Microwave background anisotropies from Alfvén waves

We investigate microwave background anisotropies in the presence of primordial magnetic fields. We show that a homogeneous field with fixed direction can amplify vector perturbations. We calculate the correlations of $\delta T/T$ explicitly and show that a large scale coherent field induces correlations between $a_{l-1,m}$ and $a_{l+1,m}.$ We discuss constraints on the amplitude and spectrum of a primordial magnetic field imposed by observations of CMB anisotropies.