  A viable class of magnetogenesis models can be constructed by coupling the
kinetic term of the hypercharge to a spectator field whose dynamics does not
affect the inflationary evolution. The magnetic power spectrum is explicitly
related to the power spectrum of (adiabatic) curvature inhomogeneities when the
quasi-de Sitter stage of expansion is driven by a single scalar degree of
freedom. Depending upon the value of the slow-roll parameters, the amplitude of
smoothed magnetic fields over a (comoving) Mpc scale can be as large as
0.01--0.1 nG at the epoch of the gravitational collapse of the protogalaxy. The
contributions of the magnetic fields to the Sachs-Wolfe plateau and to the
temperature autocorrelations in the Doppler region compare favourably with the
constraints imposed by galactic magnetogenesis. Stimulating lessons are drawn
on the interplay between magnetogenesis models and their possible CMB
signatures.
