Isual Publications 2014

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200607 51 H. U. Frey, S. A. Cummer, S. B. Mende, H. Fukunishi, Y. Takahashi, T. Adachi, H. T. Su, R. R. Hsu, and A. B. Chen( 200607 ).Sprite halos created by negative cloud to ground lightning in Central America, 2006 Western Pacific Geophysics Meeting, Beijing, china.

Sprite halos are disk-shaped, unstructured transient luminous events (TLE) that occur in association with lightning. The Imager of Sprites and Upper Atmospheric Lightning (ISUAL) on FORMOSAT-2 recorded imager and photometer data on these events, with photometer data spanning the range from far-ultraviolet to near-infrared. About 40 sprite halos that occurred in Central America could be related to simultaneously recorded ELF/VLF data. All of them were created by negative cloud to ground lightning (-CG) almost exclusively over the open water of the Caribbean Sea or the Eastern Pacific ocean. We will present their geographic distribution and spectral signatures, that distinguish them from positive CG created sprites.

200607 52 Y. Takahashi, S. Chikada, H. Ohya, T. Adachi, K. Yamamoto, H. Fukunishi, H. T. Su, A. B. Chen, and R. R. Hsu( 200607 ).Characteristics of elves and D-region structures, 2006 Western Pacific Geophysics Meeting, Beijing, china.

Elves is one of the transient luminous events (TLEs) in the upper atmosphere induced by electromagnetic pulse from lightning dischrage, which would cause significant heating and ionization of the atmosphere. In order to examine their chemical and electromagnetic effects on the lower thermosphere/ionosphere quantitatively, it is required to measure the spectral characteristics of elves with absolute scales as well as its size and forms. Spacecraft is the ideal platform to realize the well-calibrated spectral measurement. ISUAL instruments onboard FORMOSAT-2 satellite, launched in May 2005, provided us the first opportunity to measure the absolute optical characteristics of TLEs with sufficient high time resolution. The array photometer (AP), one of three ISUAL instruments, is capable of measurements of absolute luminosity with 16 FOVs aligned in vertical at two wavelength ranges, namely, 360-470 nm for N2 2nd Positive and N2 1st Negative bands and 520-750 nm for N2 1st Positive and N2 Meinel bands. The intensity ratio of those two colors reflects the energy distribution of electrons produced in the elves by impulse electric field from parent lightning discharge in the troposphere. ISUAL already captured over 2000 elves. We selected out few tens of elves events from the events captured by ISUAL imager, on the condition that they have enough intensity both in blue and red channels without saturations or contamination by cloud flash. It is also required that the whole emissions of elves are in the coverage of APfs FOV and that the parent CG/IC discharges are identified by our ground ELF network. We examined the relationships between the intensity ratio and intensity of each channel, ELF power and geographical location. We couldnft find out any strong relationship for them. On the other hand, the high blue/red ratio events are likely to occur in the summer hemisphere while the low blue/red ratio events in the winter hemisphere. We examined the electron density profile data obtained by MF radar located at Yamagawa, Japan. The profile shows strong seasonal variations, namely, in summer season from June to August the averaged altitude of the lower boundary of D region decreases to 80-85 km near local midnight while in other seasons it is about 95 km. We consider this could be a main reason for the color variation of elves.

200607 53 T. Adachi, H. Fukunishi, Y. Takahashi, K. Yamamoto, H. T. Su, and R. R. Hsu( 200607 ).Occurrence conditions of sprite streamers and halos derived from ISUAL measurements and ELF magnetic field measurements, 2006 Western Pacific Geophysics Meeting, Beijing, china.

