Photoionization effects in streamer discharges
  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. 
M. Arrayas, M. A. Fontelos, J. L. Trueba