Isual Publications 2014

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200612 61 S. Chikada, H. Fukunishi, Y. Takahashi, T. Adachi, R. R. Hsu, H. T. Su, A. B. Chen, H. U. Frey, S. B. Mende, and L.C. Lee( 200612 ).Property of parent lightning discharges of elves, American Geophysical Union, Fall Meeting 2006, San Francisco, USA.

Elves are optical flashes in the lower ionosphere caused by electro-magnetic pulses (EMP) radiated from lightning discharges, nowadays, being observed by the FORMOSAT-2/ISUAL from the space. More than 3000 elve events have been observed from July 2004 to July 2006. In addition to it, we observe radio waveforms radiated from lightning discharges at the range of ELF (1-100 Hz) and VLF (1k-40k Hz) to obtain characteristics of parent lightning discharges and to distinguish lightning events into Cloud-to-Ground (CG) and Intra-Cloud (IC) lightning. We have investigated the seasonal dependent distribution map of elves observed from July 2004 to June 2005. It is different from that of lightning activity obtained by the OTD instrument. Especially, remarkable point is that continental lightning are more active than oceanic lightning, however, most of elves have occurred over the ocean. In order to investigate a characteristic of lightning inducing elve, we distinguished 22 lightning events observed near the Onagawa observatory into 6 CGs and 16 ICs from ELF data. Then, the ratio of the ISUAL AP data, which we define as AP blue divided by AP red, have maximum value at the initial stage of emission of lightning for the CG case and the decaying stage for the IC case. This result enables us to distinguish almost all lightning events into CG/IC and to investigate requirement for occurrence of elve. We expect that a property of lightning contributing for elve generation is time constant which means fast discharges can radiate intense EMP. So, we will estimate the maximum EMP values from time-derivation of SP or AP data. Here, we assume that current time series is consistent with the emission of lightning. In this presentation, we will suggest the relation between maximum EMP value of parent lightning discharges and elve generation.

200612 62 C. L. Kuo, A. B. Chen, J. K. Chou, L. Y. Tsai, C. Y. Chiang, R. R. Hsu, H. T. Su, and L.C. Lee( 200612 ).Numerical modeling of elves recorded by the ISUAL payload on the FORMOSAT-2 satellite, American Geophysical Union, Fall Meeting 2006, San Francisco, USA.

ISUAL experiment on the FORMOSAT-2 satellite has confirmed the existence of ionization and Lyman-Birge- Hopfield (LBH) band emissions in elves [Mende et al, 2005]. These exciting results testify the advantage of a space-borne experiment on the exploration of elves. In this presentation, we demonstrate the numerical simulation results of elves based on the EMP model [Inan et al., 1996; Pasko et al., 1998; Veronis et al., 1999; Barrington-Leigh, 2001] for the optical emission between 200-800 nm and for the spatial-temporal evolution. To account for the effect of atmospheric attenuation, three major attenuation mechanisms: O2, O3 and molecular Rayleigh scattering are considered. Direct comparison of modeling results and the ISUAL recorded elves will be given. Teoretical and ISUAL integrated photon counts of elves and their correlation with the peak current of the causative CGs, which was recorded by NLDN (National Lightning Detection Network), will also be discussed. This work was supported in part by the National Space Center and National Science Council in Taiwan under grant numbers: 94-NSPO(B)-ISUAL-FA09-01, NSC94-2112-M-006-017, and NSC94-2111-M-006-001.

200612 63 H. U. Frey, S. B. Mende, H. T. Su, R. R. Hsu, A. B. Chen, H. Fukunishi, and Y. Takahashi( 200612 ).Detection of sprites and elves from space through the 762.0 nm filter, American Geophysical Union, Fall Meeting 2006, San Francisco, USA.

One of the filters on the ISUAL CCD-camera is centered at the (0-0) transition of the molecular oxygen atmospheric band at 761.9 nm. It is primarily intended for the observation of gravity wave effects on the airglow emission distribution. This particular emission is absorbed by oxygen molecules in their ground state which makes the lower atmosphere completely opaque to it. For observations from space this behavior suppresses any light at that wavelength coming from the ground like city lights or coming from lightning. Occasionally this filter was also used in the observation mode for transient luminous events (TLEs). TLEs are primarily identified by their spectral signature in the six-channel ISUAL spectrophotometer. However, many of them can then also be identified in the imager data with their signatures above about 60 km altitude. This confirmation has great implications for future space missions which will attempt to observe TLEs through such a filter in a nadir view.

200612 64 A. Yoshida, M. Sato, H. Fukunishi, Y. Takahashi, S. Chikada, T. Adachi, R. R. Hsu, H. T. Su, A. B. Chen, H. U. Frey, S. B. Mende, and L.C. Lee( 200612 ).Improvements in accuracy of ELF lightning geolocation system using ISUAL data, American Geophysical Union, Fall Meeting 2006, San Francisco, USA.

