C. L. Kuo

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.

Further investigations of lightning‐induced transient emissions in the OH airglow layer

[1] A previous study of lightning-induced transient emissions in and below the OH airglow layer using observations by the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) CCD camera onboard the FORMOSAT-II satellite showed that intensity enhancements occurred more frequently in the OH airglow layer. Here we show the results of new observations made in December 2009 and January 2010 using a narrowband 630 nm filter and spectrophotometer and present further analysis. We estimated the N 2 1P intensity enhancements to be ~65% and 53% of the total intensity enhancements for the two events we analyzed using ISUAL and the spectrophotometer data in conjunction with a model for emissions of light and VLF perturbations from electromagnetic pulse sources (elves). Our analysis indicates that there is still somewhat considerable intensity enhancement (~1.25 kR) unaccounted for after the N 2 1 P contribution has been removed. Our study suggests that there might be OH emissions in elves and that OH species might also be involved in the lightning-induced process and contribute to the intensity enhancements that we observed.

ANALYZING ISUAL SPECTROPHOTOMETER DATA USING A TWO-COLOR DIAGRAM METHOD

Transient luminous events (TLEs; sprites, elves, jets and etc.) are lightning-related optical flashes occurring above thunderstorms. Since the first discovery of sprites in 1989, scientists have learned a great deal about the morphological, spectroscopic and electromagnetic characteristics of TLEs through ground and spacecraft campaigns. However, most of the TLE studies were based on events recorded over US High Plains. To elucidate the possible biasing effects, space-borne observations are needed and have their merits. Imager of sprites and Upper Atmospheric Lightning (ISUAL) on the FORMOSAT-2 satellite is the first instrument to carry out a true global measurement of TLEs from a low- earth orbit. In this short paper, we apply a common astronomical data analysis technique, two-color diagram, on the ISUAL spectrophotometer (SP) data. By choosing appropriated bandpasses and converting the measured flux of TLEs into the unit of magnitude, two-color diagrams of TLEs can be constructed. We demonstrate that two-color diagrams, which were constructed from the narrow-band spectrophotometer data, can be used to classify different types of TLEs and trace their temporal evolution. The amount of reddening due to Earth`s atmosphere can also be estimated from two-color diagrams assembled from the broad-band spectrophotometer data.

GROUND OBSERVATIONS OF SPRITES AND OTHER TLES IN TAIWAN

Sprites, elves and blue jets are collectively denominated as the upper atmospheric transient luminous events (TLEs). They are recently discovered optical flashes between active thunderstorms and the ionosphere. In this report, a brief introduction to the most important characteristics of TLEs is given. Since 2001, scientists from the National Cheng Kung University have been performing yearly summer campaigns from various locations in Taiwan. The main achievements of their yearly campaign are presented.

Reply to comment by B. E. Prokhorov and O. V. Zolotov on “An improved coupling model for the lithosphere-atmosphere-ionosphere system”

The currents flowing between the Earth and ionosphere are numerically calculated. We solve the current continuity equation by two methods: the first method is to solve ϕ in ∇ ⋅ J = ∇ ⋅ (σ∇ϕ) = 0[Kuo et al., 2011] where σ is atmospheric conductivity and ϕ is the electric potential, and the second method is to solve the current potential Ψ in ∇ ⋅ J = − ∇2Ψ = 0by assuming J = − ∇Ψ, a curl-free current density [Kuo et al., 2014]. Two solutions of ∇ ⋅ J = 0for current flow in the atmosphere between Earth surface and ionosphere are compared in this reply. The differences of two results are <4%, which may be partly caused by the numerical errors in the presence of large variations in the σprofile. The current potential can provide a good approximation for upward currents that flow into the ionosphere. In addition, we compare two models for the current flow from lithosphere to atmosphere: (a) two charge layers embedded in the lithosphere, and (b) a dynamo in the lithosphere. The presence of a dynamo in the lithosphere for Model (b) can lead to the presence of current and electric field in the atmosphere and hence in the ionosphere. In Model (a), the electrostatic charge layers in the lithosphere cannot lead to the presence of electric field in the atmosphere.

Simultaneous observations of storm‐generated sprite and gravity wave over Bangladesh

We report simultaneous observations of sprite and gravity wave generated by a storm over Bangladesh. The origin of a concentric gravity wave can be traced to the storm region on Apr 27, 2014 over Bangladesh with a low cloud top surface temperature (175 K). After data analysis, the time period of the concentric gravity wave is found to be 8.8-8.9 minutes. The horizontal wavelength is found to be ~50 km for red emissions (~55 km for green emissions) and the horizontal phase velocity is 94.4 ± 31.7 m s-1 for red emissions (102.6 ± 29.4 m s-1 for green emissions). Using the dispersion relation of gravity wave, the elevation angle of wave propagation direction is found to be ~ 53.3 °. The sprite associated with the gravity wave was also recorded at 1534 UT on Apr 27, 2014. The initiation time of storm-generated gravity wave is estimated to be 1454 UT at which lightning activity was relatively low using lightning data. At time 1534 UT of the recorded sprite, the lightning rate was close to its maximum value. The storm-generated gravity wave could be thought as a precursor phenomenon for lightning and sprites since one of the necessary conditions for gravity wave, lightning and sprites is strong convection inside storms.

Investigations of Lightning‐Induced Sudden Brightening in the OH Airglow Layer Observed By ISUAL Onboard FORMOSAT‐II Satellite

We report observations of lightning‐induced transient emissions (LITEs) in or below the OH airglow layer observed from the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) onboard the FORMOSAT‐II satellite. The ISUAL team conducted a nine‐day campaign in January 2007 using both the broadband and narrowband filters on the CCD imager. It is found that LITEs occur more frequently in the OH airglow layer than below. It is also found that the observations of LITEs below the OH airglow layer by the narrow‐band filter are rare. The observations of LITEs by the narrowband filter are the first of its kind in existence. Our data analysis shows that there is a significant intensity increase in the OH airglow layer when there is lightning and a residual intensity increase post the lightning activity. We propose that LITEs in the OH airglow layer could be due to OH species.

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