Properties of pulsed corona streamers are measured and
simulated in full three spatial dimensions (3D). Stereo photography is used
to measure branching angles and to investigate whether apparent streamers
reconnections are real. 3D simulations of two parallel streamers show that
they can repel each other electrostatically, but that they also can merge
due to photoionization. The electrostatic interaction of several streamers
becomes evident through theoretical investigations of a periodic array of
streamers.