Logarithmic amplitude field of reconstruction I, as defined in Eq. (20), and its components. Panel a: field ɛ defined in Eqs. (20) and (18), constrained by free–free and Faraday data. This is also the reconstructed log free–free map. Panel b: second amplitude field δ defined in Eq. (20). The red crosses indicate the approximate angular positions of the Orion arm, as given by Vázquez et al. (2008) and Xu et al. (2009). Panel c: logarithmic amplitude of the Faraday sky, as defined in Eq. (19) without the additional δ contribution, while it is included in d.