Isual Publications 2014

COM_FABRIK_ORDERpublish time # Citation abstract
200512 41 J. B. Nee, I. Reddy, H. T. Su, A. B. Chen, R. R. Hsu, L.C. Lee, H. U. Frey, and S. B. Mende( 200512 ).Observations of the O, O2, and OH airglows by the ISUAL instrument onboard the FORMOSAT 2 satellite, American Geophysical Union, Fall Meeting 2005, San Francisco, USA. The mesospheric airglows produced by O(1D), O(1S), O2(b) A band , O2(A) Herzberg band,and OH in Meinel bands have been observed by using the ISUAL instrument on board the FORMOSAT 2 satellite. The ISUAL instrument consists of a CCD imager which carries six narrow and broad band filters centered at 630-750 nm, 762nm, 630 nm, 557,7 nm, and 4278,8 nm, which are used to map the spatial and temporal distributions of upper atmospheric emissions. Airglow layers in the 60-120 km have been usually observed. Occasionally, the airglow in the thermosphere at 200 km has also have been observed such that multilayer airglows from 80-200 km were measured. Measurements of oxygen and OH airglows showed their global variations. We will discuss about the measurements and analyses of the airglow and their global distributions. Specifically, the red line at 630 nm, oxygen A band at 762 nm and Meinel bands of OH in 630nm and also in 633-750 nm, O2 Herzberg band at 427.8 nm were investigated.
200512 42 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( 200512 ).Elves spectrum based on the ISUAL photometric data, American Geophysical Union, Fall Meeting 2005, San Francisco, USA. Elves are lower ionospheric optical emissions induced by lightnings' electromagnetic pulses (EMP) [Inan et al., 1996; Fukunishi et al, 1996]. Their short luminous duration (~1 msec) and low brightness (0.1-1 MR) severely limit the success in obtaining the spectroscopic infromation from ground-based observations [Barrington-Leigh and Inan, 2001]. With the launch of ISUAL payload on the FORMOSAT-2 satellite, a new opportunity has opened up to elucidate the nature of the fleeting elves. From the recorded ISUAL events, the following characteristics of elves have been obtained (1) the existence of highly absorbed Lyman-Birge-Hopfield band and ionized 1NN2+ band, (2) a higher brightness of ~40 mega-Rayleigh at 1PN2 band, and (3) an associated E-field of 15 Volt/m at 90 km. Using the ISUAL spectrophotometric data (SP1 at 150-290 nm, SP2 centered at 337 nm, SP3 cetered at 391.4 nm, SP4 at 608.9-753.4 nm, SP6 at 228.2-410.2 nm; 0.1 ms time resolution) and Array Photometer data (blue band 370-450 nm, red band 530-650 nm; 0.05 ms time resolution), 57 hehind-the-limb elves with their parent lightning blocked by the solid Earth are analyzed. The possible spectrum of elves will be proposed.
200512 43 T. Adachi, H. Fukunishi, Y. Takahashi, Y. Hiraki, K. Yamamoto, R. R. Hsu, H. T. Su, A. B. Chen, S. B. Mende, H. U. Frey, and L.C. Lee( 200512 ).Spatiotemporal and Spectral Characteristics of Sprites and Gigantic Jets Derived from ISUAL/Array Photometer Measurements, American Geophysical Union, Fall Meeting 2005, San Francisco, USA.

Sprites are electrical discharges above active thunderstorms occurring at mesospheric altitudes [Stanley et al., 1999; Stenbaek-Nielsen et al., 2000] while gigantic jets span from the top of thundercloud to the lower ionosphere [Pasko et al., 2002; Su et al., 2003] In this study, we report the spatiotemporal and spectral characteristics of sprites and gigantic jets observed with the ISUAL/array photometer on the FORMOSAT-2 satellite. The array photometer provides us spectral information by measuring two wave length ranges of 360-470 nm and 520-750 nm selected by blue and red filters, respectively. Each photometer has sixteen channels arrayed in vertical and sampling frequency of 20 kHz for the initial 18 ms and 2kHz for the following 292 ms, with which we can detect fast vertical motion of optical emissions. During the period from July 2004 to July 2005, the ISUAL observed ~130 sprites and a gigantic jet. By analyzing the array photometer data, we clarify that sprites are induced within 1-6 ms after transient emissions of lightning discharges while a gigantic jet is not accompanied with such transient lightning emissions as seen in the sprite events. The initial optical emissions of sprites occur at mesospheric altitudes and propagate upward/downward direction with vertical velocities of 2-11x107 m/s, in close agreement with past observational results [Stanley et al., 1999; McHarg et al., 2002]. On the other hand, optical emissions of a gigantic jet propagate upward from the cloud top with a vertical speed of <1x107 m/s at the initial leading-jet stage [Su et al., 2003]. At the following stage, the upward propagation speed exceeds 4x107 m/s and emissions extend to the lower ionosphere. The obtained results are discussed in association with past observational and theoretical results.