Tohoku University operates worldwide ELF observation network, consisting of 4 observation sites located at Syowa in Antarctica, Onagawa in Japan, Kiruna in Sweden, and California in US. Each sensor records ELF waveforms in the wavelength range of 1-100 Hz continuously. We can estimate the lightning location and charge moments for intense lightning events, which are the important parameters to investigate the characteristics of parent lightning discharges of TLEs such as sprites and jets. However, the estimation error in geolocation was about 2000 km, which causes a serious problem in TLE studies. Here we show the results obtained by a new method which was improved based on ISUAL optical data, considering not only the arrival direction of radio waves but also the precise timing of radio wave arrivals at observation sites. Also the sensitivity ratios between the sensors are corrected empirically. The achieved error of 500 km in geolocation is good enough to estimate the charge moment and to discuss the regional dependence of lightning characteristics. Seasonal and regional variations of TLE occurrence ratios suggest some different characteristics of parent lightning depending on season and/or location. For example, different conductivities between land and ocean could cause different time constant of lightning discharge, which in turn modulate the occurrence of TLEs. The significant improvements in geolocation accuracy enable us to distinguish between the lightning over the land and that over the ocean. We will discuss regional variations of TLE occurrence and forms comparing with the characteristics of parent discharge, such as charge moment and time constant.

200612 65 Y. Takahashi, T. Adachi, H. Fukunishi, S. Chikada, A. Yoshida, K. Yamamoto, R. R. Hsu, H. T. Su, A. B. Chen, S. B. Mende, H. U. Frey, and L.C. Lee( 200612 ).Characteristics of TLEs observed by ISUAL/AP onboard FORMOSAT-2, American Geophysical Union, Fall Meeting 2006, San Francisco, USA.

Tohoku University developed and installed array photometer (AP) as one of ISUAL equipments onboard FORMOSAT-2 satellite, collaborating with National Space Organization (NSPO) and National Cheng Kung University (NCKU) in Taiwan and U.C. Berkeley in United States, in order to get information on the time developments of absolute intensities in transient luminous events (TLEs) including sprites and elves, and parent lightning flashes. AP consists of blue and red photometers with vertical aligned 16 FOVs, which have sensitivity in the wavelength range of 360-470 nm and 520-750 nm, respectively. The sampling rate is 20kHz at maximum, which enable us to reproduce light curves of TLE/lightning flashes at each altitude. From the intensity ratio between blue and red colors (B/R ratio), we can estimate the characteristic electron energy in TLEs, assuming simple maxellian distribution. Tohoku University group has been studying the spectral and spatial characteristics of TLEs with AP and other ISUAL equipments, combining with data sets obtained by ground based facilities, such as the world-wide ELF network constructed and operated by ourselves. In this talk the latest important findings based on the ISUAL analysis are reviewed, including topics listed below: 1) time development of B/R ratio in sprites and halos, 2) seasonal dependence of color of elves which may reflect electron density profile in D/E region, 3) world distribution and optical emission intensities of sprite halos compared with ELF waveforms and charge moments, 4) morphology of elves and relationship to the parent lightning, 5) world distribution of elves and characteristics of parent discharge, 5) time developments spectral characteristics in CG and IC estimated by AP.

200612 66 L. Y. Tsai, C. L. Kuo, Y. J. Chen, Y. C. Chen, Y. J. Lee, J. K. Chou, W. C. Lai, R. R. Hsu, A. B. Chen, H. T. Su, H. U. Frey, and L.C. Lee( 200612 ).2006 Taiwan TLE campaign, American Geophysical Union, Fall Meeting 2006, San Francisco, USA.

Ground campaigns of transit luminous events (TLEs) in Taiwan started from 2001 and were repeated yearly ever since. In 2002, a new type of TLEs called gigantic jet was discovered above a convective system. During 2001-2005, there were six additional gigantic jets and more than six hundreds sprites were documented. In this paper, some interested results and progress of the 2006 Taiwan TLE campaign will be reported: 1. Two gigantic jets, which occurred above a frontal system over Taiwan, were recorded inside the NCKU campus which is located at the center of Tainan metropolitan area with more than one million residents. 2. A few sprites were recorded by a low-light-level CCD equipped with a blue filter which has a cutoff at 500nm. These sprites were very bright and occurred about 150-250km away from the observation site. Therefore, the attenuation of blue emission is less severe. In these blue band images the ionization channels in sprites can be discerned clearly. 3. A low-light level spectrograph was built and has captured two sprite spectra. However, the goal of capturing the spectrum of gigantic jets is yet to be fulfilled. 4. ISUAL images indicated that blue jets and gigantic jets may occur preferably above convective systems near typhoon. A compact imaging system was set up and deployed on an airplane operated by the Taiwan DOTSTAR projects. Only one nightly run was carried out this year and no event is recorded.

