High-Speed Schlieren Photographic Observations of Space Stems in Negative and Positive Sparks

The space stem has raised wide and long-standing concerns from researchers around the world since it was found in natural lightning first, while needs to be further studied. This paper aims at providing details of space stem morphology and development in both negative and positive sparks in a 1.27 m point-plane gap in the laboratory, using a high-speed Schlieren system. It is found that the stem areas captured in the traditional high-speed images were overestimated and the Schlieren images were preferable for the detailed observation. There was an evidence of possible (but small) positive space stem. By comparison, the negative space stem was much more obvious than the positive space stem. The average length of negative space stem was about 1.83 mm. During the step process, the space stem propagated both bidirectionally and unidirectionally. The positive portion developed at an average speed of 1.19×10
³
m/s. In only one case for bidirectional develoment, the negative portion had a faster speed than the positive portion, which means that negative portion develops slower than the positive one as reported in other papers may be not always right. In addition, two scenarios were found for the connection between the space stem and the main channel, i.e., one to the channel tip directly, and the other to the channel side.