J. K. Chou

Discharge processes, electric field, and electron energy in ISUAL‐recorded gigantic jets

� 10 7 ms � 1 , which is similar to that observed for downward sprite streamers. Analysis of spectral ratios for the fully developed jet emissions gives a reduced E field of 400–655 Td and average electron energy of 8.5–12.3 eV. These values are higher than those in the sprites but are similar to those predicted by streamer models, which implies the existence of streamer tips in fully developed jets. The gigantic jets studied here all contained two distinct photometric peaks. The first peak is from the fully developed jet, which steadily propagates from the cloud top (� 20 km) to the lower ionosphere at � 90 km. We suggest that the second photometric peak, which occurs � 1 ms after the first peak, is from a current wave or potential wave–enhanced emissions that originate at an altitude of � 50 km and extend toward the cloud top. We propose that the fully developed jet serves as an extension of the local ionosphere and produces a lowered ionosphere boundary. As the attachment processes remove the charges, the boundary of the local ionosphere moves up. The current in the channel persists and its contact point with the ionosphere moves upward, which produces the upward surging trailing jets. Imager and photometer data indicate that the lightning activity associated with the gigantic jets likely is in-cloud, and thus the initiation of the gigantic jets is not directly associated with cloud-to-ground discharges.

Radiative emission and energy deposition in transient luminous events

The Imager of Sprites and Upper Atmospheric Lightning (ISUAL) experiment on the FORMOSAT-2 satellite has recently reported that an elve is the most dominant type of transient luminous events (TLEs) and deduced the global occurrence rates of sprites, halos and elves to be ~1, ~1 and 35 events/min, respectively (Chen et al 2008 J. Geophys. Res. 113 A08306). In this paper, we report the computed radiative emission and energy precipitation of the TLEs in the upper atmosphere. By analysing 1415 ISUAL TLEs, we found that for sprites, halos and elves the spatially averaged brightness are 1.5, 0.3 and 0.17 MR, and the energy deposition is 22, 14 and 19 MJ per event. After factoring in the global occurrence rates, the global energy deposition rates in the upper atmosphere are 22, 14 and 665 MJ min−1 from sprites, halos and elves.

Transient luminous event coordinated observations using FORMOSAT-2 satellite and Taiwan sprites campaign

The Imager of Sprites and Upper Atmospheric Lightning (ISUAL), on board the FORMOSAT-2 satellite launched in 2004, is the first instrument primarily dedicated to transient luminous event (TLE) surveys from space. The ISUAL TLE geolocation accuracy and detection efficiency are validated in this paper through two coordinated observation events using ISUAL and the Taiwan TLE ground campaign, the first coordinated space and ground observations. Due to the long distances between TLEs and the observation stations, triangulation required taking the Earth’s curvature into account using spherical trigonometry. After a series of systematic triangulation procedures, the results indicate that the coordinated ISUAL and ground station geolation accuracy is less than 7 km. Moreover, three columniform sprites were recorded at multiple sites during the ground campaign. The triangulation of individual elements shows that the map projection of columniform sprites is nearly circular, parallel and fan shaped. The aforementioned events demonstrate that the columniform sprite distribution patterns are diverse. The average base altitude of the columns is 72.4 ± 2.4 km, and the average terminal altitude is 82.3 ± 1.8 km. The sprite column width is estimated to be < 0.5 km. Article history: Received 22 February 2016 Revised 12 September 2016 Accepted 21 September 2016

ISUAL Imager and far-ultraviolet spectrophotometer degradation

The Imager of Sprites and Upper Atmospheric Lightning (ISUAL) has been operated for more than ten years. The onboard spectrophotometer channel-1 (SP1) was designed to detect the far-ultraviolet (FUV) emissions (N2 Lyman-Birge-Hopfield band) from space. It was found that most of the transient luminous events (TLEs), such as sprites, halos, elves, and gigantic jets, are usually accompanied by FUV emissions. The FUV emission could be utilized as an indicator for searching TLEs because the FUV emission of lightning is severely attenuated in the atmosphere during propagation, although the TLE FUV emissions are still detectable. Furthermore, the elves FUV emission photon flux can also be used to estimate the parent lightning peak current. Therefore, SP1 sensitivity degradation assessment, which affects the observed intensity of FUV emissions, is important. Although ISUAL has a lightemitting diode (LED) which is preset as a standard light source, the emission wavelength range of the LED does not cover the FUV band. We established a method in this study to determine the ISUAL Imager and SP1 sensitivity degradation. We found that the annual degradation rates of the ISUAL Imager and the SP1 are 8.4 and 9.7%, respectively. Article history: Received 29 February 2016 Revised 19 August 2016 Accepted 26 August 2016