postscript
MDI simultaneous observations of granulation and magnetic fields has allowed detailed comparisons of supergranular flow and magnetic fields. The fields are seen to be in the supergranule boundaries. The field is seen to exist in small clumps which migrate along the flow-convergence lines and collect at the vertices. Small bipolar field elements can appear at any location but quickly the two components migrate to the supergranule boundaries where they either cancel existing field or replace it. Some details are here.
