Panel a: difference between field ɛ, as defined in Eq. (18) and inferred only by free–free data, and the same field resulting from the joint inference with Faraday data (reconstruction I). Panel b: uncertainty of the ɛ field in reconstruction I. This demonstrates that the ɛ field is predominately determined by the free–free data irrespective of the simultaneous inference of Faraday sky. There are considerable deviations between the reconstructions near the Galactic poles. Panel b shows that the reconstruction is also rather uncertain in these regions.