### 2.3 Kinematic models

A first drastic simplification of the MHD system of equations consists in dropping Equation (2)
altogether by specifying a priori the form of the flow field u. This kinematic regime remained
until relatively recently the workhorse of solar dynamo modelling. Note that with u given,
the MHD induction equation becomes truly linear in B. Moreover, helioseismology has now
pinned down with good accuracy two important solar large-scale flow components, namely
differential rotation throughout the interior, and meridional circulation in the outer half of the solar
convection zone (for a recent and comprehensive review, see Christensen-Dalsgaard, 2002). Given
the low amplitude of observed torsional oscillations in the solar convective envelope, and the
lack of significant cycle-related changes in the internal solar differential rotation inferred by
helioseismology to this date, the kinematic approximation is perhaps not as bad a working
assumption as one may have thought, at least for the differential rotation part of the mean flow
u.