First Time-Resolved Spectroscopic Observation of Upward Positive Leader Initiated from Tall Tower

Using a slitless spectrograph, we recorded for the first time the time-resolved spectra of an upward positive leader (UPL) from the 600-m Canton Tower, together with synchronized current and electric field measurements. Triggered by a positive cloud-to-ground flash, the UPL showed singly ionized nitrogen (NII) emissions confined to the rapidly advancing leader tip, while neutral atomic lines (e.g., Hα, OI) dominated the trailing channel below the tip. This has not been reported in spectral studies of downward leaders. Based on ionic emission presence, propagation of the upward positive leader could be divided into three stages: neutral-dominated heating stage, onset stage of NII at the tip, and decay stage of ionization. NII emissions were short-lived and confined to the tip, whereas neutral atom emissions persisted along the channel. These results reflect the microscopic physical processes of ionization, excitation, and channel cooling during UPL propagation.