AE31B-0162 0800h
Blue jet and grafted sprites observed in summertime, Japan
Jet-like phenomena associated with cloud-to-ground or intracloud lightning discharges were detected by normal video frame rate CCD cameras installed at two observation sites in Tokyo using a real time data processing software, named UFO Capture, on July 8, 2004. A blue starter event was found 0.2 after cloud flash. The blue jet event started 0.35 sec after the same cloud flash and extended upward, lasting for 0.38 sec with few re-enhancements in brightness. On a field image taken when the luminosity of jet peaked in time, two sprite clusters appeared at the top of the jet, like as a grafted tree. The estimated bottom and top heights of blue jet are 17 km and 46 km, respectively, while the top altitude of sprites is 80 km. These characteristics are consistent with parameters of blue jet, blue starter and sprites, reported in previous studies but the combination of the jet and grafted sprite feature would be the first case. We examined the polarity and charge moment of the lightning or possibly jet/sprites discharge using VLF and ELF waveforms recorded within a few hundred km from the events. The results suggest that both the grafted sprites and blue jet play some role to charge up the lower ionosphere, rather than to reduce the electric potential like as normal sprites caused by positive cloud to ground discharge.
AE31B-0163 0800h
Characteristics of Elves Observed with the Array Photometer on board the ROCSAT-2 Satellite
Elves are the very short (<few ms) transient luminous phenomena at altitude of the lower ionosphere (90-100 km) with a horizontal extent of 200 _| 500 km in diameter induced by strong electromagnetic pulse from lightning discharge. It has been confirmed by multi-channel photometric measurements that the apparent motion observed from the ground site at a distance over the radius of elves is downward at quite high speed, which is well consistent with numerical calculations of electromagnetic model. Simulation studies predict that in the optical emission region of elves the electron density could be modulated by one order or more. In order to estimate the quantitative effect of elves on the ambient atmosphere/ionosphere, measurements of absolute luminosity and spectral intensity ratio between emission bands are required. For this purpose we developed high-speed multi-anode array photometers (MAPs), which have 16 channel anodes aligned in the vertical or horizontal direction with two band pass filters, and have conducted ground observations for almost one decayed. Based on the observational result we estimated the optical emission altitude precisely and found that in the very initial phase of elves events the electron energy show considerably high values compared to the time period of maximum luminosity. However, the optical extinctions and scatters by atmosphere and aerosols prevent us to calculate the absolute values of physical parameters. The well-calibrated ISUAL instruments onboard the ROCSAT-2 satellite provide us quantitative brightness information about transient luminous events in the upper and middle atmosphere. The array photometer (AP), one of the ISUAL instrument, is an application of MAP to space use, which measures intensity ratio of two colors, namely, 360-470 nm and 520-750 nm, with absolute brightness. It is found using the preliminary observational data obtained by AP that elves consist of both red (N2 1st Positive and N2 Meinel) and blue(N2 2nd Positive and N2 1st Negative) colors at magnitude of almost same order. The ISUAL recorded about 100 elves for first three months. We discuss the possible ionizing and heating effects of elves based on the initial observational results.
AE31B-0164 0800h
ISUAL calibration and optimal ground coverage for detecting transient luminous events
ISUAL, the scientific payload of ROCSAT-2 satellite began its operation since July 1, 2004. ISUAL consists of an intensified imager, a 6-channel spectrometer and a dual-channel/16-anodes array photometer. The mission goals are to acquire spatial, spectral and temporal informations of transient luminous events (TLEs) from an 891 km sun-sync polar orbit. A series of post-launch on-orbit tests were performed to extract crucial instrumental parameters for data processing. We will report some of the results. To maximize the efficiency for global survey, the ground coverage of ISUAL at different spacecraft roll and yaw angles as well as in different seasons was computed. By comparing the computed ground coverage and the global lightning distribution maps obtained from satellites like TRMM, we can determine the best altitude of imaging in order to maximize the chance in capturing TLEs. In this presentation, the salient informations for the groups that wish to perform future ground/space coordinated observations of TLEs will also be provided. *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.
