  Starting from the minimal model for the electrically interacting particle
densities in negative streamer discharges, we derive a moving boundary
approximation for the ionization fronts. The boundary condition on the moving
front is found to be of 'kinetic undercooling' type. The boundary
approximation, whose first results have been published in [Meulenbroek et al.,
PRL 95, 195004 (2005)], is then tested against 2-dimensional simulations of the
density model. The results suggest that our moving boundary approximation
adequately represents the essential dynamics of negative streamer fronts.
