The primary unknown at this writing is the degree to which meridional circulation is affected by the Lorentz force associated with the dynamo-generated magnetic field. Recent calculations by M. Rempel and collaborators (Rempel, 2004) suggest that the backreaction is limited to regions of strongest toroidal fields, so that the “conveyor belt” is still operating in the bulk of the convective envelope. Such calculations must be vigorously pursued.
Another pressing matter is to establish the form and speed of the equatorward return flow in the lower convective envelope, and to determine how much - if any - penetration takes place in the radiative layers below. While Nandy and Choudhuri (2002) have argued that deep penetration (i.e., down to ) is essential for a proper butterfly diagram to be produced, the boundary layer analysis of Gilman and Miesch (2004) indicates no significant penetration below the base of the convective envelope. This is an issue where the final word will likely come from helioseismology, hopefully in the not too distant future.
© Max Planck Society and the author(s)