Isual Publications 2014

COM_FABRIK_ORDERpublish time # Citation abstract
200705 71 A. B. Chen, C. L. Kuo, Y. J. Lee, H. T. Su, R. R. Hsu, J. L. Chern, H. U. Frey, S. B. Mende, Y. Takahashi, H. Fukunishi, and L.C. Lee( 200705 ).Global distribution and occurrence rate of transient luminous events, 2007 TGA, Taoyuan, Taiwan.

Based on 2.5-year survey by ISUAL experiment, the transient luminous event (TLE) global distribution and occurrence rate of TLEs including sprite, elve, and halo are derived. 80% of recorded TLEs were identified as elves, and only 20% are recognized as sprites and halos. The results show that sprites mainly congregate over continent as lightnings do, whereas elves scatter mostly over oceans. Theoretical calculation indicates that only less than 1% continental CG lightnings are able to generate elves which are detectable by ISUAL. The strong dependency of elve occurrence on the sea surface temperature and updraft flow of global atmosphere circulation implies that the warm tropical oceans provide the heat source to drive vertical convection and produce intense oceanic lightnings, which induce a large fraction of observed elves. This finding suggests that oceans, atmosphere and ionosphere are coupled.

200705 72 C. L. Kuo, A. B. Chen, L. Y. Tsai, J. K. Chou, R. R. Hsu, H. T. Su, and L.C. Lee( 200705 ).Peak current of the elves-generating causative CGs, 2007 TGA, Taoyuan, Taiwan.

Elve is one type of transient luminous events (TLEs; sprites, elves, electric jets) between thundercloud and the ionosphere. Elves were first discovered in the space shuttle images [Boeck et al., 1992], and later identified by ground observation [Fukunishi et al., 1996]. The generating mechanism of elves is the heating of electrons by the electromagnetic pulses (EMPs) emitted by cloud-to-ground discharge [Inan et al., 1996]. The E-field driven electrons collide with the major species of atmospheric molecules (nitrogen and oxygen), including excitation and ionization, and eventually resulting in the expanding luminous emissions in the lower ionosphere. Typical altitude of elves is in the range of 80-95 km and their lateral dimensions is 200-500 km. The short luminous duration (~1ms) and severe atmospheric attenuation of short wavelength emission have limited the success in recording the full spectroscopy of elves from ground observation. In 2004, ISUAL experiment on the FORMOSAT-2 has successfully confirmed the existence of FUV emission in elves [Mende et al, 2005]. In this paper, an in-depth study of the ISUAL recorded elves is carried out. Numerical simulation results of elves based on an EMP model of the emissions
between 185-800 nm and of their spatial-temporal evolution are presented. To account for the effect of atmospheric attenuation, three major attenuation mechanisms: O2, O3 and molecular Rayleigh scattering are considered. Numerical results show the elves intensity is a function of the peak current of the causative CG. Among the 105 behind-the-limb elves analyzed in this work, the causative CGs is estimated to be between 170 kA and 400 kA, much larger than peak currents in typical CGs (~30 kA). Therefore, the ISUAL can be viewed as a probe of elves induced by intense lightning events.

200705 73 H. T. Su, C. L. Kuo, A. B. Chen, J. K. Chou, and R. R. Hsu( 200705 ).150-280 nm FUV Emissions from Lightning, 2007 TGA, Taoyuan, Taiwan.

Lightning is known to emit electromagnetic radiation in radio, HF, VHF, UHF, IR, optical, mid-UV, VLF, ELF bands. Recently, lightning is also found to induce terrestrial gamma-ray emissions and transient luminous events (sprites, elves, halos, blue jets and gigantic jets) above thundercloud tops. One of the main mission goals for ISUAL is investigate the occurrence and spatial-temporal evolutions of TLEs from
space. However in the ISUAL data, SP channel 1 (band pass 150-280 nm) often recorded strong FUV emissions from lightning. In this talk, we will present the observation data and discuss how it was possible for the FUV emissions to penetrate the thick atmosphere and to escape into space.