In order to clarify the essential parameters of lightning discharges in the production of sprite streamers and halos, we analyze optical data observed with the ISUAL payload onboard the FORMOSAT-2 satellite and magnetic field data observed with the ground-based ELF network system. The ISUAL consists of an imager, an array photometer, and a spectrophotometer. We primarily analyze data obtained from the array photometer which possesses two photometers measuring the two wavelength ranges of 360-470 nm and 520-750 nm. Each photometer has a vertical resolution of 8-13 km and a temporal resolution of 50 or 500 microseconds The ELF magnetic field observation system is installed at three sites: Syowa station ($39.50\deg$E, $69.018\deg$S), Onagawa observatory ($141.483\deg$E, $38.433\deg$N), and Esrange observatory ($21.100\deg$E, $67.883\deg$N). These observation systems measure electromagnetic waves in the frequency range of 1-100 Hz which are radiated from lightning discharges. We analyze 77 sprite events observed during the period from July 4 2004 to November 20 2005. Based on the imager data, we classify these sprite events into three categories: 15 halo events, 29 streamer events, and 23 halo with streamer events. The average charge moment values of causative lightning discharges in halo, streamer, and halo with streamer events are 400, 1100, and 1200 C-km respectively. It is clear that the halo events have smaller charge moment values compared with other two categories. In addition, we estimate the intensity of reduced electric fields producing sprites from blue/red emission ratios measured with the array photometer. The estimated values at the initiation of halos and streamers are 60-80 Td and 100-260 Td, respectively. Since the ionization rate coefficient exceeds the attachment coefficient at an electric field larger than the conventional breakdown field of $\sim$128 Td, strong ionization processes would occur in the streamer events while ionization would be negligible in the halo events. In the presentation, we will report furthermore on the brightness of sprites and halos and the electrical features of causative lightning discharges such as the polarity and time constant.

200607 54 K. Yamamoto, H. Fukunishi, M. Sato, Y. Takahashi, T. Adachi, R. R. Hsu, H. T. Su, A. B. Chen, H. U. Frey, S. B. Mende, and L.C. Lou( 200607 ).Characteristics of sprite-inducing lightning discharges and their global distribution, 2006 Western Pacific Geophysics Meeting, Beijing, china.

Sprites are recently discovered optical evidence of electrical discharges in the mesosphere and lower ionosphere induced by cloud-to-ground lightning discharges (CGs). We also used the optical data obtained from the ISUAL (Imager of Sprites and Upper Atmospheric Lightning) instrument on board the FORMOSAT-2 satellite launched on May 20, 2004. The ISUAL data provide us global occurrence distributions of sprites. To investigate the characteristics of global lightning activity producing sprites, the most useful method is to measure ELF electromagnetic waves radiated from CGs since ELF waves can propagate long distances with extremely low attenuation rates. Using ELF data, we can obtain information on electric characteristics of lightning discharges on a global scale. We have analyzed ELF transients associated with sprites observed by the ISUAL for a period from July 2004 to November 2005. We divided 106 sprites into three types: 22 halo events, 49 sprite streamer events, and 35 halo-sprite streamer events. It is found that 16 halo events (72.3 %) are generated by -CGs and these -CGs are concentrated over Caribbean Sea and South-East Asia. It is also found that sprite streamer-inducing CGs are positive (98.8 %) except for 1 event and that CGs with the charge moment exceeding 1000 C km are concentrated over Central Africa.

200607 55 S. Chikada, H. Fukunishi, Y. Takahashi, T. Adachi, K. Yamamoto, R. R. Hsu, H. T. Su, A. B. Chen, H. U. Frey, S. B. Mende, and L.C. Lee( 200607 ).Relationships between elves observed by the FORMOSAT-2 / ISUAL and their parent lightning discharges, 2006 Western Pacific Geophysics Meeting, Beijing, china.

