Isual Publications 2014

COM_FABRIK_ORDERpublish time # Citation abstract
201508 201 Yen-Jung Wu, Earle Williams, Gorden Shephard, Martin Friedrich, Shu-Chun Chang, Jung-Kuang Chou, Alfred Bing-Chi Chen, Han-Tzong Su,Rue-Ron Hsu( 201508 ).Electrical and chemical linkage in the nighttime mesosphere to explain the collocation of OH* Meinel band nightglow and elves, 12 th Annual Meeting of Asia Oceania Geosciences Society, 新加坡, 新加坡. AS08-A013
Elves in the mesosphere produced by intense lightning return strokes in the troposphere are the dominate type of transient luminous event (TLE) and predominate near 85-87 km of altitude. The OH* Meinel band airglow is a conspicuous optical phenomenon in the same height range. By analyzing the limb-viewed images from the Imager of Sprite and Upper Atmosphere Lightning (ISUAL) onboard Formosat-2 in a wavelength range 623-754 nm, it is found that ~70% of elves are collocated in altitude with the OH* nightglow peak brightness within +/- 1 pixel at the mean altitude 85.6 +/- 4km. This collocation has not been discussed extensively and is frequently dismissed as coincidence, since the physical mechanisms for the formation of elves and the OH* Meinel band are macroscopically quite different. Elve radiance requires the presence of free electrons, though not so many as to exclude the lightning-produced electric field by high conductivity. These requirements make the characteristic ledge in nighttime electron density a favorable location for elves. However, the sharp increase in monatomic oxygen density in the same height range as this ledge and the OH* concentration is a hint that the OH* nightglow and the nighttime ledge in electron density are physically/chemically linked. In this study, we will discuss correlations between the key chemical species and the electron density with a focus on their diurnal and annual variations, and also the wavenumber-4 structure in OH* airglow intensity at local midnight. These comparisons of marked changes in key quantities over a narrow range of special altitudes lend support to O being the main player in linking electrical and optical behavior in elve and nightglow emission.
201508 202 Alfred CHEN, Yi-Bei CHEN( 201508 ).The Spatial Distribution of the Discharge Polarity of Lightning and Transient Luminous Events Speaker, 12 th Annual Meeting of Asia Oceania Geosciences Society, 新加坡, 新加坡. AS08-A007

"Transient Luminous Events (TLEs) are triggered by inter-cloud lightning (IC) or cloud to ground (CG) lightning. The polarity of lightning and TLEs have been studied for decades. For example, more than 99% of sprites are initiated by +CG lightning, but the polarity of elve-associated lightning is still unclear now. We can explore the connection between TLEs occurrence and lightning polarity by investigating the spatial distribution of the discharge polarity. Since there is no single-source data providing all necessary information for this study, multiple sources including the magnetic field measurements at extremely low frequency (ELF) band-pass, the database of the World Wide Lightning Location Network (WWLLN) and the ISUAL TLEs list are used to explore the discharge polarities of lightning and TLEs. The polarities of the lightning and TLE events are investigated by an efficient algorithm developed in this study. The results show that the successful detection of lightning polarity can be up to 70% within 3000 km from Taiwan. In this region, -CG lightning accounts for approximately 90% of the total polarity-resolved events. Moreover, the polarity detection rate decays along the distance, as well as the ratio of –CG lightning also drops to about 50%. This result implies that +CG discharge is averagely more energetic than –CG events. The in-depth analysis exhibits that the observed sprite in this period are dominantly triggered by +CG events (95%), only one -CG sprite out of 48 is confirmed and this result is comparable with previous findings. It is surprisingly founded that elves are majorly initiated by –CG lightning (95%), and this result is new and confident because the accuracy of ISUAL onboard trigger time is improved by other ELF study, and the uncertainty of ISUAL event trigger time shrinks from 25 milliseconds to 3 milliseconds. In addition, it is revealed that the density of TLE is proportional to lightning occurrence in the seasonal distribution of lightning and TLEs, but the polarity of lightning seems no significant seasonal variation is observed. This result also suggests that the distribution of TLEs is mainly governed by lightning density and energy, and less correlated with polarity."

