  We address the problem of angular momentum transport in stellar radiative
interiors with a novel semi-analytic spectral technique, using an eigenfunction
series expansion, that can be used to derive benchmark solutions in
hydromagnetic regimes with very high Reynolds number (10^7 - 10^8). The error
arising from the truncation of the series is evaluated analytically. The main
simplifying assumptions are the neglect of meridional circulation and of
non-axisymmetric magnetic fields. The advantages of our approach are shown by
applying it to a spin-down model for a 1 M_sun main-sequence star. The
evolution of the coupling between core and envelope is investigated for
different values of the viscosity and different geometries and values of the
poloidal field. We confirm that a viscosity enhancement by 10^4 with respect to
the molecular value is required to attain a rigid rotation in the core of the
Sun within its present age. We suggest that a quadrupolar poloidal field may
explain the short coupling time-scale needed to model the observed rotational
evolution of fast rotators on the ZAMS, while a dipolar geometry is indicated
in the case of slow rotators. Our novel semi-analytic spectral method provides
a conceptually simple and rigorous treatment of a classic MHD problem and
allows us to explore the influence of various parameters on the rotational
history of radiative interiors.
