Isual Publications 2014

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200412 31 S. B. Mende, H. U. Frey, R. R. Hsu, H. T. Su, A. B. Chen, L.C. Lee, H. Fukunishi, and Y. Takahashi( 200412 ).Sprite Imaging Results from the ROCSAT2 ISUAL Instrument, American Geophysical Union, Fall Meeting 2004, San Francisco, USA. AE51A-02(invited) For the systematic observations of Transient Luminous Events (TLE-s, sprites, elve-s, blue jets etc.) and the study of their global distribution, a satellite based camera was launched as part of the "Imager for Sprites and Upper Atmospheric Lightning" (ISUAL) instrument on the Taiwanese ROCSAT 2 satellite. In addition to being a high quality imager of TLE-s the camera is suitable for the observation of aurora and airglow. The imager is bore-sighted with two photometers and the combined instrument package images the atmospheric phenomena and measures their spectral and rapid temporal properties. The imager has a field of view of 20 degrees horizontal and 5 degrees vertical covering a region that is approximately 1000 x 250 km at the earth limb. The view direction is perpendicular to the orbital plane. The imager carries a filter wheel and it can be operated in a number of wavelength regions uninhibited by atmospheric absorption. The spectral ranges are: 427.8 nm for the detection of energetic electron induced phenomena in TLE-s (e.g. blue jets) and aurora, 557.7 nm for the observation of aurora and airglow, 630 nm for the observation of ionospheric irregularities and aurora, wide red/IR N2 1st positive band region for the quantitative observation of sprites and elve-s and 762 nm for the observation of the airglow and especially gravity wave modulated intensity fluctuations. This filter is also effective in suppressing the low altitude part of TLE-s because of the atmospheric O2 absorption. Numerous examples of elve-s were observed both in the N2 1st positive and the 762 nm filter bands. The nicest data on fully developed sprites was obtained in the N2 1st positive band. High intensity gravity waves were seen in the 762 nm band. Examples of various kinds of TLE-s, aurora and airglow that were observed by the imager illustrate its wide variety of application in aeronomy research.
200412 32 H. T. Su, T. H. Huang, C. L. Kuo, A. B. Chen, R. R. Hsu, S. B. Mende, H. U. Frey, H. Fukunishi, Y. Takahashi, and L.C. Lee( 200412 ).Global distribution of TLEs based on the preliminary ISUAL data, American Geophysical Union, Fall Meeting 2004, San Francisco, USA. AE51A-03(invited) The ISUAL payload on the ROCSAT-2 satellite successfully passed the instrument checkout phase, and has started its global survey of upper atmospheric transient luminous events since early July of 2004. As of the mid-September of 2004, ISUAL has captured 140 TLE events in the first 70 days of operation. In this talk, the preliminary global distribution of TLEs will be presented. Also the correlation between TLEs and global lightning distributions will be evaluated. Comparisons of the occurrence rates for different types of TLEs between ground observation data and ISUAL data will also be presented. *Works performed at National Cheng Kung University were supported in part by grants from NSPO (93-NSPO(B)-ISUAL-FA09-01) and NSC (NSC93-2112-M-006-007, NSC93-2111-M-006-001) in Taiwan.
200412 33 H. Fukunishi, Y. Takahashi, T. Adachi, R. R. Hsu, H. T. Su, A. B. Chen, S. B. Mende, H. U. Frey, and L.C. Lee( 200412 ).Observations of Sprites and Elves with the ISUAL Array Photometer, American Geophysical Union, Fall Meeting 2004, San Francisco, USA. AE51A-04(invited) The ROCSAT-2 satellite with a scientific payload named ISUAL and a remote sensing payload was launched on May 20, 2004 from the Vandenberg range in California. The ISUAL (Imager of Sprites/Upper Atmospheric Lightning) instrument is composed of an imager, a spectrophotometer, and an array photometer. The ISUAL project is an international collaboration of National Cheng Kung University and National Space Program Office in Taiwan, University of California in USA, and Tohoku University in Japan. The goals of ISUAL are to complete a global survey of transient luminous events (TLEs: sprites, elves, blue jets, gigantic jets etc.) and also to investigate the global distributions of airglow and aurora and their dynamical processes. The ISUAL instrument observes optical emissions at the limb of the Earth with a range of 3373 km to the limb at the 60 km altitude tangent point. The array photometer developed by the Tohoku University group can capture spatial and temporal evolution of TLEs at the two wavelength ranges of 370-450 and 530-650 nm selected by blue and red filters, ewspectively. Each photometer consists of 16 channels arrayed in vertical with a field of view of 0.23 degrees (vertical) by 22.5 degrees (horizontal). The vertical resolution of 0.23 degrees corresponds to about 11 km at the limb. The array photometer is operated in an event trigger mode for TLE measurement. The length of data for one event is 220 ms, and the sampling frequency is 20 kHz for the first 18 ms and 2 kHz for the following 202 ms. The ISUAL has been operated on a routine basis from the middle of July in 2004. The total number of lightning cloud flashes measured by the ISUAL for 40 days from the middle of July to the end of August is 2210, while the numbers of sprites and elves for the same period are 20 and 100, respectively. A high time resolution (50 microseconds) and a high vertical resolution (11 km) of the array photometer enable us to investigate rapid intensity variations of sprites and elves as a function of altitude. Furthermore, blue/red intensity ratio measurements at 16 channels enable us to estimate the energies of electrons inducing optical emissions as a function of altitude. Typical examples of sprites and elves captured by the array photometer are presented, and their space-time structures and spectral features are discussed.
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.
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