Isual Publications 2014

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200510 11 Kuo, C. L.*, R. R. Hsu, A. B. Chen, H. T. Su, L. C. Lee, S. B. Mende, H. U. Frey, H. Fukunishi and Y. Takahashi ( 200510 ), Electric fields and electron energies inferred from the ISUAL recorded sprites, Geophys. Res. Lett., Volume 33, Issue 17, L19103, (SCI, IF=3.982, RANK=11/172,GEOSCIENCES, MULTIDISCIPLINARY, Cited=41)

Electron energies and the strength of electric fields in sprites are deduced for five selected events, which were recorded by the space-borne instrument called ISUAL (Imager of Sprites and Upper Atmospheric Lightning; a payload on the FORMOSAT-2 satellite). From the derived peak intensity ratios of spectrophotometer channel 2 (centered at 337 nm) and channel 3 (centered at 391.4 nm) of these sprites, the average and characteristic electron energies were found to be in the range of 6.2-9.2 eV and 4.5-6.5 eV, respectively. The estimated E/N at 40-60 km is 243-443 Td and the strength of the electric field is 2.1-3.7 times that of the atmosphere breakdown E-field at these altitudes. The inferred electron energies and the strength of electric fields are about two times higher than those inferred from ground campaigns. However, they are consistent with the prediction of the sprite streamer model.

200511 12 Mende, S. B.*, H. U. Frey, R. R. Hsu, H. T. Su, A. B. Chen, L. C. Lee, D. D. Sentman, Y. Takahashi and H. Fukunishi ( 200511 ), D region ionization by lightning-induced electromagnetic pulses, Journal of Geophysical Research: Space Physics, Volume 110, Issue A11, A11312, (SCI, IF=3.174, RANK=23/172,GEOSCIENCES, MULTIDISCIPLINARY, Cited=56)

The electromagnetic pulses (EMP) from tropospheric lightning produce transient
luminous events (TLEs), known as elves, in the 80–90 km region above the lightning. The luminosity is evidence that the EMP carries sufficient electric field to excite optical emissions at these altitudes; however, it is still unknown whether the field is sufficient to ionize the atmosphere. The first multiwavelength, quantitative observatory on a free-flying satellite dedicated to observing TLEs, the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) instrument on FORMOSAT-2, formerly called ROCSAT-2, confirmed that a significant number of lightning events are accompanied by elves. The instrument consists of a low light level imager and a set of multichannel photometers. In a few cases where the lightning occurred beyond the solid Earth limb, pure spectral measurements of the elves were obtained. Here we analyze such an event and show that the elves contained significant 391.4 nm emission of the N2+ ion. This is clear evidence that ionization takes place in elves. The ratio of cross sections for N2 ionization and the production of the upper state of 391.4 nm emission is not a constant for low-energy electrons found in TLEs. We attempted to find this ratio by comparing our photometric measurements of the TLE produced emissions to theoretically derived emission intensities. The electron energy distribution and the ratios of the modeled N2 Lyman-Birge-Hopfield (LBH) and the N2+ first negative to the second positive were computed as a function of the reduced electric field. From these ratios it was possible to obtain the reduced electric field from the ratios. We estimated that the reduced electric field, which characterizes the local electron energy distribution, was >200 Td. We also made comparisons of the theoretically derived intensities to our measurements of the N2+ first positive and Lyman-Birge-Hopfield (LBH) band emission in the elves. On the basis of the ratio between the N2+ first negative emission and the time-integrated ionization production,
we estimate that the elves produced an average electron density of 210 electrons cm3 over a large (165 km diameter) circular region having an assumed 10 km altitude extent. These observations indicate that thunderstorms are a significant source of ionization in the low- to midlatitude nighttime D region.

200601 13 Liu, N. Y.*, V. P. Pasko, D. H. Burkhardt, H. U. Frey, S. B. Mende, H. T. Su, A. B. Chen, R. R. Hsu, L. C. Lee, H. Fukunishi and Y. Takahashi ( 200601 ), Comparison of results from sprite streamer modeling with spectrophotometric measurements by ISUAL instrument on FORMOSAT-2 satellite, Geophys. Res. Lett., Volume 33, Issue 1, L01101, (SCI, IF=3.982, RANK=11/172,GEOSCIENCES, MULTIDISCIPLINARY, Cited=31)

Three intensity ratios of the second positive band system of N2 to N2 LBH band system, the first negative band system of N2+, and the first positive band system of N2 are obtained separately from sprite streamer modeling and from measurements by ISUAL instrument on FORMOSAT-2 satellite. The direct comparison between the obtained ratios indicates that in order to explain the ISUAL spectrophotometric data the maximum field driving the emissions of an observed sprite event must be greater than three times of the conventional breakdown threshold field. These findings are consistent with an assumption that most of the observed sprite emissions during initial sprite development originate from localized high field regions associated with tips of sprite streamers.

