The linear polarization convention used historically in CMB data sets differs from the IAU convention. In the polarization convention defined by the IAU, the local x-axis points North, the local y-axis points East, and the local z-axis points inwards to the observer for a right-handed system. In the polarization convention used historically in the CMB community, the local x-axis points South, the local y-axis points East, and the local z-axis points outwards from the observer for a right-handed system. Figures showing the two conventions are shown above. As a result, there is a difference in the sign of the U Stokes parameter. A more in-depth discussion of both conventions is described in the HEALPix documentation. Figures describing the two conventions are shown at the top of the page. SVG versions can be downloaded by clicking on the figures.
On December 8, 2015, the IAU requested that scientists, including those working on CMB data, follow the IAU standard for polarization angle in their publications.
HEALPix files contain the FITS keyword POLCCONV to describe the polarization convention. Its value is 'COSMO' for files following the CMB convention and 'IAU' for those following the IAU convention. An absence of this keyword is interpreted as meaning 'COSMO'. Most HEALPix files will follow the COSMO convention.
The convention of non-HEALPix files will be specified in their detailed data description.
No archival data has been retroactively changed to the IAU convention.