2007 67 C. L. Kuo, A. B. Chen, R. R. Hsu, H. T. Su, L.C. Lee, S. B. Mende, H. U. Frey, H. Fukunishi, and Y. Takahashi( 200700 ).Analysis of ISUAL recorded gigantic jets, Workshop at Leiden University (2007), Leiden, Nederland.
200705 68 T. Y. Huang, C. Y. Chiang, C. L. Kuo, J. B. Nee, A. B. Chen, H. T. Su, and R. R. Hsu( 200705 ).Investigations of the Observed Lightning Induced enhancements at the OH nightglow altitude during the ISUAL January 2007 Campaign, 2007 TGA, Taoyuan, Taiwan.

Observations of TLEs by ISUAL broadband filter onboard FORMOSAT-II have sometimes shown an enhancement accompanied by lightning activity at the OH nightglow altitude. Due to too much overlapping of OH and N2 spectrum within the bandwidth of the broadband filter, it was not possible to determine whether the enhancements were elves or lightning induced sudden OH brightness. Simulations of the spectrum of elves (Kuo et al., 2006) reveal that N2 1P makes very small contribution to the intensity at 630 nm wavelength. The simulations of sprite-induced OH nightglow emission (Huang, 2006) have indicated that the column-integrated OH intensity could be enhanced quite significantly when lightning occurs. In response to the need to discern the causes for such enhancements, ISUAL conducted a 9-day campaign in January 2007, with an orbit devoted exclusively to observing TLEs with the 630 nm filter for such an investigation. Some of the observations did show some enhancement in the images, however, the enhancements were not as strong as those observed by the broadband filter. We will present the analysis of the observations and discuss the implications of the results.

200705 69 W. S. Chen, J. Y. Liu, P. K. Rajesh, A. B. Chen, R. R. Hsu, H. T. Su, and L.C. Lee( 200705 ).The limb observation of ionospheric 630 nm airglow by using FORMOSAT-2/ISUAL, 2007 TGA, Taoyuan, Taiwan.

The newest limb observation of high altitude emission from FORMOSAT-2/ISUAL is reported in this study. The FORMOSAT-2/ISUAL is located in 891 km altitudes to observe 630 nm emissions along 125°E longitudes during December 18-21, 2006. The four day observations show that the two layers existence around 90 and 250 km altitudes. The characteristics of the layers and simulation results suggest that the lower one is OH emission, and the higher one is caused by OI emission. Moreover, the simulation recommends that luminous sources are mostly around the tangent height plane. The OI emission shows 2-days variations during observations, and it possible relates to quasi-2-day (QTD) oscillations. Moreover, the unusual emission altitudinal extension is founded. The simulation suggests that it is partially caused by overlap with high altitude emission in other longitude.

200705 70 T. F. Chang, C. Z. Cheng, C. Y. Chiang, A. B. Chen, R. R. Hsu, H. T. Su( 200705 ).FORMOSAT-2 satellite Observations of auroral arcs and auroral substorms, 2007 TGA, Taoyuan, Taiwan.

The FORMOSAT-2 satellite was launched into a polar orbit on May 21, 2004 and the scientific payload, ISUAL (The Imager of Sprites and Upper Atmospheric Lightnings ), has been used to investigate auroral arcs and auroral substorms. With an altitude of 891km and an inclination of 99.1 degree, it takes images of optical phenomena from the sideway. We analyzed CCD imager data and choose 557.7nm and 630.0nm as our filter wavelengths. The 557.7nm green transition line of atomic oxygen from the 1S state to 1D state has life-time of 0.74s and is mainly produced by energetic electrons with energy≧ 1KeV. The 630nm red-line emission is the atomic oxygen transition from the 1D state to 1P state (triplet) with a life-time of 110s and is mainly produced by electrons with energy ~O(100eV). The 630nm red auroral emission is more diffuse than the green emission at 557.7nm. Several events of auroral arcs and auroral substorms were analyzed. Typically the observation lasted about 3 minutes, and it covered ~ 1200 km wide by limb viewing. For auroral arcs, there is a thin bright enhanced layer at the bottom of the aurora arc. In the bright bottom layer, there are equally spaced brighter spots streaming from east to west. The separation between brighter spots is about 1 degree. We have also observed a variety of auroral forms, such as, substorms, vertex, etc. The conditions of aurora observations will be described and some explanations of observed auroral arcs and substorms will be given.

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