200609 14 Mika, A.*, C. Haldoupis, T. Neubert, H. T. Su, R. R. Hsu, R. J. Steiner and R. A. Marshall ( 200609 ), Early VLF perturbations observed in association with elves, Annales Geophysicae, Volume 24, Issue 8,, 2179-2189, (SCI, IF=1.518, RANK=80/172,GEOSCIENCES, MULTIDISCIPLINARY, Cited=17)

VLF remote sensing is used to detect lower-ionospheric electron density changes associated with a certain type of transient luminous events known as elves. Both ground- and satellite-based observations of elves are analysed in relation to VLF data acquired at various receiver sites in Europe, the United States and Antarctica. Ground-based observations were performed during the EuroSprite2003 campaign, when five elves were captured by low-light cameras located in the Pyrenees. Analysis of VLF recordings from Crete shows early VLF perturbations accompanying all of the elves. A large dataset consisting of elves captured by the ISUAL (Imager of Sprites and Upper Atmospheric Lightning) payload on Taiwan's FORMOSAT-2 satellite over Europe and North America has also been analysed. Early/fast VLF perturbations were found to accompany some of the elves observed over Europe. However, no VLF perturbations were detected in relation to the elves observed by ISUAL over North America. The present analysis - based on the largest database of optical elve observations used for VLF studies so far - constitutes evidence of processes initiated by the lightning EMP (electromagnetic pulse) causing electron density changes in the lower ionosphere in line with theoretical predictions. It also proves that sub-ionospheric electron density changes associated with elves can intrude to lower heights and thus perturb VLF transmissions. The possibility of VLF detection, however, depends on several factors, e.g., the distance of the elve from the receiver and the transmitter-receiver great circle path (GCP), the altitude of the ionised region and the characteristics of the VLF transmitter, as well as the EMP energy, which occasionally may be sufficient to cause optical emissions but not ionisation.

200609 15 Adachi, T.*, H. Fukunishi, Y. Takahashi, Y. Hiraki, R. R. Hsu, H. T. Su, A. B. Chen, S. B. Mende, H. U. Frey and L. C. Lee ( 200609 ), Electric field transition between the diffuse and streamer regions of sprites estimated from ISUAL/array photometer measurements, Geophys. Res. Lett., Volume 33, Issue 17, L17803, (SCI, IF=3.982, RANK=11/172,GEOSCIENCES, MULTIDISCIPLINARY, Cited=24)
200610 16 Cummer, S. A.*, H. U. Frey, S. B. Mende, R. R. Hsu, H. T. Su, A. B. Chen, H. Fukunishi and Y. Takahashi ( 200610 ), Simultaneous radio and satellite optical measurements of high-altitude sprite current and lightning continuing current, Journal of Geophysical Research: Space Physics, Volume 111, Issue A10, A10315, (SCI, IF=3.174, RANK=23/172,GEOSCIENCES, MULTIDISCIPLINARY, Cited=19)
200706 17 Cheng, Z., S. A. Cummer*, H. T. Su and R. R. Hsu ( 200706 ), Broadband very low frequency measurement of D region ionospheric perturbations caused by lightning electromagnetic pulses, Journal of Geophysical Research: Space Physics, Volume 112, Issue A6, A06318, (SCI, IF=3.174, RANK=23/172,GEOSCIENCES, MULTIDISCIPLINARY, Cited=18)
200709 18 Frey, H. U.*, S. B. Mende, S. A. Cummer, J. Li, T. Adachi, H. Fukunishi, Y. Takahashi, A. B. Chen, R. R. Hsu, H. T. Su and Y. S. Chang ( 200709 ), Halos generated by negative cloud-to-ground lightning, Geophys. Res. Lett., Volume 34, Issue 18, L18801, (SCI, IF=3.982, RANK=11/172,GEOSCIENCES, MULTIDISCIPLINARY, Cited=21)
200711 19 Kuo, C. L.*, A. B. Chen, Y. J. Lee, L. Y. Tsai, R. K. Chou, R. R. Hsu, H. T. Su, L. C. Lee, S. A. Cummer, H. U. Frey, S. B. Mende, Y. Takahashi and H. Fukunishi ( 200711 ), Modeling elves observed by FORMOSAT-2 satellite, Journal of Geophysical Research: Space Physics, Volume 112, Issue A11, A11312, (SCI, IF=3.174, RANK=23/172,GEOSCIENCES, MULTIDISCIPLINARY, Cited=20)
200802 20 Enell, C. F.*, E. Arnone, T. Adachi, O. Chanrion, P. T. Verronen, A. Seppala, T. Neubert, T. Ulich, E. Turunen, Y. Takahashi and R. R. Hsu ( 200802 ), Parameterisation of the chemical effect of sprites in the middle atmosphere, Annales Geophysicae, Volume 26, Issue 1, 13-27, (SCI, IF=1.518, RANK=80/172,GEOSCIENCES, MULTIDISCIPLINARY, Cited=1)
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