Isual Publications 2014

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200212 1 Mende, S. B., H. U. Frey, R. L. Rairden, H. T. Su, R. R. Hsu, T. H. Allin, T. Neubert, E. A. Gerken and U. S. Inan ( 2002 ), Fine structure of sprites and proposed global observations, Cospar Colloquium Series, Volume 12:Space Weather Study using Multipoint Techniques, 275-282, (SCI, IF=, RANK=, Cited=)

In order to understand sprite processes we, have to explain the phenomena from spatial scales of a few meters to the scale of thunderstorm cells. The intricate small-scale vertical structuring of sprites or the so called beads are particularly difficult to understand. From a two-station triangulation featuring observations from Kitt Peak, Arizona and Socorro, New Mexico, it was possible to make high resolution observations of the sprite structure when the sprite events occurred within the field of view of the narrow field imager. In several cases the lower altitude luminous filamentary structures of columniform sprites (C sprites) consisted of slant directed, nearly vertically aligned columns of intense pinpoint like beads. The distance of the sprites from the observer was measured and the altitude and vertical spacing of the beads were estimated. The distribution of beads showed that the most frequently observed bead spacing is between 0.6 and I km. The vertical and horizontal size of the bright luminous beads was about 80 m or less. The bead spacing showed a trend to increase with altitude and the e folding distance or attitude "scale-height" of bead spacing was found to be 20 and in another case 25 km. In order to make systematic observations of the large-scale sprite morphology a satellite based instrument the Imager for Sprites and Upper Atmospheric Lightning (ISUAL) instrument is planned to fly on the Taiwanese satellite, ROCSAT 2. The instrument will consist of an imager and two bore-sighted photometers. The imager will locate the sprites near the earth limb and make global synoptic measurements while the photometers will measure the spectral and temporal properties of sprites and other upper atmospheric luminous phenomena in a number of different wavelength regions uninhibited by atmospheric absorption.

200212 2 Su, H. T., R. R. Hsu, A. B. Chen, S. F. Chen, S. B. Mende, R. L. Rairden, T. H. Allin and T. Neubert ( 2002 ), Observation of angel sprites, Cospar Colloquium Series, Volume 12:Space Weather Study using Multipoint Techniques, 289-294, (SCI, IF=, RANK=, Cited=)

Angel sprites are column-like sprites, which are capped with diffuse hair regions and trailed by tendrils. This paper presents some interesting properties of the angel sprites observed in the SPRITES'99 campaign from Kitt Peak, Arizona. From the observed images, it is noted that there are faint columniform glows preceding the angel sprites, but not for other types of sprites. For the angle sprites, the top of the leading glows and the top of the tendrils roughly match in altitude, but the leading glows extend to lower altitude below the tendrils. At its brightest stage, the sprite streamers are topped with diffused hair regions and trailed by branches of luminous beads and smaller, fainter and slant side branches. These luminous beads and the side branches are the main components of the tendrils. Some sprites occurred at the same location separated in time by hundreds of milliseconds to a few minutes. In these recurrent sprite events, the earlier sprites seem to have some influence on the dynamical behavior of the later sprites.

200202 3 Su, H. T.*, R. R. Hsu, A. B. C. Chen, Y. J. Lee and L. C. Lee ( 200202 ), Observation of sprites over the Asian continent and over oceans around Taiwan, Geophys. Res. Lett., Volume 29, Issue 4, 1044, (SCI, IF=3.982, RANK=11/172,GEOSCIENCES, MULTIDISCIPLINARY, Cited=18)

Sprites were observed over thunderstorms in the southern China and in oceans around Taiwan. The observation sites were on the Ali Mountain of Taiwan's Central Ridge area with an altitude of 2413 m and in the campus of National Cheng Kung University with an altitude of 50 m. For the observed land sprites, 90% of them were either carrots or columniforms and 64% of the sprites occurred in groups. Among the observed oceanic sprites, 89% of them were carrots but only 22% of the sprites occurred in groups. We define a sprite active system as a thunderstorm that continuously produces at least one sprite in a 10-minute interval. The active sprites generating periods for the observed thunderstorms were typically shorter than 30 minutes. The sprite production rates for these Asian thunderstorms are estimated to be between I approximate to 2 x 10(-4) events/km(2)/hr and I approximate to 1 x 10(-3) events/km(2)/hr.

200303 4 Chern, J. L., R. R. Hsu, H. T. Su*, S. B. Mende, H. Fukunishi, Y. Takahashi and L. C. Lee ( 200303 ), Global survey of upper atmospheric transient luminous events on the ROCSAT-2 satellite, J. Atmos. Sol.-Terr. Phys., Volume 65, Issue 5, 647-659, (SCI, IF=1.417, RANK=38/76,GEOCHEMISTRY & GEOPHYSICS, Cited=53)

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 Taiwan's 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.

