Isual Publications 2014

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200412 34 H. U. Frey, S. B. Mende, R. R. Hsu, H. T. Su, A. B. Chen, L.C. Lee, H. Fukunishi, and Y. Takahashi( 200412 ).The spectral signature of transient luminous events (TLE, sprite, elve, halo) as observed by ISUAL, American Geophysical Union, Fall Meeting 2004, San Francisco, USA. AE51A-05 The Imager for Upper Atmospheric Lightning (ISUAL) contains a Spectrophotometer with six individual photometers covering the spectral range from the far ultraviolet to the near infrared. The photometers point towards the limb and integrate the light in a field of view of 20x5 degrees. Sudden changes in the amplitude of the output signal are used to trigger the other ISUAL components (Imager and Array Photometer) to collect data. The photometers cover well known spectral ranges of TLE as for instance the N2-1P band at 623-750 nm or the lightning signature at 777.4 nm. In addition there are two photometers for the far-UV (150-280 nm) and near-UV (250-390 nm) that are aimed at spectral signatures of TLE that are only observable from space due to the absorption by atmospheric O2 towards ground-based instruments. TLE that were observed during the first months of routine observation will be analyzed and their spectral characteristics, similarities and differences will be discussed. The most important result is a strong far-UV signature of the high altitude elves that is not disturbed by the lower atmospheric absorption. We will discuss the most likely emission feature and the consequence in terms of the energetics of the excitation process.
200412 35 K. Wang, Y. C. Wang, R. R. Hsu, H. T. Su, and L.C. Lee( 200412 ).Low-latitude ELF Whistler-like Events Observed in Taiwan, American Geophysical Union, Fall Meeting 2004, San Francisco, USA. SM22A-02 Whistler-like events between 60 to 100 Hz have been detected in the ELF station operated by NCKU-ISUAL team at the Lulin Observatory, Taiwan (120o52'E, 23o28' N, 2862m high) from August 20, 2003 to July 29, 2004. The most distinguished feature is the frequency descent in the frequency-time spectrograms, resembling terrestrial whistlers. Other features of the ELF events include(a) a long event duration up to 2 minutes, (b) a daytime diurnal maximum occurring around 10 am, (c) a dominant magnetic field polarization in the north-south direction with strength at a few to tens of pT, and (d) no detection of vertical electric fields...etc. For the past 20 years, similar events were only reported twice, one at the auroral latitude (Heacock, 1974) and the other at the mid-latitude (Sentman and Ehring, 1994). Possible source mechanisms have been discussed are (a) magenetosheath lion roars propagating along the TEM-mode waveguide in the earth-ionosphere cavity, and (b) hiss detected in the equator. Lightning-generated whistlers are not favored in previous studies because of being unable to interpret the observed long dispersions with the electron density environment but all the mechanisms remain open questions. References Heacock, R.R., Whistler-like pulsation events in the frequency range 20-200Hz, Geophys. Res. Lett., 2, 77, 1974. Sentman, D. D., D. A. Ehring, Midlatitude detection of ELF whistlers. J. Geophys. Res., 99, 2183, 1994.
200505 36 A. B. Chen, R. R. Hsu, H. T. Su, T. H. Huang, C. L. Kuo, Y. J. Lee, S. C. Wang, M. H. Chang, J. L. Chern, F. Cheng, Y. S. Chang, T. Y. Liu, S. B. Mende, H. U. Frey, H. Fukunishi, Y. Takahashi, T. Adachi, and L.C. Lee( 200505 ).Global distribution of TLEs and new results from the ISUAL experiment, 2005 Japan Earth and Planetary Science meeting, chiba, japan.
200506 37 H.T. Su et al.( 200506 ).Initial results from the ISUAL experiment, Asia Oceania Geosciences Society 2005, suntec, Singapore.
200510 38 H. T. Su, R. R. Hsu, A. B. Chen, C. L. Kuo, Y. J. Lee, L.C. Lee, S. B. Mende, H. U. Frey, H. Fukunishi, and Y. Takahashi( 200510 ).Space surveys of transient luminous events, URSI, new-Delhi, India.
200512 39 K. Wang, Y. C. Wang, H. T. Su, and R. R. Hsu( 200512 ).Low-Latitude ELF Emissions below 100Hz Observed in Taiwan, American Geophysical Union, Fall Meeting 2005, San Francisco, USA. ELF antennas have been mounted at the Lulin Observatory (23.47oN, 120.87oE; 2862m) and National Cheng Kung University (23.00oN, 120.22oE, 32m) in Taiwan for study of global lightning activities and ELF events. We have previously reported 10-month ELF-Whistlers observations from Aug. 26, 2003 to July 13, 2004. [Wang et al., 2005]. In addition to these events, other forms of ELF emissions were also detected. In this study, an Atlas of these observed ELF emissions below 100Hz for the same period of observation is presented. Total numbers of more than 100 detected events are categorized into six groups: discrete emissions, periodic emissions, quasi-periodic emissions, hiss, chorus, and triggered emissions, according to the system of classification for VLF emissions in [Helliwell, 1965]. Nevertheless, there are still some emissions hardly to be classified. Diurnal and seasonal variations of occurrences for these ELF emission events are analyzed. Correlation between these events and storm indices will also be discussed. References Helliwell, R. A., VLF Emission, in Whistlers and Related Ionospheric Phenomena, Stanford University Press, Stanford, California, 1965. Wang . Y. C., K. Wang, H. T. Su, R. R. Hsu, Low-Latitude ELF-Whistlers observed in Taiwan
200512 40 C. L. Kuo, A. B. Chen, R. R. Hsu, H. T. Su, L.C. Lee, S. B. Mende, 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 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.

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