  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 re-derive this power spectrum using modern conventions,
provide a simplified result, and identify some of the discrepancies in the
literature.
