Stacked Q_r image around temperature hot spots selected above the null threshold (ν = 0) in the SMICA sky map. The quantity Q_r (and its partner U_r, introduced in Kamionkowski et al. 1997) is a transformed version of the Stokes parameters Q and U, where Q_r measures the tangential-radial component of the polarization relative to the centre and U_r measures the polarization at ±45° relative to a radial vector. Left panel: observed data, and right panel: ensemble average of CMB-only maps for the fiducial cosmology. The axes are in degrees, and the image units are μK. The black solid lines show the polarization directions for stacked Q and U, with lengths proportional to the polarization amplitude P. From Planck Collaboration XVI (2016).