Microwave background anisotropies from Alfven 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. [S0556-2821(98)06224-9].