### 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 (Christensen-Dalsgaard, 2002) 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
reviews, see Gizon, 2004; Howe, 2009). 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.