  An appropriate gauge-invariant framework for the treatment of magnetized
curvature and entropy modes is developed. It is shown that large-scale magnetic
fields, present after neutrino decoupling, affect curvature and entropy
perturbations. The evolution of different magnetized modes is then studied
across the matter-radiation transition both analytically and numerically. From
the observation that, after equality (but before decoupling) the (scalar)
Sachs-Wolfe contribution must be (predominantly) adiabatic, constraints on the
magnetic power spectra are deduced. The present results motivate the
experimental analysis of more general initial conditions of CMB anisotropies
(i.e. mixtures of magnetized adiabatic and isocurvature modes during the
pre-decoupling phase). The role of the possible correlations between the
different components of the fluctuations is partially discussed.
