Rue-Ron Hsu

Gigantic jets between a thundercloud and the ionosphere

Transient luminous events in the atmosphere, such as lighting-induced sprites and upwardly discharging blue jets, were discovered recently in the region between thunderclouds and the ionosphere. In the conventional picture, the main components of Earths global electric circuit include thunderstorms, the conducting ionosphere, the downward fair-weather currents and the conducting Earth. Thunderstorms serve as one of the generators that drive current upward from cloud tops to the ionosphere, where the electric potential is hundreds of kilovolts higher than Earths surface. It has not been clear, however, whether all the important components of the global circuit have even been identified. Here we report observations of five gigantic jets that establish a direct link between a thundercloud (altitude ∼16 km) and the ionosphere at 90 km elevation. Extremely-low-frequency radio waves in four events were detected, while no cloud-to-ground lightning was observed to trigger these events. Our result indicates that the extremely-low-frequency waves were generated by negative cloud-to-ionosphere discharges, which would reduce the electrical potential between ionosphere and ground. Therefore, the conventional picture of the global electric circuit needs to be modified to include the contributions of gigantic jets and possibly sprites.

Global survey of upper atmospheric transient luminous events on the ROCSAT-2 satellite

Abstract Upper atmospheric transient luminous events (TLEs; sprite, elves, blue jet, etc.) are recently discovered thunderstorm-induced phenomena. Imager of sprites/upper atmospheric lightning (ISUAL) is a scientific payload on the Taiwans ROCSAT-2 satellite that aims primarily to provide crucial observation data on these TLEs from space. The ISUAL payload includes an intensified CCD imager, a six-channel spectrophotometer, and two array photometers. All the instruments are mounted on a common platform and boresighted in the same direction. The imager is equipped with six selectable filters, which have bandpasses covering the visible spectrum. The spectrophotometer contains six photometers, and each photometer is fitted with a special bandpass filter ranging from ultraviolet to red regions. The two array photometers are identical in every aspect, except one is fitted with a blue band filter and another one is equipped with a red band filter. With this set of well-chosen instrument, this project seeks to determine the location and timing of upper atmospheric transient events above thunderclouds, to investigate their spatial, temporal and spectral properties, to obtain a global survey of them, and to perform a global study of aurora and airglow. ISUAL project is an international collaboration supported by the National Space Program Office in Taiwan, with additional contributions from the National Cheng Kung University, Taiwan, the Space Science Laboratory of the University of California at Berkeley, and Tohoku University, Japan. ROCSAT-2, the platform of ISUAL, is scheduled to launch around October 2003.

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.

Modeling elves observed by FORMOSAT‐2 satellite

Received 18 March 2007; revised 20 July 2007; accepted 31 July 2007; published 22 November 2007. [1] The ISUAL experiment on the FORMOSAT-2 satellite has confirmed the existence of ionization and Lyman-Birge-Hopfield (LBH) band emissions in elves. In this paper, an in-depth study of the ISUAL recorded elves was carried out. Numerical simulation results of elves based on an electromagnetic finite difference time domain (FDTD) model of the emissions between 185–800 nm and of their spatial-temporal evolution are presented. To account for the effect of atmospheric attenuation, three major attenuation mechanisms: O2 ,O 3, and molecular Rayleigh scattering are considered. Validations of the electromagnetic FDTD model were conducted in three ways: by comparing the calculated and observed photon fluxes in the ISUAL spectrophotometric channels, by directly comparing the simulated and observed morphologies of elves, and by comparing the computed photon counts of the ISUAL Imager based on the derived peak currents for two elve-associated NLDN (National Lightning Detection Network) cloud-to-ground discharges (CGs) with those recorded by the ISUAL Imager. In all three ways, very good agreement was achieved.

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.

Electric fields and electron energies in sprites and temporal evolutions of lightning charge moment

The fundamental electrodynamical coupling processes between lightning and sprites are investigated. By combining the observed spectral data with the Monte Carlo swarm experiments, reduced electric fields and electron energies in sprite streamers and halos are estimated. The obtained fields inside sprite halos (70‐97 Td with an analysis error of ±5 Td) are lower than the conventional breakdown field, Ek ∼ 128 Td, indicating a significant reduction of electrons associated with halos while those in sprite streamers (98‐380 Td with an error of ±50 Td) are higher than Ek, suggesting that a significant ionization process drives their formation and development. A combined analysis of photometric and electromagnetic data makes it possible to estimate temporal evolutions of lightning charge moment. It is found that lightning discharges with a short time scale (∼1 ms) and a moderate amount of charge moment (∼400 C km) produce discernible halos. On the other hand, lightning discharges with a large amount of charge moment (∼1300 C km) produce streamers regardless of their time scale. The results obtained are comprehensively interpreted with both the conventional breakdown field necessary for the formation of streamers and the electric field necessary for the production of optical emissions of halo which is sensitive to the time scale of the thundercloud field due to the significant reduction of electrons.

