Current helicity pattern of large-scale magnetic field on the photosphere

Following Pevtsov and Latushko, we study the current helicity pattern of the large-scale magnetic field on the photosphere. We use the same technique as theirs to derive the vector magnetic field (Br, Bθ, Bϕ) from full-disk longitudinal magnetograms based on the assumption that large-scale magnetic fields evolve rather slowly and the variations of the longitudinal magnetic fields within certain time duration are caused by the changing position angles only. Different from their study, we have calculated the current helicity maps directly from the derived vector magnetograms, rather than from obtaining the latitudinal variation of hc by ignoring the role of Bθ component and averaging Br and Bϕ over all solar longitudes. This approach significantly strengthens the evidence of the hemispheric rule presented in the reconstructed vector magnetic field. Our study shows that the established hemispheric sign rule, that is, positive helicity sign in the southern hemisphere and negative helicity sign in the northern hemisphere, is applicable everywhere in the global magnetic field, namely, also evident in weak fields outside active regions, and that the obtained sign pattern is independent of the longitudinal magnetograms and the parameters that we have used.