Occurrence Conditions for Gigantic Jets Connecting the Thundercloud and the Ionosphere

Recently a new type of TLEs called gigantic jets which connect the top of thunderclouds and the lower ionosphere was discovered by ground-based imaging observations. The upper body of gigantic jets is very similar to sprites, while their lower body resembles blue jets. Pasko et al. [2002] reported a video recording of blue jet propagating upward from a thudercloud to an altitude of about 70 km, taken at Arecibo Observatory, Puerto Rico. Su et al. [2003] reported their observations of five gigantic jets at Kenting, Taiwan. These events occurred above the South China Sea near Luzon Island, Philippines, and spanned the thundercloud top at 16 km and the ionosphere at 90 km. Although they found no CGs with one-to-one correspondence to these gigantic jets, Tohoku University ELF network data provided evidence for occurrences of associated ELF transients. Based on these results, Su et al. [2003] concluded that a negative cloud to ionosphere discharge could lead to the generation of these ELF waves. More recently, the ISUAL instrument on board the FORMOSAT-2 satellite succeeded in observing a typical gigantic jet near Madagascar on March 21, 2005 [R. -R. Hsu et al. 2005]. Open questions are what is the generation mechanism of gigantic jets and what role gigantic jets play in the global electric circuit. Tong et al. [2004] investigated the initiation of gigantic jets by numerical simulation method and showed that gigantic jets are initiated at an altitude of 18-24 km as negative streamers. Furthermore, Tong et al. [2005] developed a randomly stepped leader propagation model to explain gigantic jets as a leader discharge process after a steamer-leader transition. Their simulation presented a three dimensional overall picture of leader propagation, which appears to be in agreement with the tree structure of observed gigantic jets. In this study we examine the conditions triggering gigantic jets. It is important to note that all events observed so far occurred in the subtropics (around 18 degrees in latitude) and above ocean close to the continent. Based on the model presented by Tong et al. [2005], we demonstrate that the low electrical conductivity in the subtropical mesosphere plays an important role in the upward propagation of leaders.