Streamer discharges can move perpendicularly to the electric field

Streamer discharges are a primary mode of electric breakdown in thunderstorms and high voltage technology; they are generally believed to grow along electric field lines. However, we here give experimental and numerical evidence that streamers can propagate nearly perpendicularly to the background electric field. These streamers are guided by pre-ionization that is orders of magnitude lower than the ionization density in a streamer channel, hardly affecting the background field. Positive streamers could be guided in nitrogen with 0.5% of oxygen or less, but not in air. This observation also tests the role of photo-ionization in gas mixtures with varying nitrogen-oxygen ratio.