Evidence for line-of-sight frequency decorrelation of polarized dust emission in <i>Planck</i> data

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