AE31B-0165 0800h
Spatial and Temporal Structures of Sprites Observed with the Array Photometer on board the ROCSAT-2 Satellite
Sprites are transient luminous events at the altitude of the mesosphere mostly induced by positive cloud-to-ground lightning discharges (+CGs). Past theoretical and experimental studies put forward the quasi-electrostatic (QE) model in which energetic electrons accelerated by the QE fields excite and ionize ambient neutral particles such as nitrogen molecules through collision processes. By measuring the spatio-temporal evolution and spectral characteristics of optical emissions, it is possible to investigate the physical and chemical processes occurring in sprites quantitatively. Miyasato et al. [2004] estimated electron energies by analyzing sprite data obtained from ground-based observations with a dual-color photometer, and suggested that strong ionizations occur at the initial stage of sprite halo. Ground spectral information of sprites, however, has large uncertainty due to the attenuation effects by atmospheric molecules and aerosol particles along the light path from the sprite emission region to the ground observation site. For that reason, optical observations from space are essential for the precise estimation of absolute luminosity. The ROCSAT-2 satellite with a scientific payload named ISUAL (Imager of Sprites/Upper Atmospheric Lightning) is the first satellite which observes transient luminous events such as sprites from space. The ISUAL is composed of an imager, a spectrophotometer, and an array photometer (AP). The AP 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. The AP has 16 channels arrayed in vertical and spatial resolution corresponds to ~11 km in the case of sprites occurring at the limb point 3315 km away from the satellite. The time resolution of 50 or 500 Es enables us to detect vertical motions of sprites which have an average duration of several to tens of ms. From the middle of July to the end of August in 2004, we observed ~2210 lightning flashes, ~20 sprites, and ~100 elves. The average observation frequency of lightning and sprite flashes are 55 and 0.5 per day, respectively. The AP succeeded in detecting clear upward/downward vertical motions of sprites in both the blue and red channels with quite high signal-to-noise ratios. In this study, we discuss the spatio-temporal evolution and spectral characteristics of sprites in detail.
AE31B-0166 0800h
Ground based ISUAL validation with the Los Alamos Sferic Array
The Los Alamos Sferic Array (LASA) is an array of fast electric field change meters which has been used for validation studies of satellite lightning observations. The array was established to support the FORTE satellite (with both radio and optical sensors), and therefore the LASA/FORTE coincident database has been the most heavily studied. LASA has also been used to interpret and validate the observations of LIS and OTD (with optical detectors). The EDOT data set is currently being used to compliment the ROCSAT-2 ISUAL payload observations of sprites. In this presentation, we will briefly review some of the surprising results which have been documented or validated with LASA in FORTE and LIS/OTD studies, specifically the comparison of radio and optical observations of lightning. We will present a comparison of ISUAL observations with LASA electric field measurements and discuss the coincident observations based our experience with past LASA satellite validation studies.
AE31B-0167 0800h
Role of intracloud lightning discharges in occurrences of winter sprites
Sprite often occur as far as 50 kilometers horizontally away from the location of the parent CG discharges [Lyons, 1996], and long time delays more than 100 milliseconds from the parent CGs to sprite onset are also often reported [Bell et al., 1998; Fuellekrug and Reising, 1998]. These results suggest the role of horizontal current channels in lightning discharges producing sprites. However, the role of horizontal components is still an open question. To capture the evolution of lightning discharge inducing sprites, we carried out broadband ELF/VLF sferic measurement during the winter sprite campaign at Tohoku University Iitate observatory (37.7oN, 140.7oE) in Japan. The campaign period is December 15, 2003 to the middle of February. We employed an II-CCD camera and two sets of multi-anode array photometers (MAPs) for optical measurement, and a VLF receiver with a vertical electric field dipole antenna and two horizontal crossed loop magnetic field antennas for VLF sferics measurement. The frequency range of the VLF receiver is 1 to 40 kHz for all antennas and output signals are recorded as an event trigger mode using an A/D board with a sampling rate of 100 kHz. Furthermore we use ELF data recorded at Onagawa Observatory of Tohoku University, Japan [Sato et al., 2003]. Orthogonal search coil magnetometers are installed and the output signals which are amplified and band pass filtered in the range of 1-100 Hz are recorded continuously using an analog-to-digital converter with a sampling rate of 400 Hz. It is found that VLF data associated with sprites show the occurrences of sferic clusters characterized by long durations (10 to 100 milliseconds) and higher frequency components (10 - 40 kHz) without exception. In addition, ELF waveform data show occurrences of positive transient bursts related to continuing currents in lightning discharges just coincidently with the occurrences of sferic clusters. Since sfeirc clusters are likely to be produced by intracloud discharges inducing horizontal current channels, we will discuss about the role of intracloud discharges and continuing horizontal currents in the sprite generation.