200708 74 H. T. Su, A. B. Chen, C. L. Kuo, Y. J. Lee, R. R. Hsu, J. L. Chern, H. U. Frey, S. B. Mende, Y. Takahashi, H. Fukunishi, and L.C. Lee( 200708 ).Global Occurrence of TLEs and Their Effects, 13th International Conference on Atmopheric Electricity (ICAE), Beijing, china.
200712 75 A. B. Chen, C. L. Kuo, Y. J. Lee, R. R. Hsu, H. T. Su, J. L. Chern, H. U. Frey, S. B. Mende, H. Fukunishi, Y. Takahashi, T. Y. Liu, Y. S. Chang, and L.C. Lee( 200712 ).Occurrence and distribution of global TLE activities and their effects, American Geophysical Union, Fall Meeting 2007, San Francisco, USA. AE42A-01(invited)

During the 3-year survey of ISUAL experiment on the FORMOSAT-2 satellite, thousands of TLEs including sprites, elves, halos and jets have been recorded. The most dominated type (~80%) of TLEs was recognized as elves, and only 20% of the recorded events are sprites and halos. In addition, ISUAL also recorded 13 gigantic jets during this time span. Distributions of the ISUAL TLEs show that sprites congregate mainly over continents as lightning does, whereas elves scatter mostly over oceans. The global occurrence rates of TLEs are also derived. However, a detailed study of the peak CG current distribution and the ISUAL imager detection threshold indicates that only around 1% CG lightning are able to generate ISUAL detectable elves. The true occurrence rates of TLEs could range from a factor of two to an order magnitude higher than detected rates. The strong dependency of elve occurrence on the sea surface temperature, updraft flow of global atmosphere circulation, and precipitation implies that the warm tropical oceans act as the heat reservoirs to drive vertical convection and accelerate charge accumulation by rainfall, thus produce intense oceanic lightnings, which induce a large fraction of observed elves. This process suggests that oceans, atmosphere and ionosphere are coupled. Sprites and jets are able to transport electrons between thunderclouds and lower ionosphere, and elves contribute substantial free electrons and ions at ~90km height from EMP-heated ionization. In this presentation we also discuss the role of TLE in the global circuit and atmospheric chemistry quantitatively.

200712 76 M. Yamamoto, S. Watanabe, T. Ono, T. Abe, M. Yamamoto, T. Adachi, A. Saito, A. B. Chen, R. R. Hsu, and P. Bernhardt( 200712 ).WIND, FERIX-2 and ISUAL F-region imaging: Ionospheric Observation Campaigns over Japan in 2007, American Geophysical Union, Fall Meeting 2007, San Francisco, USA. SA12A-01

In the middle/low latitude ionosphere, coupling processes between plasma and neutral atmosphere is important, which is a key to understand ionospheric structures of the spatial scale of 10-1000km. Under international collaboration between scientists from Japan, Taiwan and USA, we are now conducting number of observation campaigns over Japan region. WIND (Wind measurement for Ionized and Neutral atmospheric Dynamics study) is a F-region rocket experiment to study neutral-plasma coupling processes. ISAS/JAXA successfully launched the S-520-23 sounding rocket from Uchinoura Space Center on September 2, 2007. The rocket was equipped with in-situ instruments for plasma-parameters. The Li-release experiment was conducted at the rocket downleg to measure the neutral wind. A dual-band beacon (DBB) transmitter was on board of the rocket, and we received the signal at five stations including one boat under the rocket trajectory. FERIX-2 (F- and E-Region Ionosphere Coupling Study-2) is multi-radar experiment to reveal F-region and E- region coupling processes in the ionosphere. Simultaneous measurement of F-region and E-region FAIs with the MU radar and a portable VHF radar obtained much data showing the coupling through geomagnetic field line. In parallel to these observations we conducted limb imaging of the airglow in the F-region by means of ISUAL on the FORMOSAT-2 satellite in December 2006 and June 2007. Data from FORMOSAT-3/COSCMIC satellites are valuable to support these observations. In the presentation we will overview preliminary results from these observation campaigns, and discuss future collaboration for the study of middle-low latitude ionosphere.

200712 77 J. B. Nee, R. R. Hsu, A. B. Chen, H. T. Su, H. U. Frey, S. B. Mende, and T. Y. Huang( 200712 ).OH and O(1D) Airglow measurements at 630 nm from FORMOSAT 2 satellite, American Geophysical Union, Fall Meeting 2007, San Francisco, USA.

By using a 630 nm filter, airglow emissions in 60-250 km range were measured by the FORMOSAT-2 satellite using the ISUAL instrument (Imager of Sprites and Upper Atmospheric Lightning ). Measurements in Oct 2004 and March-April 2007 revealed two layers of airglow produced by OH and O(1D). The emissions of O(1D) and OH had maximum intensity in the equatorial regions, but with maximum latitude varied from day to day. The OH intensity showed an oscillation of two day period, but O(1D) varied by many factors. By comparing the relative intensity and height separation of these two airglow layers, we found the intensity of O(1D) was generally stronger than OH in the southern hemisphere but weaker or comparable with OH in the northern hemisphere.

