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Pictures show that streamer or sprite discharge channels emerging from thesame electrode sometimes seem to reconnect or merge though their heads carryelectric charge of the same polarity; one might therefore suspect thatreconnections are an artifact of the two-dimensional projection in thepictures. Here we use stereo-photography to investigate the fullthree-dimensional structure of such events. We analyze reconnection, possiblyan electrostatic effect in which a late thin streamer reconnects to an earlierthick streamer channel, and merging, a suggested photoionization effect inwhich two simultaneously propagating streamer heads merge into one newstreamer. We use four different anode geometries (one tip, two tips, twoasymmetric protrusions in a plate, and a wire), placed 40 mm above a flatcathode plate in ambient air. A positive high voltage pulse is applied to theanode, creating a positive corona discharge. This discharge is studied with afast ICCD camera, in many cases combined with optics to enable stereoscopicimaging. We find that reconnections as defined above occur frequently. Mergingon the other hand was only observed at a pressure of 25 mbar and a tipseparation 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 morethan 10 times as large as the tip separation. At higher pressures or with awire anode, merging was not observed.

Reconnection and merging of positive streamers in air