Elves are optical flashes in the lower ionosphere caused by electromagnetic pulses radiated from lightning discharges. It is reported that the lateral scale of elves is ~300-600 km and the duration of whole optical emission is smaller than 2 ms. The ISUAL instrument onboard the FORMOSAT-2 satellite observed more than 2000 elve events for the period from July 2004 to January 2006. These events show various shapes: symmetric doughnut shape, symmetric disc shape and asymmetric doughnut shape elves. From the EMP model, it is suggested that vertical, horizontal and oblique lightning discharges generate these different shapes. To investigate the characteristics of parent lightning discharges producing these different shapes of elves, we examined the magnetic field component of sferics in ELF band (1-100 Hz). Using these ELF data, we can get information on the polarity and charge moment values of parent lightning discharges. We have calculated the polarities and charge moment values for 324 elve events observed by the ISUAL in the period between July 2004 and February 2005. It is found that the number of elve events induced by negative CGs and positive CGs are 249 and 75, respectively. The occurrence probability of negative CGs and positive CGs inducing elves are almost the same in the regions of North America and the Atlantic Ocean, while the number of elves induced by negative CGs is greater than that of positive CGs in the region of the Indian Ocean, South-East Asia, and the Pacific Ocean. The average charge moment value of positive CGs inducing elves is 790 Ckm, while that of -CG is 446 Ckm. Furthermore, we will report the results on the regional and seasonal dependences of polarities and charge moment values, and will discuss the relationships between the parameters such as shapes, diameters, etc., characterizing elves observed by ROCSAT-2 / ISUAL, and the parameters, such as polarities and charge moment values, characterizing parent lightning discharges.

200607 56 I. Reddy, J. B. Nee, H. T. Su, A. B. Chen, H. H. Hsu, L.C. Lee, H. U. Frey, and S. B. Mende( 200607 ).Observations of the O(1D), O2(1??) and OH airglow by the ISUAL instrument onboard the FORMOSAT-2 satellite, 36th COSPAR Scientific Assembly, Beijing, china.

In the past using different observational techniques the mesospheric airglow emissions were documented There is a quantitative comparison between the theoretical and experimental brightness is rather poor because of short time scale gravity waves strong variability of tides and the short duration of the observations The ISUAL instrument on board the FORMOSAT-2 satellite was launched on May 21 2004 to investigate the upper atmospheric emissions produced by lightning airglow and aurora The mesospheric airglow produced by O 1 S O 2 b 1 Sigma g O 2 A 3 Sigma u and OH Meinel bands has been observed By using 630 nm filter O 1 D airglow emission was also observed at 200-240 km along with OH emission at 85-88 km In this paper the FORMOSAT-2 observations of October 2004 were presented to show the characteristics of the three emissions including the peak emission altitude emission brightness and their spatial variabilities The results show the variability of the airglow and their global distributions Here we found multiple peaks in the OH and O 2 Sigma air glow emissions The global distribution of the airglow enhancement in the mid latitudes of the northern hemisphere NH is observed

200607 57 J. B. Nee, I. Reddy, H. T. Su, A. B. Chen, R. R. Hsu, H. U. Frey, S. B. Mende, D. Wang, W. Ward, and L. Lyu( 200607 ).O(1D) and OH airglows observed at 630 nm from the, 36th COSPAR Scientific Assembly, Beijing, china.

Emissions at 630 nm in the upper atmosphere produced by oxygen atom and OH have been respectively measured by the instrument ISUAL on board FORMOSAT 2 satellite which was launched in May 2004 to a sun synchronous orbit of about 890 km A CCD imager and photometers combined with several filters on board the satellite have investigated airglows in the upper atmosphere Observations have been made by using filters centered at the wavelengths 557 7 nm 630 nm 762 nm and 630-750 nm corresponding to emissions produced by O 1 S O 1 D O 2 b 1 Sigma g and OH in the height region 80-200 km By using the 630 nm filter airglows from O 1 D at 200 km and OH at 90 km have been both measured separately and simultaneous in October 2004 Results in the tropical regions show various dynamic processes such as tidal influences Modeling simulating airglow observations by including global temperature MSIS horizontal wind as well as a gravity wave will be discussed

200612 58 K. Wang, Y. C. Wang, H. T. Su, and R. R. Hsu( 200612 ).Analysis on Source Mechanisms of Low-Latitude ELF-Whistlers, American Geophysical Union, Fall Meeting 2006, San Francisco, USA.

