  We discuss tensor metric perturbations in a magnetic field around the
homogeneous Juttner equilibrium of massless particles in an expanding universe.
We solve the Liouville equation and derive the energy-momentum tensor up to
linear terms in the metric and in the magnetic field.The term linear in the
magnetic field is different from zero if the total charge of the primordial
plasma is non-zero. We obtain an analytic formula for temperature fluctuations
treating the tensor metric perturbations and the magnetic field as independent
random variables. Assuming a cutoff on large momenta of the magnetic spectral
function we show that the presence of the magnetic field can discriminate only
low multipoles in the multipole expansion of temperature fluctuations. In such
a case the term linear in the magnetic field can be more important than the
quadratic one (corresponding to the fluctuations of the pure magnetic field).
