  The interaction of streamers in nitrogen-oxygen mixtures such as air is
studied. First, an efficient method for fully three-dimensional streamer
simulations in multiprocessor machines is introduced. With its help, we find
two competing mechanisms how two adjacent streamers can interact: through
electrostatic repulsion and through attraction due to nonlocal
photo-ionization. The non-intuitive effects of pressure and of the
nitrogen-oxygen ratio are discussed. As photo-ionization is experimentally
difficult to access, we finally suggest to measure it indirectly through
streamer interactions.