Broadband very low frequency measurement of D region ionospheric perturbations caused by lightning electromagnetic pulses

[1] Prompt early/fast perturbations on narrowband sub-ionospherically propagating very low frequency (VLF) signals are the primary evidence for the direct coupling of energy released by lightning discharge to the lower ionosphere. Different mechanisms have been advanced to explain the fast ionospheric perturbations, such as heating and ionization from the lightning electromagnetic pulse (EMP) associated with elves or from quasielectrostatic fields associated with sprites and halos. By comparing the broadband VLF spectra (3-25 kHz) of lightning discharges that shortly followed high peak current lightning discharges with the spectra of lightning discharges that did not, we detect D region perturbations caused by these intense lightning strokes over the U.S. East Coast and the U.S. High Plains. The electron density changes are measured by analyzing the broadband VLF propagation changes, and the perturbed electron density profiles from both regions are found to be consistent with those theoretically predicted for strong lightning EMP. In one case, a D region perturbation was detected following a lightning stroke that produced an isolated elve recorded by the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) instrument on the FORMOSAT-2 satellite, confirming the EMP origins of these ionospheric perturbations. For this case, we measure electron density enhancements of 460 cm -3 averaged over a 220-km radius and 10-km-high perturbation region, in good agreement with the 210 cm -3 measured optically by Mende et al. (2005) for a different elve event. The characteristics of the lightning responsible for these ionospheric perturbations are investigated by comparing high peak current lighting strokes that do and do not generate detectable ionospheric perturbations.

Simultaneous radio and satellite optical measurements of high-altitude sprite current and lightning continuing current

Received 21 April 2006; revised 17 July 2006; accepted 18 July 2006; published 19 October 2006. [1] We report coordinated measurements of lightning and resulting sprites using ground-level magnetic field sensors (<0.1 Hz to 30 kHz bandwidth) and the ISUAL instrument on the FORMOSAT-2 satellite. These measurements demonstrate two distinct elements of the connection between the radio and optical emissions. First, the quasi-static magnetic field signature is tightly correlated with the low-altitude optical emissions from the lightning flash, indicating that this radio signature is produced by continuing lightning current. Second, in two events with strong postreturn stroke extremely low frequency (ELF) magnetic pulses, the optical emissions demonstrate that there are no observable intensifications of low-altitude optical emissions associated with those pulses. If they were produced by a lightning process, such as an M-component, the connection between optical emissions and current seen in the return stroke and the continuing current suggests they should be visible. However, as has been observed previously, the bright, high-altitude optical emissions associated with the sprite are simultaneous with the ELF pulse. This is strong evidence that these ELF pulses originate in high-altitude electric current in the sprite itself and are not produced by a low-altitude lightning process.

Transient luminous events in the vicinity of Taiwan

Abstract Sprites were observed over Asian continent and over oceans around Taiwan in the summer of 2001 (Geophys. Res. Lett. 29(4) 2002). In this article, we report some characteristic differences between the oceanic and the land sprites. Qualitatively, the oceanic sprites are tended to be brighter than the land sprites. Also some of the oceanic sprites have very peculiar forms, which do not match any of the existing types. In two of the recorded sprites, the diffuse hair region contains a distinct short streak. We suspect that they probably are tracks left behind by micrometeorites, which might also have helped in lowering the threshold of the electric field needed for sprite generation and produced exceptionally bright sprites. For the optical survey, one of the observation sites was situated in the campus of the National Cheng Kung University, which is located at the center of the Tainan metropolitan area with more than one million residents. However, on June 7, 2001, we were able to recorded 6 oceanic sprites from this highly light-polluted site. Our experience exemplifies the exceptional brightness of the oceanic sprites. The Asian TLEs recorded in this survey were observed to occur above thunderstorms at stationary fronts and over localized thunderstorms. The sole attempt to observe TLEs over typhoon Chebi failed, hence, whether they exist over tropical storms in the Asian region is yet to be confirmed.

SPACECRAFT BASED STUDIES OF TRANSIENT LUMINOUS EVENTS

The Imager of Sprites and Upper Atmospheric Lightning (ISUAL) is a scien- tific payload on Taiwans FORMOSAT-2 (previously known as ROCSAT-2) that provides new observations of transient luminous events (TLEs) from space. The ISUAL project is an international collaboration between the National Cheng Kung University, Taiwan, Tohoku University, Japan and the instrument devel- opment team from the University of California, Berkeley. The project was sup- ported by the National Space Program Office in Taiwan. The ISUAL payload includes a visible wavelength intensified CCD imager, a boresighted six wave- length spectrophotometer, and a two channel Array Photometer (AP) with 16 vertically spaced horizontally wide sensitive regions. The imager is equipped with 5 selectable filters on a filter wheel and a 6 th open position. The spec- trophotometer contains six filter photometer channels, their bandpasses ranging from the far ultraviolet to the near infrared regions. The two channel AP is fitted with broadband blue and red filters. The orbiting platform with this set of instru- ments will provide the first comprehensive global latitude and longitude survey of TLEs near the midnight local time region. One of the great advantages of spaceborne observations is the lack of the intervening atmosphere between the TLEs and the observer. Ground based observations are often adversely affected by clouds, atmospheric extinction or scattering whereas the space-borne ISUAL instrument measurements provide true emission ratios unobstructed by the vari- able atmospheric extinction. The channels of the spectrophotometer channels cover the far and mid ultraviolet in addition to channels that respond to various excitation levels of the neutral and ionized nitrogen molecule atmosphere and to emissions from oxygen. The preliminary data shows that the ratio of the emis- sions is highly variable during the lightning and the associated TLEs. The data is qualitatively consistent with harder characteristic energy electron production