds is sufficient to reproduce themeasured effect. Our finding underlines the importance of ongoing studies tomap the 3D structure of the magnetized dusty ISM that could help componentseparation methods to account for frequency decorrelation effects in CMBpolarization studies.

Evidence for Line-of-Sight Frequency Decorrelation of Polarized Dust Emission in $Planck$ Data