201508 203 Kang-Ming PENG, Ruei-Ron HSU, Han-Tzong SU, Alfred CHEN, Jung-Kung CHOU, Shu-Chun CHANG, Yen-Jung WU, Chien-Lun HUNG, I-Ching YANG, Sung-Hui TSAI( 201508 ).Triangulation of Gigantic Jets and Its Applications, 12 th Annual Meeting of Asia Oceania Geosciences Society, 新加坡, 新加坡. AS08-A011
Coordinate optical observation campaign on TLE’s near Taiwan are held since 2011 with the aim to triangulate TLEs. Currently, there are four stations with baseline varying from 100 to 400 km between them. Due to the length of baselines and the TLE occurring locations, the earth curvature needed to be taken into account by means of spherical trigonometry method. The projecting location on the earth, initiating height, branching points and the top of gigantic jets are retrieved through the azimuthal and elevation angle obtaining from the video frames. Our optical observation system recorded 48 various types of TLEs on the night of 20 August 2014, with five of them being gigantic jets that were recorded by at least three stations. The preliminary results shows the gigantic jets occurred in northern part of Taiwan and the accuracy of positioning is less than 10 km. The accuracy of the retrieval height on the key structures could be less than 1 km. In the future the same method will be extended to other gigantic jets recorded in the past years and to elucidate the important properties of GJs.
201508 204 Cheng-Liing KUO, Tai-Yin HUANG, Han-Tzong SU, Ruei-Ron HSU( 201508 ).Numerical Modeling of Sprite Streamers at an Altitude of 70 Km, 12 th Annual Meeting of Asia Oceania Geosciences Society, 新加坡, 新加坡. AS08-A004

We develop a numerical code to study the plasma streamer observed in the sprite events. The plasma streamer of sprites at an altitude of 40-90 km is the ionization wave propagating through the atmosphere. Just after the lightning occurs, the associated electric field could drive the development of plasma streamers in the sprite altitudes. A possible explanation is described as follows. For sources of ionizing by cosmic rays, the electrons and ions will be accelerated in the opposite directions in a background electric field. That causes the positively/negatively charged shell in both the opposite edges of the ionization source. In the positively/negatively charged shell, the local electron field plus background electric field can exceed the breakdown electric field for ionizing gases. The high electric field will ionize more electrons and ions ahead the positively/negatively charged edges. The positively/negatively charged head with a peak electric field will propagate through the atmosphere, like an ionization wave. In this presentation, we will study the chemically active particles behind the ionization wave and estimate the degree of ionization after the passage of streamers. The sprite occurring rate and detailed chemical reactions are poorly known. With the FORMOSAT-2 ISUAL data and our modeling results of the sprite streamers, we try to resolve the problem of chemical and ionization effect by sprites in the middle atmosphere.

201508 205 Tai-Yin HUANG, Cheng-Liing KUO, Chih-Yu CHIANG, Alfred CHEN, Han-Tzong SU, Ruei-Ron HSU( 201508 ).An Overview of Lightning-Induced Airglow Intensity Enhancements, 12 th Annual Meeting of Asia Oceania Geosciences Society, 新加坡, 新加坡. AS08-A001
Previous studies using both observations and modeling approach have shown that airglow intensity of some airglow emissions can be enhanced in the occurrence of lightning. These lightning-induced airglow intensity enhancements were often spotted in the form of elves. In this talk, we will give an overview of these enhancements and discuss the current status of this work. We will first present ISUAL’s narrow-band observations on 630-nm, 557-nm, and 762-nm emissions and the analysis. We will then describe our modeling work and compare our simulation results to the observations. Based on our study, it is found that the observed maximum brightness in all narrow-band filters are larger than the brightness from the model simulations. This suggests that there might be other chemical reactions or processes that have not been accounted for in the model.
201512 206 Yen-Jung Wu, Earle R Williams, Martin Friedrich, Shu-Chun Chang, Jung-Kuang Chou, Alfred Bing-Chih Chen, Han-Tzong Su, Rue-Ron Hsu, Harald U Frey, Yukihiro Takahashi and Lou-Chuang Lee,( 201512 ).Atomic Oxygen as the controlling factor in collocation of Elves and OH* Meinel band nightglow, American Geophysical Union, Fall Meeting 2015, San Francisco, USA. AE33B-0491