AE31B-0168 0800h
Automated, Remote-Controlled Optical System for TLE Observations: The SpriteWatch
A camera system for observations of Transient Luminous Events (TLE) was mounted on Observatoire du Pic du Midi in the Pyrenees during the summer months of 2003 and 2004. The system was operated remotely over the Internet by a team of scientists located at research institutions around Europe, automatically detecting TLEs and transmitting event data to be viewed by the operator. The experiment was highly successful, with more than 130 TLEs observed in 2003, providing a reference data set for the multitude of complementary TLE observations taken simultaneously in Europe and South Africa. The heart of the system is the real time feed of video data into a digital processing station, running the SpriteWatch software - a tool for analysis of video data on Linux computers. Taking advantage of the highly integrated networking functionality of the Linux kernel and its multitasking capabilities, the 2004 system was able to analyze four video streams in real time, and store one or more images from each stream when events are detected. In this paper, a complete description of the Spritewatch system is given, with Internet reference to the software. It is the hope, that the freely available software will inspire others to establish semi-automated observations sites, adding further momentum to ground-based observations of TLEs.
AE31B-0169 0800h
Cloud-to-ground and Intracloud lightning activity associated with Sprite events observed over France on 23 July 2003
As a part of the European research program CAL, sprite observations were carried out from the Pic du Midi observatory in the French Pyrenees during the summer 2003. Images of the sprites were taken by two remotely controlled CCD cameras. The 23 July observations are considered particularly interesting because we then had access to data concerning both cloud-to-ground and intracloud lightning activity. A large thunderstorm with two convective cores, one to the north and the other to the south, developed over the South of France during the late afternoon, and about two hours after sunset, the first sprite was detected. During a little more than three hours, 13 sprites were observed, 7 over the northern system and 6 over the southern system. The images enabled us to determine the azimuth angle of each sprite from the OMP observatory. 12 of the 13 sprites could be associated to positive cloud-to-ground flashes, and one could be associated with intracloud flash. Satellite images were included at this point of the study, and it appeared clear that sprites tend to occur over the stratiform region of the storm system in the area with the coldest (highest) cloud tops. The associated positive flashes were also within or close to this portion of the storm. The sprite occurrences are studied in relation to the cloud-to-ground and to the intracloud activity. We find that sprites seem to occur in a late stage of each storm system, when the rate of negative cloud-to-ground flashes has considerably decreased, and when the ratio of positive cloud-to-ground flashes is much higher than during the most active phase of the storm. Globally, the intracloud activity is also low during the sprite-producing periods, but sudden "bursts" of intracloud lightning could frequently be observed at the moment of the sprite. The area coverage of the radar echo was calculated. The result supports the idea that sprite events tend to appear almost exclusively over large thunderstorm systems.
AE31B-0170 0800h
Possible Detection of a Conjugate Sprite
We report the detection of a transient luminous event (TLE) exhibiting an uncharacteristic large time delay and lateral displacement from any near-by thunderstorm. The event was imaged from the space shuttle Columbia during the MEIDEX sprite campaign in January 2003 (Yair et al., 2004) south of Madagascar above the Indian Ocean. It was delayed 0.23 seconds from a preceding visual flash and had a horizontal displacement $>$ 1000 km from the nearest lightning activity, values far larger than previously reported. The calculated brightness in the 860 (±40) nm filter was 310kR, and the morphology of the emitting volume did not resemble any known class of TLE (i.e. sprites, ELVES or halos). We propose that this emission was the result of a relativistic electron beam emitted from a thunderstorm at the magnetic conjugate hemisphere and impacting the lower mesosphere (Lehtinen et al., 2000). Analysis of meteorological data, satellite images and ground-based lightning locations suggests that lightning occurring in an eastern Mediterranean storm near Turkey may be the cause of this event south of Madagascar, which we interpret as a possible conjugate sprite.
