  In this paper we study the effects of photoionization processes on the
propagation of both negative and positive streamer discharges. We show that
negative fronts accelerate in the presence of photoionization events. The
appearance and propagation of positive streamers travelling with constant
velocity is explained as the result of the combined effects of photoionization
and electron diffusion. The photoionization range plays an important role for
the selection of the velocity of the streamer as we show in this work.
