AE51A-01 08:00h
Overview of the ROCSAT-2 Satellite and ISUAL Instruments
After six years of preparation, ROCSAT-2 - the second satellite from Taiwan was successfully launched on 20 May 2004 from Vandenberg Air Force Base in California. ROCSAT-2 carries two payloads onboard: the Remote Sensing Imager (RSI) to image the ground Earth, and the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) to study the transient luminous phenomena. The ISUAL instruments consist of a low-light level camera, a six-channel spectrophotometer and a red/blue band array photometer. With this set of instruments, ISUAL project seeks to determine the location and timing of upper atmospheric transient events above thunderstorms, to investigate their spatial, temporal and spectral properties, to obtain a global survey of upper atmospheric flashes, and to perform an additional global survey of auroras and airglows. In this talk, the characteristics of the ISUAL payload, the key parameters of ROCSAT-2, the observation strategy of the experiment and some preliminary results will be presented. ISUAL project is an international collaboration supported by the National Space Program Office in Taiwan, with additional contributions from the National Cheng Kung University (NCKU), Taiwan, the Space Science Laboratory of the University of California at Berkeley, and Tohoku University, Japan. The NCKU team is supported in part by research grants from National Space program Office (93-NSPO(B)-ISUAL-FA09-01) and National Science Council (NSC93-2112-M-006-007,, NSC93-2111-M-006-001) in Taiwan.
AE51A-02 INVITED 08:12h
Sprite Imaging Results from the ROCSAT2 ISUAL Instrument
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 N$_2$ 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 O$_2$ absorption. Numerous examples of elve-s were observed both in the N$_2$ 1st positive and the 762 nm filter bands. The nicest data on fully developed sprites was obtained in the N$_2$ 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.
AE51A-03 INVITED 08:27h
Global distribution of TLEs based on the preliminary ISUAL data
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.
AE51A-04 INVITED 08:42h
Observations of Sprites and Elves with the ISUAL Array Photometer
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.
AE51A-05 08:57h
The spectral signature of transient luminous events (TLE, sprite, elve, halo) as observed by ISUAL
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 N$_2$-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 O$_2$ 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.
AE51A-06 09:09h
Results From the European Sprite2003 Campaign
During the northern hemisphere summer of 2003, a sprite observation campaign was conducted with co-ordinated measurements from Southern Europe and from the magnetically conjugate region in South Africa. The goals of the campaign were to investigate the effects of sprites on the mesosphere, and look for signatures of the relativistic electron acceleration process in the magnetically conjugate hemisphere. Measurements in Europe included optical video imaging from a remotely controlled, semi-automatic camera system at the Observatoire du Pic du Midi, in the Pyrenee mountains in Southern France, infra-sound observations from a location north-west of Paris, and ULF-HF electromagnetic observations from a number of locations. The measurements in South Africa included observations of optical emissions by an array of 6 photometers and of VLF electromagnetic waves. The campaign was successful, with more than 130 sprites observed during 10 storms. The presentation will give an overview of the campaign and present results on (1) the optical observations, (2)infra-sound from sprites, (3) sprites possibly generated by intra-cloud lightning,(4)the relationship of sprites with early VLF ground transmitter signal perturbation events, and (5) conjugate signateres of sprites.
AE51A-07 INVITED 09:21h
Detection of Acoustic Emission From Sprites
During the summer of 2003, complementary measurements were taken in Europe during the Sprite2003 campaign to study sprites and associated processes. More than 70 sprites observed by light-sensitive optical cameras located at the Observatoire du Pic du Midi in the Pyrenees mountains were found to correlate with chirp-like signals of several tens seconds to one minute duration measured by an infrasound station at Flers (France) which was distant from the thunderstorms of 300-1200 km. Using an acoustic wave propagation model, the observed propagation delays, frequency dispersion, and duration of the infrasound bursts are found to be consistent with source altitudes at 60-80 km with horizontal dimensions equal to the optical width of the sprites. The chirp-like dispersion is a result of the propagation properties in the earth-thermosphere waveguide. The sprite activity identified by the infrasound signature continued past sunrise into the daytime where optical systems cannot be used, documenting the occurrence of daytime sprites.
AE51A-08 INVITED 09:36h
Sprite Spectra at 1000 fps
The physical processes involved in sprites develop on very fast time scales, and high speed spectroscopy may be the key to unraveling them. For this purpose we conducted a field campaign at the Langmuir Laboratory in Soccorro, NM, during the New Moon period in August, 2004. Spectra with millisecond resolution were recorded by a large aperture imaging spectrograph with the 1000 fps imager previously used by Stenbaek-Nielsen et al. (2002) to image sprites. The recordings were made over two wavelength ranges, 550-730 nm, and 620-800 nm, covering the brightest molecular nitrogen bands in the optical/near infrared emissions previously reported by Hampton et al. (1996). Preliminary analysis of the ms spectra show these band structures to be well resolved, but at the time of this writing it is uncertain if any ionized lines were present. Additional spectra at up to 40 kHz resolution were recorded by a newly developed spectral photometer. Analysis of this data set is ongoing.
AE51A-09 09:48h
High Time Resolution Telescopic Imaging of Fine Structure in Sprites
Experimental documentation of the dynamics of streamer formation and propagation is crucially important for a complete understanding of sprites and their effects on the mesosphere and the lower ionosphere. In July and August 2004, sprites were observed from Langmuir Laboratory using a 1000+ frames-per-second CCD imager mounted to a Dobsonian telescope. Additionally, photometer data was recorded using an array of six photometers with 40 us resolution, and broadband ELF and VLF data was recorded at multiple locations. We present telescopic video observations of sprites at high frame rates, demonstrating very complex structures in both space and time, including bead and streamer formation and propagation. Comparisons show that beads are seen to often propagate at much slower speeds than streamers, and at other times are static, while streamers often propagate at velocities on the order of one-tenth the speed of light. Streamers that appear and disappear within a single frame (i.e., 1 ms), with lateral sizes often in the range of tens of meters are documented. Streamers rarely persist for more than 1-2 ms, while beads persist up to tens of ms and often propagate at slow speeds. Additionally, images show that streamers are often made up of many bead-like structures. Photometer data and ELF/VLF data are also presented, as evidence for sprite halos and early/fast events associated with sprites.