The Imager of Sprite and Upper Atmospheric Lightning (ISUAL) onboard the Formosat-2 satellite has monitored Transient Luminous Events (TLEs) and lightning activity within the latitude interval +/- 60o since May 2004. Channel 1 of the Imager with a bandpass from 623 to 754 nm is the channel usually used for recording TLEs. However, it also covers the wavelength range of the OH* Meinel Band nightglow (8,3) in the infrared region, and this circumstance opens a window for us to study the relationship between OH* nightglow and TLEs. The result shows that over 95% of the elves are within +/- 2 pixels in altitude of the brightest OH* emission. The abrupt increase of atomic oxygen with altitude serves to release electrons from O2- thereby providing electrons the opportunity to attach to meteoric dust eventually. Atomic O is also vital to OH* nightglow as the necessary species to make O3. Additionally , from the fundamental reaction , O+ O2- -> O3 + e- , it can be seen that this is not only a key process to move the free electrons to the ablation dust toward making the electron density ledge, but is also an effective way to make O3 for the OH* nightglow. We suggest that the meteoric dust with abrupt appearance in the electron density ledge forms the upper boundary for the VLF waveguide. This circumstance may justify the use of the VLF waveguide cutoff height as a proxy for the dominant altitude of elves. When the global map of such cutoff heights (Toledo-Redondo et al., 2012) and the OH nightglow (WINDII) heights are compared, both show a distinct wavenumber-4 structure in the low-latitude region. The collocation of elves and the OH* nightglow layer is not coincidental and reveals new information about the structure of the D region.

201512 207 Alfred Bing-Chih Chen, Po-Hsun Huang, Han-Tzong Su, Rue-Ron Hsu and Scientific Team of ISUAL Mission( 201512 ).The early ELF signals of the gigantic jets captured by the Taiwan ground observation network, American Geophysical Union, Fall Meeting 2015, San Francisco, USA. AE33C-0506

The in-cloud ignition process of gigantic jets and blue jets receives attentions and discussions in the past years. The polarity and the position of their breakdown were proposed by Krehbiel et al. [2008] but no concrete observational evidence to support it directly. ELF spectrogram is a good tool to explore the electric activities, but traditional spectrograms are generated by a Fourier transform which obtain the frequency information through an integration operation. However the integration greatly limits the lowest frequency revealed by spectrogram and buries the important transient features. In this study, we applied a new but widely-used method, the Hilbert-Huang transform (HHT), to explore the spectrogram. Instead of the integration, HHT obtains the frequency information by differentiating on the phase angle, and become a powerful tool to reveal the fast frequency variation associated with transient luminous events. More than 100 transient luminous events including 25 gigantic jets observed by Taiwan ground optical observation network were analyzed. The results indicate that approximately 70% of gigantic jets can identify a rapid frequency variation in the interval of 300-600 milliseconds before main surge discharge, and this early feature can not find a clear corresponding amplitude variation in its sferic. Since this early signal can not be identified from the traditional Fourier spectrogram, but clear in-cloud lightning was registered correspondingly by the ground optical observation. In contrast to gigantic jets, this feature of early frequency change can be seen only in less than 30% of sprites and elves. These observational evidences are able to provide new constraints on the early discharge process of gigantic jets in clouds.

201512 208 Jung-Kuang Chou, Rue-Ron Hsu, Han-Tzong Su, Alfred Bing-Chih Chen, Cheng-Ling Kuo, Sung-Ming Huang, Shu-Chun Chang, Yen-Jung Wu, Peng Kang-Ming, Harald U Frey, Yukihiro Takahashi and Lou-Chuang Lee( 201512 ).The Associated Sferics of the ISUAL Blue Luminous Events, American Geophysical Union, Fall Meeting 2015, San Francisco, USA. AE33C-0510

