Speaker
Axel Brandenburg
(Nordita)
Description
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).
