First Time‐Resolved Spectroscopic Observation of Upward Positive Leader Initiated From Tall Tower

Abstract Using a slitless spectrograph operated at 14,400 frames per second (with a spatial resolution of 3.3 m per pixel), 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. In this event, based on the evolution of ionic emissions, we divided the initial propagation of the UPL into three stages: (a) a neutral‐dominated heating stage lasting about 0.97 ms, during which the leader tip rose from 600 to 1,079 m AGL; (b) an ionization burst stage with detectable NII at the tip lasting about 1.11 ms, extending from 1,158 to 1,983 m AGL; and (c) an ionization decay stage starting at about 2,000 m AGL, during which singly ionized lines became undetectable again and only a few neutral atomic lines remained detectable. Detectable NII emissions were confined to the tip, whereas neutral atomic emissions persisted along the channel. The three spectral stages exhibited distinct current characteristics, with pronounced impulsive pulses in the first two stages but smoother fluctuations during the third stage, suggesting that only sufficiently strong, stepwise leader advances can produce detectable ionic line emission.