On 31 August 2010, more than 100 transient luminous events were observed to occur over Typhoon Lionrock when it passed at ∼210 km to the southwest of the NCKU site in Taiwan. Among them, 14 negative gigantic jets (GJs) with clear recognizable morphologies and radio frequency signals are analyzed. These GJs are all found to have negative discharge polarity and thus are type I GJs. Morphologically, they are grouped into three forms: tree‐like, carrot‐like, and a new intermediate type called tree‐carrot‐like GJs. The ULF and ELF/VLF band signals of these events contain clear signatures associated with GJ development stages, including the initiating lightning, the leading jet, the fully developed jet, and the trailing jet. Though the radio waveform for each group of GJs always contains a fast descending pulse linked with the surge current upon the GJ‐ionosphere contact, the detailed waveforms actually vary substantially. Cross analysis of the optical and radio frequency signals for these GJs indicates that a large surge current moment (CM) (>60 kA‐km) appears to be essentially associated with the tree‐like GJs. In contrast, the carrot‐like and the tree‐carrot‐like GJs are both related to a surge CM less than 36 kA‐km, and a continuing CM less than 27 kA‐km further separates the carrot‐like GJs from the tree‐carrot‐like GJs. Furthermore, on the peak CM versus charge moment change diagram for the initiating lightning, different groups of GJs seem to exhibit different trends. This feature suggests that the eventual forms of negative GJs may have been determined at the initiating lightning stage.

[1] Secondary transient luminous events (TLEs) recorded by the ISUAL-FORMOSAT2 mission can either be secondary jets or secondary gigantic jets (GJs), depending on their terminal altitudes. The secondary jets emerge from the cloud top beneath the preceding sprites and extend upward to the base of the sprites at $50 km. The secondary jets likely are negative electric discharges with vertically straight luminous columns, morphologically resembling the trailing jet of the type-I GJs. The number of luminous columns in a secondary jet seems to be affected by the size of the effective capacitor plate formed near the base of the preceding sprites and the charge distribution left behind by the sprite-inducing positive cloud-to-ground discharges. The secondary GJs originate from the cloud top under the shielding area of the preceding sprites, and develop upward to reach the lower ionosphere at $90 km. The observed morphology of the secondary GJs can either be the curvy shifted secondary GJs extending outside the region occupied by the preceding sprites or the straight pop-through secondary GJs developing through the center of the preceding circular sprites. A key factor in determining the terminal height of the secondary TLEs appears to be the local ionosphere boundary height that established by the preceding sprites. The abundance and the distribution of the negative charge in the thundercloud following the sprite-inducing positive cloud-to-ground discharges may play important role in the generation of the secondary TLEs.

[1] From analyzing the distribution of the transient luminous events (TLEs) registered by the Imager of Sprites and Upper Atmospheric Lightning payload on the FORMOSAT-2 satellite, we deduced the synoptic-scale factors that control the occurrence of TLEs. For the low-latitude tropical regions (25°S ∼ 25°N), 84% of the TLEs were found to occur over the Intertropical Convergence Zone and the South Pacific Convergence Zone and exhibited a seasonal variation that migrates north and south with respect to the equator. For the midlatitude regions (latitudes beyond ±30°), the occurrence of TLEs congregated over the Pacific Ocean, the Atlantic Ocean, and the Mediterranean Sea during the winter seasons. From studying the distributions of the daily winter storm centers and the winter TLEs, the winter TLEs are usually found to occur near the cold fronts and thus are closely related to the winter storms. Our study shows that 88% of the northern winter TLEs and 72% of the southern winter TLEs occurred near the midlatitude cyclones. The winter TLE occurrence density and the storm-track frequency share similar trends with the distribution of the winter TLEs offset by 10°-15°. Additionally, this study compares the luminous intensities of elves and sprites from the tropical and winter midlatitude regions. The results show that the convective systems in the tropical regions are presumably more capable of producing bright TLEs in comparison to their winter counterparts.

Observing from southern Taiwan on 2 August 2010, a thunderstorm near Luzon Island, Philippines, about 500 km away was found to produce 72 transient luminous events (TLEs). Besides optical images, ULF and VLF sferics of lightning from this thunderstorm were also recorded. This work examines the characteristics of TLE-producing lightning through studying their ULF and VLF sferics. The attenuation of ULF and VLF sferics in the Earth cavity is obtained through analyzing the sferics associated with Imager of Sprite and Upper Atmospheric Lightning elves that occurred within~1500 km of Taiwan. Amplitudes of the ULF and the VLF sferics are found to vary as D À0.871 and D À1.207 , respectively, where D is the source distance from the sferic stations. After normalizing the sferics from the 2 August 2010 storm to 500 km distance, the ratio of the peak ULF and the VLF magnetic fields is found to be distinct for different TLE-producing lightning. The ratio for the halo-producing lightning is nearly 3 times that of the elve-producing lightning, but it is comparable to that of the halo-sprite-producing lightning, although the ULF strength for the halo-sprite lightning is significantly larger than that for the halo lightning. Therefore, it is possible to distinguish between the TLE-types using the ULF to VLF peak ratio or the strength of ULF/VLF band emissions of the parent lightning. Comparison of numerically simulated and the observed lightning radiation fields indicates that the best fit discharge time of the elve lightning is about 100 μs, while that for the halo-producing lightning is between 200 and 500 μs.

[1] Three multi-transient luminous events, which were recorded by ISUAL on FORMOSAT-2 and shared a similar generating sequence, are reported. Each event began with a positive cloud-to-ground lightning discharge (CG)-induced sprite, and a secondary gigantic jet (GJ) followed withiñ 30-50 ms. Then, 1 ms after the GJ, a new sprite occurred near the GJ without associated impulsive lightning signal. The associated electromagnetic signal for one of the events indicates that the GJ is a negative cloud-to-ionosphere discharge carrying a high peak current moment. Crossanalysis of the spectral, image, and electromagnetic data of these three events indicates that the new sprites are likely induced by the secondary GJs, and the high current moment of the secondary GJs appears to be a crucial factor for the induction of the new sprites. Hence, these secondary GJs may have played a role in inducing sprites as much as the negative CGs do for the occurrence of negative sprites.