200712 78 C. Y. Chiang, T. F. Chang, T. Y. Huang, Y. J. Wu, A. B. Chen, R. R. Hsu, and H. T. Su( 200712 ).Side-viewing observations of OI(1D) and OH night airglows by ISUAL, American Geophysical Union, Fall Meeting 2007, San Francisco, USA.

Optical and side-viewing images obtained from airglow surveys by ISUAL/FORMOSAT-2 often showed local enhancements in the F region and in the 90 km OH-layer. Detailed inspection of the latitudinal distributions of airglows in these two layers indicates the enhancements may be correlated. In some cases, the enhancements fell on two sides of the magnetic equator and could have been invoked by the Equatorial Ionization Anomaly (EIA) effect with an additional displacement contributed by the wind field. However in most cases, the cause(s) are far from clear. In addition, nonuniform luminescence, called spread F, was often observed in the pre-midnight regions and appears to exhibit longitudinal and seasonal variations. In the paper, the possible mechanisms of the enhancements will be discussed systematically.

200712 79 T. F. Chang, C. Z. Cheng, C. Y. Chiang, S. W. Tam, A. B. Chen, R. R. Hsu, and H. T. Su( 200712 ).Azimuthal structure of auroral arcs associated with various disturbed magnetic conditions, American Geophysical Union, Fall Meeting 2007, San Francisco, USA.

We investigated the mode numbers of azimuthally spaced auroral forms in various disturbed magnetic conditions using limb-view images of ISUAL (The Imager of Sprites and Upper Atmospheric Lightnings ) on board FORMOSAT-2 satellite. We choose 557.7nm and 630.0nm as our filter wavelengths. The 557.7nm green transition line of OI(1S→1D) is mainly produced by energetic electrons with energy ≥ 1keV and the 630.0nm red-line emission of OI(1D→3P) (triplet) is mainly produced by electrons with energy ~O(100eV). The auroral arcs have bright spots with approximately equal-spaced separation, in the azimuthal direction, which allows estimate of the azimuthal mode number. The azimuthal mode number of quiet time arcs is higher (m~720) and becomes lower at moderately disturbed times (m~360). For the breakup arcs at onset, the azimuthal mode number is lower with m~220. After onset, the observed breakup arcs move explosively in poleward direction from 64° to 70° latitude in about 2 minutes. The observed azimuthal structure of the substorm breakup arc at onset is similar to the events observed by THEMIS All Sky Imagers at Fort Yukon (m~250) and is consistent with the kinetic theory of ballooning modes [Cheng, Zaharia and Gorelenkov, 2004], which showed the most unstable mode at about -8RE with azimuthal mode number m~300.

200712 80 J. K. Chou, L. Y. Tsai, C. L. Kuo, Y. J. Lee, Y. C. Chen, C. P. Hu, A. B. Chen, H. T. Su, R. R. Hsu, and L.C. Lee( 200712 ).Blue And Gigantic Jets From Taiwan 2007 TLE Campaign, American Geophysical Union, Fall Meeting 2007, San Francisco, USA.

Blue and gigantic jets are believed to be blue luminous phenomena. In Taiwan 2007 TLE campaign, a multi- wavelength imaging system was deployed with the aim to elucidate the physical and chemical characteristics of TLEs. On 22 July 2007, twenty blue/gigantic jets and four sprites were observed to occur over a frontal system in Fujian province of China, about 400km away from our observation site at Lulin Observatory, Taiwan. All the observed jets showed little blue band (380-510nm) emissions but had easily recognizable signals in red band (570-2700nm). This result indicates that jets observed from ground are reddish and most of the blue emissions are extinct. One of the jets was observed to propagate upward to ~75km elevation, thus it can be identified as a GJ and is similar to the GJ-event observed by Pasko et al (2001). This GJ was launched 200ms after a small jet from the same cloud top. This implies that the smaller jet could be regarded as the leader of this GJ, similar to the stepped/dart leader in a CG flash. Since the blue luminous events in ISUAL data (Su, et. al. 2005 AGU) have similar features as the jets in this ground observation. Thus, we can conclude that the ISUAL blue luminous events also are blue jets or blue starters. Even though blue jets and blue starter have different ISUAL SP2 (N2 2P, 337.0nm), SP3 (N2 1N, 391.4nm) and SP6 (250-390nm) intensities, but the peak ratio between SP2 and SP6 are ~ 0.5 and the peak ratio between SP3 and SP2 are ~ 0.07. This means blue jets and starters possess the same spectral properties and the same degree of ionization. Finally, the relation between jets and the nearby lightning will also be addressed.

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