Axel Brandenburg (Nordita)
To produce strong flux concentrations, there has to be a downflow that pulls gas down. Since the flow is constrained to be along field lines, there must be inflow from above and the sides, which concentrates the field further. Thermal convection associated with the hierarchical nature of the supergranulation may be one such mechanism. Another one is the negative effective magnetic pressure instability. Given that the field becomes dynamically important, the interplay between flow and magnetic field must be important. In the context of magneto-convection, a segregation between magnetized and unmagnetized regions has been seen for some time (Tao et al. 1998, ApJL 496, L39), and is often associated with flux expulsion, although no predictive model has yet been constructed. Flux segregation leads to larger scale structures, as is also predicted by the negative effective magnetic pressure instability. However, there is a striking analogy with the inverse cascade behavior of the underlying dynamo process: in both cases, pseudo-scalars are involved (g.Omega and A.B in the dynamo process and g.B and u.B in the flux segregation process) and in both cases one sees the growth of large-scale magnetic structures. It is therefore tempting to argue that both are associated with the presence of quadratic invariants of the underlying equations (magnetic helicity A.B and cross helicity u.B).