  A 2D approach is used to simulate the properties of positive and negative
streamers emerging from a high-voltage electrode in a long (14 cm) air gap for
standard pressure and temperature. The applied voltage varies from 100 to 500
kV. To reveal the influence of photoionization, the calculations are made for
various rates of seed electron generation in front of the streamer head. The
difference between the properties of positive and negative streamers is
associated with the different directions of the electron drift ahead of the
streamer head. As a result, the peak electric field at the streamer head and
the streamer velocity are higher for positive voltage polarity. The average
electric field in the negative streamer channel is approximately twice that in
the positive streamer channel, in agreement with available measurements in long
air gaps. It is shown that photoionization in front of the streamer head is
important not only for the development of strong positive discharges, but for
the development of strong negative discharges as well. An increase in the
photoionization rate increases the propagation velocity of the positive
streamer and retards the propagation of the negative streamer.
