Statistics of CMB polarization angles
  We study the distribution functions of the CMB polarization angle $\psi$,
focusing on the Planck 2018 CMB maps. We extend the model of Preece & Battye
(2014) of Gaussian correlated $Q$ and $U$ Stokes parameters to allow nonzero
means. When the variances of $Q$ and $U$ are equal and their covariance and
means are zero, the polarization angle is uniformly distributed. Otherwise the
uniform distribution will be modulated by harmonics with $2\psi$ and $4\psi$
phases. These modulations are visible in the Planck 2018 CMB maps. Furthermore,
the mean value of $U$ is peculiar compared to the power spectrum.
