  The tight coupling expansion, appropriately generalized to include
large-scale magnetic fields, allows the estimate of the brightness
perturbations of CMB anisotropies for typical wavelengths that are larger than
the Hubble radius after matter-radiation equality. After discussing the basic
features of the the pre-decoupling initial conditions in the presence of fully
inhomogeneous magnetic fields, the tight coupling expansion is studied both
analytically and numerically. From the requirement that the amplitudes and
phases of Sakharov oscillations are (predominantly) adiabatic and from the
inferred value of the plateau in the temperature autocorrelation, the effects
of the magnetized contribution can be systematically investigated and
constrained.
