  While vector modes are usually ignored in cosmology since they are not
produced during inflation they are inevitably produced from the interaction of
density fluctuations of differing wavelengths. This effect may be calculated
via a second-order perturbative expansion. We investigate this effect during
the radiation era. We discuss the generation mechanism by investigating two
scalar modes interacting, and we calculate the power of vector modes generated
by a power-law spectrum of density perturbations on all scales.
