Simulation of leader speeds at gigantic jet altitudes

Lightning leaders advance in space by creating a heating conversion zone in their tips (i.e., streamer-to-leader transition) in which Joule heating produced by currents of many non-thermal corona streamers transforms into a hot and conducting leader channel. It is believed that the initial stages of transient luminous events termed gigantic jets (GJs) propagating toward the lower ionosphere are directly related to leaders initiated by conventional intra-cloud lightning discharges and escaping upward from thundercloud tops. In the present work we provide quantitative description of speeds of these leaders as a function of leader current and ambient air density (altitude). The direct comparisons with available experimental data indicate that the initial speeds of GJs of ̃50 km/s are consistent with leaders possessing currents 2-8 A. The observed acceleration of GJs can be explained by growth of the leader current, and at high altitudes (low air densities) may be significantly affected by predominance of non-thermal (i.e., streamer) discharge forms.