200512 44 R. R. Hsu, A. B. Chen, C. L. Kuo, Y. J. Lee, H. T. Su, H. Fukunishi, Y. Takahashi, T. Adachi, K. Yamamoto, H. U. Frey, S. B. Mende, and L.C. Lee( 200512 ).Gigantic jet observation by the ISUAL payload of FORMOSAT-2 satellite, American Geophysical Union, Fall Meeting 2005, San Francisco, USA. Gigantic jet is a kind of transient luminous events (TLEs), which was first discovered in 2002 [Su et al]. Based on the 16ms time resolution black/white images, three luminous stages, called leading jet, fully-developed jet, and trailing jet, were identified. At the fully-developed stage, the fully-extended luminous body connected the cloud top at 15km with the E-layer ionosphere at 90km. The leading jet, which can be viewed as the pre-stage of the fully- developed jet, behaved like a stepped leader of cloud-to-ground (CG) lightning. The trailing jets have similar dynamical evolution features as the blue jets, where a funnel-shape jet emerged from the cloud-top and propagating upward. The luminous period of these jets lasted from 300 ms to 500 ms, which is longer than typical sprites but is shorter than that of the large blue jets observed by Pasko et al (2002). From the associated ELF emissions, the charge moment changes of GJs were estimated to be 1000-2000C-km and the polarity was determined to be negative. Both optical and radio signals suggested that GJs were not CG-induced events, and likely are true discharges from thundercloud to the ionosphere. Among the TLE events recorded by the ISUAL payload on the FORMOSAT-2 satellite, a gigantic jet event was identified. The spectrophotometric data recorded by the ISUAL SP reveals that the GJ is more energetic than sprites. The expected spatial-temporal properties of GJ were confirmed by the ISUAL array photometer. The ISUAL AP data indicates that the electric field intensity of GJ is much larger than the sprite QE field intensity above 70 km. Both ISUAL imager and AP data demonstrate that the fully-developed stage in GJ truly is an upward discharge from the cloud-top to ionosphere and plays the similar role as the return stroke in cloud-to-ground lightning. In this paper, the ISUAL data on this fantastic event will be presented, including its spectroscopic data and dynamical properties at 1ms time resolution. The corresponding ELF signals, observed by Onagawa station in Japan and Esrange station in Sweden, will also be shown.
200512 45 A. B. Chen, C. L. Kuo, R. R. Hsu, H. T. Su, T. H. Huang, Y. J. Lee, S. Wang, P. S. Chiang, M. Chang, S. B. Mende, H. U. Frey, H. Fukunishi, Y. Takahashi, and L.C. Lee( 200512 ).Gloabl Distribution and Seasonal Distribution Variaion of Transient Luminous Events, American Geophysical Union, Fall Meeting 2005, San Francisco, USA. ISUAL, the scientific payload of FORMOSAT-2 satellite, recorded approximately 2,000 transient luminous events (TLEs) in the first 1.5 years of operation after launch. By geolocating the ISUAL registered events using the spacecraft position and attitude data, the global distribution and seasonal distribution variation of the TLE events can be obtained. Several hot zones of high TLE appearance rate are identified, including South China Sea, central Africa, Caribbean Sea and central Pacific Ocean. The first three hot zones were reported by previous lightning or ELF surveys, but the last one, central Pacific Ocean, was not noted before. Our results show the ocean vs land occurrence rates for sprites and elves are quite different. Elves preferentially occur above ocean, whereas sprites tend to be triggered by lightning over the land. A more indepth comparison with results from other global surveys will also be reported.
200512 46 Y. Takahashi, S. Chikada, T. Adachi, K. Yamamoto, H. Fukunishi, H. T. Su, A. B. Chen, R. R. Hsu, S. B. Mende, H. U. Frey, and L.C. Lee( 200512 ).Spectral characteristics of elves observed by FORMOSAT-2/ISUAL, American Geophysical Union, Fall Meeting 2005, San Francisco, USA. Elves is one of the transient luminous events (TLEs) in the upper atmosphere, 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. However, on the ground observations atmospheric attenuation that is time variable and strongly dependent on wavelength prevents us to obtain the true spectral information. Spacecraft is the only perfect platform to realize the well-calibrated spectral measurement. ISUAL instruments onboard FORMOSAT-2 satellite 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 800 elves. It is recognized that shape of elves is not always doughnut-like but sometimes disk without center hole. Such variety could be caused by intra cloud discharge especially by horizontal component. We selected out 25 elves events from over 700 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 AP_fs 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. Indeed the number of 25 is not statistically sufficient, we couldn_ft find out any clear relationship for them at this moment. However, there is a tendency that high blue/red ratio events occur in the summer hemisphere while low blue/red ratio events in the winter hemisphere. This fact suggests that the lower ionospheric structure could affect the characteristics of elves.
200512 47 H. T. Su, R. R. Hsu, C. L. Kuo, A. B. Chen, Y. J. Lee, P. S. Chiang, H. U. Frey, S. B. Mende, Y. Takahashi, H. Fukunishi, and L.C. Lee( 200512 ).Blue jets, blue starters and other blue luminous events observed by ISUAL payload on the FORMOSAT-2 satellite, American Geophysical Union, Fall Meeting 2005, San Francisco, USA.