Events of ELF-whistlers between 60Hz to 100Hz were observed at Lulin Observatory in Taiwan from 2003 to 2004 [\it Wang \it et \it al., 2005]. The correlation between occurrences, local time, and Ap index were analyzed. In this meeting we will present more statistical analysis on these events such as occurrences with Dst index, durations with local time and seasons,...etc. The durations of events are shown to vary with local time and seasons too. From above analysis, the ionospheric plasma environment probably plays a key role to determine whether these waves can propagate or not. Ion-cyclotron whistlers between O+ and He+ gyrofrequencies are deduced to be possibly the source waves. Reference Wang, Y-C, K. Wang, H.-T. Su; R.-R. Hsu, Low-latitude ELF-whistlers observed in Taiwan

200612 59 T. Adachi, H. Fukunishi, Y. Takahashi, S. Chikada, A. Yoshida, R. R. Hsu, H. T. Su, A. B. Chen, S. B. Mende, H. U. Frey, and L.C. Lee( 200612 ).Electrical properties of lightning discharges producing sprite streamers and halos measured with the ISUAL/array photometer and ELF magnetometers, American Geophysical Union, Fall Meeting 2006, San Francisco, USA.

In order to clarify electrical properties of lighting discharges producing sprite streamers and halos, we analyze optical data observed with the ISUAL/array photometer onboard FORMOSAT-2 satellite and magnetic field data observed with the ground-based ELF network system. The array photometer possesses two photometers measuring two wavelength ranges of 360-470 nm and 520-750 nm, with a vertical resolution of 8-13 km and a temporal resolution of 50/500 microseconds. The ELF observation system installed at Onagawa observatory (141.483°E, 38.433°N) measures electromagnetic waves in the frequency range of 1 100 Hz that are radiated by lightning discharges. We analyze 69 sprite events observed during the period from July 4 2004 to November 20 2005. Based on the imager data, we classify these events into three categories: 15 halo events, 29 streamer events, and 25 halo with streamer events. By combining the array photometer data with the ELF magnetic field data, we estimate temporal variations of lightning charge moment changes in each category. In the case that time scales are short (~1 ms), it is found that lightning discharges with charge moment values of ~400 C-km produce sprite halos while lightning discharges with ~1000 C-km produce sprite streamers besides halos. On the other hand, lightning discharges with long time scales (~10 ms) and large charge moment values (~1000 C-km) are found to produce streamers without discernible halos. The obtained results are interpreted using the conventional breakdown field necessary for formations of streamers which is invariable with time and the critical electric field necessary for optical emissions of halos which increases with time due to the electron attachment process. Furthermore, lightning processes responsible for the productions of streamers and halos are derived from blue/red emission ratios observed with the array photometer. The inferred results are validated using coincident optical and electromagnetic data that are obtained in the 2006 balloon experiment campaign in Sanriku, Japan.

200612 60 A. B. Chen, Y. J. Lee, J. K Chou, L. Y. Tsai, C. L. Kuo, C. Y. Chiang, R. R. Hsu, H. T. Su, L.C. Lee, S. B. Mende, H. U. Frey, H. Fukunishi, and Y. Takahashi( 200612 ).Seasonal variation of the CG-induced sprites and elves, American Geophysical Union, Fall Meeting 2006, San Francisco, USA.

ISUAL mission has recorded more than 4,000 TLEs in the first two years of operation. Two major types of TLEs, sprites and elves, show a clear discordance on the occurrence rate, the distribution and the characteristics of seasonal variation, even though both of them are induced by cloud-to-ground discharges. The distribution of sprites is closely correlated with that of the lightning and concentrates mainly over land, whereas the distribtuion of elves congregates strongly over ocean. The seasonal variation of elves implies that the meteorological condition driven by atmospheric circulation may play an important role on the induction of elves. In this presentation, we will compare the global distributions and seasonal variation of sprites and elves with other meteorological factors quantitatively, to explore the difference on the physical onset processes in producing elves and sprite. The operational statistics of ISUAL TLEs will also be presented.

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