ISUAL has recorded many mystic blue luminous events (BLEs) that emit clear middle ultraviolet to blue emissions (230−450 nm) but contain dim red emissions (653−754 nm). Most BLEs appear to be dot-like on the ISUAL images, and a few BLEs develop further into blue jets or type II gigantic jets (GJs). The associated sferics of the BLEs in the extremely low frequency to the very low frequency (ELF/VLF) band and the low frequency (LF) band exhibit similar features to those of the VLF/LF waveform for the narrow bipolar events (NBEs). The ISUAL BLE is conjectured to be the accompanied light emissions from the NBE-like event. Both positive and negative discharge polarity-types for the BLEs have been found. Based on the sferics and the ISUAL optical data, a NBE-like event is found to be related to a rapidly-flowing current of the initiation discharge in the thunderstorm, while a blue jet or a type II GJ is suggested to be associated with the slowly-varying current of the ensuing discharge. The selected six blue jets and one type II GJ occurring within 3,500 km from the NCKU sferics-detecting stations are inferred to be positive upward discharges. The spectral characteristics of BLEs resemble those of the emissions mostly from the non-thermal air plasmas produced by the locally-enhanced electric field, rather than those from the hot air plasmas in local thermodynamic equilibrium (LTE).

201512 209 Peng Kang-Ming, Rue-Ron Hsu, Han-Tzong Su, Alfred Bing-Chih Chen, Jung-Kuang Chou, Shu-Chun Chang, Yen-Jung Wu, Hung Chien-Lun, I-Ching Yang and Sung-Hui Tsai( 201512 ).Triangulation of the Gigantic Jets in 20 August 2014, American Geophysical Union, Fall Meeting 2015, San Francisco, USA. AE33C-0511

Coordinate optical observation campaigns on TLEs near Taiwan are held since 2011 with the aim to triangulate TLEs. Currently, there are four stations with baseline varying from 100 to 400 km between them. Our optical observation systems recorded 48 various types of TLEs on the night of 20 August 2014, with eight of them being gigantic jets that were recorded by at least two stations. Due to the length of baselines and the TLE occurring locations, the earth curvature needed to be taken into account by means of spherical trigonometry method. The preliminary results shows the gigantic jets occurred over the northern Taiwan and the accuracy of geolocation is less than 1 km and the accuracy of the retrieval height on the key structures is less than 0.5 km. The triangulation results of the eight events indicate most of these gigantic jets terminated at 80-90km, but one of the gigantic jets is likely extend to 100 km. Three of the eight gigantic jets occurred consequently after previous one with time interval of 500ms to more than 100s. The previous gigantic jet is likely to influence the consequent gigantic jet for usually the consequent gigantic jet has more beads structures in high altitude and one of the streamer column of a consequent gigantic jets at 55 -60 km is identified to re-bright, which is more than 100s after the previous gigantic jet.

201512 210 Earle R Williams, Yen-Jung Wu, Martin Friedrich, and Rue-Ron Hsu( 201512 ).Meteor Ablation as Origin for the D-region Ledge in Electrical Conductivity, American Geophysical Union, Fall Meeting 2015, San Francisco, USA. AE22A-04

The ledge in electron density and electrical conductivity in the 80-90 km altitude range has been a consistent feature in rocket and electromagnetic observations of the D-region for half a century. Most conspicuous at nighttime and at low latitudes, the abrupt increase in electron density with altitude near 85 km often shows a scale height of less than 1 km. This observed behavior is a marked departure from the Wait-Spies exponential profile that is often used to model the D-region. Calculations show that the conduction current and displacement current are matched at the height of the observed ledge over a wide range of VLF frequencies, pinning this altitude as the sharp boundary for the global VLF waveguide. Meteor ablation involves the abrupt transformation in altitude of faint sub-millimeter-sized meteoroids to nanometer-sized dust, when the meteor boiling temperature near 2100K is attained. The ablation dust can reach concentrations comparable to electron density near the ledge height and in the polluted continental boundary layer. The electron affinity of this silicate mineral dust together with the suppression of negative molecular ions (e.g., O2- ) by monatomic O (Plane et al., 2014), serves to reduce the free electron concentration to form the ledge in conductivity. Calculations with the classical model for meteor ablation require a mean incoming meteor speed of 15 km/s. with rapid decline at higher speeds, to produce a ledge height at 85 km altitude. The key role for meteor ablation in this ionosphere context has likely not received due recognition because neither the meteoric dust nor the meteors that create it are readily detectable by remote sensing, and in situ observations of the mesosphere are scarce.

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