A visible channel core and the channel structure below the connection point for natural cloud-to-ground lightning

The channel core of natural negative cloud-to-ground lightning was recorded using a high-speed slitless spectrograph, which confirms the corona sheath model of the lightning channel. By analyzing the channel spectra and images at different stages, it is corroborated that the ionic lines in spectra are mainly due to the channel core. Based on the luminescence and spectral characteristics, the connection point of the downward moving leader and the upward connecting leader was located, and it is concluded that the current is the highest at the connection point. Above all, it is discovered that there is no obvious channel core below the connection point, which means that the current is transmitted over the whole channel cross section. This feature is an important basis for investigating the lightning connection process and the physical mechanism of a lightning strike.