  Pictures show that streamer or sprite discharge channels emerging from the
same electrode sometimes seem to reconnect or merge though their heads carry
electric charge of the same polarity; one might therefore suspect that
reconnections are an artifact of the two-dimensional projection in the
pictures. Here we use stereo-photography to investigate the full
three-dimensional structure of such events. We analyze reconnection, possibly
an electrostatic effect in which a late thin streamer reconnects to an earlier
thick streamer channel, and merging, a suggested photoionization effect in
which two simultaneously propagating streamer heads merge into one new
streamer. We use four different anode geometries (one tip, two tips, two
asymmetric protrusions in a plate, and a wire), placed 40 mm above a flat
cathode plate in ambient air. A positive high voltage pulse is applied to the
anode, creating a positive corona discharge. This discharge is studied with a
fast ICCD camera, in many cases combined with optics to enable stereoscopic
imaging. We find that reconnections as defined above occur frequently. Merging
on the other hand was only observed at a pressure of 25 mbar and a tip
separation of 2 mm, i.e., for a reduced tip distance of p*d=50 micrometer*bar.
In this case the full width at half maximum of the streamer channel is more
than 10 times as large as the tip separation. At higher pressures or with a
wire anode, merging was not observed.
