Comparison of the Planck 2018 CMB polarization maps in the BICEP2/Keck
  region
  We examine the statistical properties of polarization maps from Planck 2018
within the patch of sky observed by the BICEP2/Keck experiment using the one
point distribution function (1PDF), skewness, and kurtosis statistics. Our
analysis is performed for the Q and U Stokes parameters and for the
corresponding E- and B-modes of the CMB signals. We extend our analysis by
studying the correlations between CMB polarization maps and residual maps (the
difference between the full signal and the CMB map) for the frequency range of
100--217 GHz with both the Q/U and E/B approaches. Although all the CMB maps
reveal almost Gaussian statistical properties for Q/U and E/B domains, we have
detected very significant anomalies for cross-correlations with residuals at
100 GHz at the level of $3.7\sigma$ for the Commander map and $5.2\sigma$ for
NILC, for both the Q and U parameters. Using the NILC--Commander difference,
which does not contain a cosmological signal, we find a sub-dominant
non-Gaussian component in Q skewness and kurtosis at the level of $4.3\sigma$
and $10\sigma$, respectively. For the B-mode we have found a very high level of
cross-correlation (0.63--0.69) between the NILC/Commander maps and the 143 GHz
total signal, which cannot be associated with the cosmological component. These
strong deviations suggest that remnants of foregrounds, systematic effects, and
component separation exist in the 2018 Planck CMB polarization maps in the
BICEP2 sky area, which is far away from the Galactic plane. Our analysis also
demonstrates the preferability of the Q/U domain over E/B for determination of
the statistical properties of the derived CMB signals, due to non-locality of
the transition Q/U $\rightarrow$ E/B.
