Primordial density perturbations from magnetic fields

Perturbations to the cosmic baryon density - and thus to the total-matter density - can be induced by magnetohydronamic forces if there are primordial magnetic fields. The power spectrum for these density perturbations was first provided in 1996, but without much in the way of detail in the derivation, and there has been confusion in the intervening years about this calculation. In this brief note, we rederive this power spectrum using modern conventions, provide a simplified result, and identify some of the discrepancies in the literature.