200303 5 Hsu, R. R., H. T. Su*, A. B. Chen, L. C. Lee, M. Asfur, C. Price and Y. Yair ( 200303 ), Transient luminous events in the vicinity of Taiwan, J. Atmos. Sol.-Terr. Phys., Volume 65, Issue 5, 561-566, (SCI, IF=1.417, RANK=38/76,GEOCHEMISTRY & GEOPHYSICS, Cited=12)

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, 200 1, 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. (C) 2003 Elsevier Science Ltd. All rights reserved.

200306 6 Su, H. T.*, R. R. Hsu, A. B. Chen, Y. C. Wang, W. S. Hsiao, W. C. Lai, L. C. Lee, M. Sato and H. Fukunishi ( 200306 ), Gigantic jets between a thundercloud and the ionosphere, Nature, Volume 423, Issue 6943, 974-976, (SCI, IF=38.597, RANK=1/56,MULTIDISCIPLINARY SCIENCES, Cited=92)

Transient luminous events in the atmosphere, such as lighting-induced sprites(1-8) and upwardly discharging blue jets(9-14), were discovered recently in the region between thunderclouds and the ionosphere. In the conventional picture, the main components of Earth's global electric circuit(15,16) 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 Earth's 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 similar to16 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(17,18).

200504 7 Wang, Y. C.*, K. T. Wang, H. T. Su and R. R. Hsu ( 200504 ), Low-latitude ELF-whistlers observed in Taiwan, Geophys. Res. Lett., Volume 32, Issue 8, L08102, (SCI, IF=3.982, RANK=11/172,GEOSCIENCES, MULTIDISCIPLINARY, Cited=6)

Detections of ELF whistler-like events at a low latitude location are reported. Events with frequencies between 60 and 100 Hz were recorded by the ELF station at the Lulin Observatory in Taiwan from August 26, 2003 to July 13, 2004. The most distinguished feature for these events is the frequency descent in the frequency-time spectrograms, resembling terrestrial whistlers. Other notable features include (a) a long event duration averaging up to two minutes, (b) a daytime diurnal maximum occurring around 10 am, (c) a dominant magnetic field polarization in the north-south direction with strength of a few to tens of pT, and (d) no detection of vertical electric fields. Similar events were only reported twice for the past thirty years: one at an auroral latitude site in Alaska and the other at a mid-latitude site in California. Possible source mechanisms including magnetosheath lion roars and lightning-generated whistlers are discussed.

200506 8 Chen, A. B. C.*,P. S. Chiang,T. H. Huang,C. L. Kuo,S. C. Wang,H. T. Su,R. R. Hsu,M. H. Chang,Y. S. Chang,T. Y. Liu,S. B. Mende,H. U. Frey,H. Fukunishi,Y. Takahashi and L. C. Lee( 200506 ), Analyzing Isual Spectrophotometer Data Using a Two-Color Diagram Method, J. Korean Astron. Soc., Volume 38, Issue 2, 303-306, (SCI, IF=0.909, RANK=42/56,MULTIDISCIPLINARY SCIENCES, Cited=NA)

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.

200506 9 Wang, Y. C.*,R. R. Hsu,H. T. Su,A. B. C. Chen,Y. J. Lee,C. L. Kuo,W. S. Tsay,C. K. Chang,S. C. Wang,L. C. Lee and T. Y. Liu( 200506 ), Ground Observations of Sprites and Other TLEs in Taiwan, J. Korean Astron. Soc., Volume 38, Issue 2, 299-302, (SCI, IF=0.909, RANK=42/56,MULTIDISCIPLINARY SCIENCES, Cited=NA)

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.

200507 10 Frey, H. U.*, S. B. Mende, S. A. Cummer, A. B. Chen, R. R. Hsu, H. T. Su, Y. S. Chang, T. Adachi, H. Fukunishi and Y. Takahashi ( 200507 ), Beta-type stepped leader of elve-producing lightning, Geophys. Res. Lett., Volume 32, Issue 13, L13824, (SCI, IF=3.982, RANK=11/172,GEOSCIENCES, MULTIDISCIPLINARY, Cited=19)

The Imager for Sprites and Upper Atmospheric Lightning (ISUAL) on the Taiwanese FORMOSAT-2 (formerly ROCSAT-2) spacecraft is the first global observatory of transient luminous events (TLEs). During the first 4 months of operation a large number of elves were observed. Elves are obvious with their far ultraviolet (FUV) signature that is less attenuated by atmospheric O2 at their emission altitude compared to emission from sprites or lightning at lower altitude. About half of all elves were produced by lightning that shows a three-step signature in the photometer signal: (1) An initial brightening in all except the FUV channels, (2) a slow brightness decrease for the next 2-5 milliseconds, and (3) an impulsive increase of signal in all channels. We interpret this specific signature as the initial breakdown with a beta-type stepped leader followed by the bright return stroke of a negative cloud to ground (-CG) lightning. In contrast, sprites follow positive cloud to ground lightning (+CG) without a signature of initial breakdown and stepped leader. Many sprites are associated with the continuing current and can be delayed up to 100 ms after the lightning.

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