Blue jets and blue starters were first discovered in 1994 (Wescott, et. al. GRL, 22, 1209, 1995; GRL, 23, 2153, 1996). During 2000 STEPS campaign, other blue luminous events were also observed to emerge directly from cloud-top and were named gnomes and pixies (Lyons, et. al. BAMS, 445, 2003). The spectroscopic properties of blue starters were investigated in EXL98 campaign and no 2PN2 emission was detected. However using 1NN2+ images at 427.8nm and color TV images, the degree of ionization was estimated to be 3% (Wescott, et. al. JGR, 106, 21549, 2001). Among the thousands of TLEs registered by ISUAL payload onboard the FORMOSAT-2 (Chern et. al. JASTP 65, 647, 2003), more than two hundreds of them were identified as blue luminous events. These events have significant blue channel (370-450nm) signals in the ISUAL array photometer (AP), but have no red channel (530-650nm) emission. In the ISUAL spectrophotometer (SP), they also have strong 2PN2 (337nm) and 1NN2+ (391.4nm) emissions; but only a few also have N2-LBH (150-290nm) or N2 1PG (608.9-753.4nm) emissions and none has SP#5 channel (777.4nm) signal. The AP and SP spectrophometric data indicates that these blue luminous events are not related to lightning. The ISUAL ICCD images, which were taken through the red filter (633-750nm), show that some dim red emissions are emitted from the bottom of these blue luminous events near the cloud-top elevation; for some events the dim red emission can extend for >10km. Based the current spectroscopic information and their luminous duration (<200ms), these events are very similar to the blue jets, blue starters, gnomes or pixies. In this paper, the brightness, the characteristic and average electron energies of these events will be presented. The meteorological conditions for these blue luminous events to occur will also be discussed.

200512 48 H. U. Frey, S. B. Mende, H. T. Su, R. R. Hsu, A. B. Chen, L.C. Lee, H. Fukunishi, and Y. Takahashi( 200512 ).Characteristics of Sprites and Elves and Their Effect on the Upper Atmosphere, American Geophysical Union, Fall Meeting 2005, San Francisco, USA. The Imager for Sprites and Upper Atmospheric Lightning (ISUAL) on FORMOSAT-2 is the first space-based instrument that is dedicated to observe the spatial, spectral, and temporal characteristics of transient luminous events (TLE) like sprites and elves on a global scale. It contains a fast CCD-camera to detect the spatial characteristics of the TLE. Photometers operate in well known spectral ranges as for instance the N2-1P or N2-2P bands. In addition there are two photometers for the far-UV and near-UV that are only observable from space due to the absorption by atmospheric O2 towards ground-based instruments. The first 1.5 years of operation provided data on the characteristics of the parent lightning and the following sprites and elves. The spectral signatures of the TLE can be used to estimate the most likely electron energy in TLE and the amount of upper atmospheric ionization. This presentation will summarize the initial results from ISUAL and will discuss the potential of the collected data.
200512 49 H. T. Su, R. R. Hsu, A. B. Chen, L.C. Lee, S. B. Mende, H. U. Frey, H. Fukunishi, Y. Takahashi, and C. L. Kuo( 200512 ).Key results from the first fourteen months of ISUAL experiment, American Geophysical Union, Fall Meeting 2005, San Francisco, USA.

Imager of Sprites and Upper Atmospheric Lightning (ISUAL) is a scientific payload onboard the FORMOSAT-2 satellite. The ISUAL payload contains three sensor packages; an ICCD imager, a six-channel spectrophotometer, and a duel-band array photometer. As the FORMOSAT-2 motions from the southern to the northern night hemisphere, ISUAL looks across the track toward the local mid-night region to monitor the activity of transient luminous events (TLEs; sprites, elves, blue jets, gigantic jets, etc). In a given day, the field-of-view of ISUAL sweeps through most of the Earth's tropical and sub-tropical regions. With the long term and global observation data, ISUAL experiment strives to elucidate the characteristics, occurrence frequency, and global distribution of TLEs from space. After the first light on 1 July 2004, ISUAL has registered approximately two thousands TLE events as of the end of August 2005. In the talk, key results based on these data will be presented; including the major hot TLE zones, the spectroscopic characteristics of TLEs (sprites, elves, and jets), the electric characteristics of TLEs, the meteorological conditions for the occurrence of TLEs, etc.

200607 50 A. B. Chen, C. L. Kuo, R. R. Hsu, H. T. Su, Y. J. Lee, F. C. Cheng, L.C. Lee, S. B. Mende, H. U. Frey, H. Fukunishi, Y. Takahashi, and T. Adachi( 200607 ).Key results and current status of ISUAL experiment, 2006 Western Pacific Geophysics Meeting, Beijing, china.

After the first light on 1 July 2004, ISUAL has registered thousands of TLE events and has devoted hundreds of orbits on airglow observations in the past two years. In the talk, The current status will be briefed and key results based on these data will be presented; including the global distribution and seasonal variation of TLEs, the spectroscopic characteristics of TLEs, the electric characteristics of TLEs, as well as the latitutional variation of air glows, etc

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