AE31B-0171 0800h
Balloon Observations of the Electromagnetic Fields from Quasi-Planar Ionizing Solitary Waves Associated With Sprites
The University of Houston conducted 3 balloon flights to study sprite electrodynamics during the Sprites99 balloon campaign. During this campaign, the University of Alaska performed a series of optical sprite observations from the Wyoming Infrared Observatory (WIRO) on Jelm Mountain. One of the cameras was a white light 1000 frame/sec high-speed imager (HSI) that permitted following sprite dynamics at millisecond time scales and spatial resolutions of a few hundred meters. The best balloon flight occurred on the night 21 August 1999. A number of sprites exhibiting a variety of complex forms over a wide range of brightness were recorded the night of 18 August 1999 over two thunderstorms centered over Kansas and the South Dakota-Minnesota border, $\sim$700 km from WIRO. Twelve separate events were observed simultaneously by the balloon and by the HSI. An analysis of contrast enhanced images of sprites from this storm revealed several more examples of the previously unreported type of weak ($\sim$several hundred kR), diffuse emission associated with most events that were discussed in the previous talk. In time lapse sequences the diffuse emissions give the visual appearance of downward expanding puffs of smoke launched from the bottom ends of tendrils at $\sim$60 km altitude, reaching diameters $\sim$ 5-10 km over $\sim$5-10 ms, for an average apparent expansion speed of $\sim$1000 km/sec. Several of the events clearly exhibited an effect analogous to limb brightening, suggesting the emissions originated in expanding shells of $\sim$1-2 km thickness. Other events, unaccompanied by immediate streamers, were observed to originate near 75 km and propagate downward as quasi-planar sheets of weak optical emissions. The previous talk suggested that some of the puzzling features of ELF/VLF electromagnetic emissions associated with sprites may derive from charge transport by ionizing solitary waves. This paper will present a detailed examination of the electric and magnetic field perturbations observed by the balloon during these sprites and compare them with the HSI data.
http://www.uh.edu/research/spg/Sprites99.html
AE31B-0172 0800h
Observations of Quasi-Planar Ionizing Solitary Waves Associated With Sprites
The University of Alaska performed a series of optical sprite observations from the Wyoming Infrared Observatory (WIRO) on Jelm Mountain as part of the Sprites99 field campaign. One of the cameras was a white light 1000 frame/sec high speed imager (HSI) that permitted following sprite dynamics at millisecond time scales and spatial resolutions of a few hundred meters. A number of sprites exhibiting a variety of complex forms over a wide range of brightness were recorded the night of 18 August 1999 over a very active thunderstorm centered over nearby Nebraska, ~300 km from WIRO. An analysis of contrast enhanced images of sprites from this storm revealed a previously unreported type of weak (~several hundred kR), diffuse emission associated with most events. In time lapse sequences the diffuse emissions give the visual appearance of downward expanding puffs of smoke launched from the bottom ends of streamer tendrils at ~60 km altitude, reaching diameters ~ 5-10 km over ~5-10 ms, for an average apparent expansion speed of ~ 1000 km/sec. Several of the events clearly exhibited an effect analogous to limb brightening, suggesting the emissions originated in expanding shells of ~1-2 km thickness. Other events, unaccompanied by immediate streamers, were observed to originate near 75 km and propagate downward as quasi-planar sheets of weak optical emissions. It is proposed that these emissions are the optical signatures of large scale, ionizing solitary waves excited in the upper atmosphere by transient quasi-electrostatic lightning electric fields, and which propagate downward as shells or sheets of electric charge. A one-dimensional fluid model of ionizing solitary waves is developed appropriate to the weakly ionized collisional plasma environment of the upper atmosphere. Both negative and positive charge density solitary waves may occur, depending on the polarity of the applied transient electric field. The solitary waves propagate as thin, self-consistent charge structures at speeds typically much larger than the electron drift speed. It is found that electron attachment processes play a strong role in determining both the internal charge structure of the solitary wave and the plasma composition in its trailing wake. When taken in combination with streamers, which may be thought of as thin columnar versions of ionizing solitary waves, such events may help account in part for the enormous spatial complexity that typically characterizes sprites. It is suggested that some of the puzzling features of ELF/VLF electromagnetic emissions associated with sprites may derive from charge transport by ionizing solitary waves.
AE31B-0173 0800h
Measurement of D region ionospheric perturbations caused by direct lightning-ionosphere interactions
The first experimental evidence of the impulsive direct coupling of energy released by lightning discharge to the lower ionosphere was reported in the form of early/fast perturbation on sub-ionospherically propagating very low frequency (VLF) signals. Since then, based on modeling and measurements, different mechanisms have been advanced to explain the early/fast sub-ionospheric VLF perturbations such as electrical breakdown associated with sprites and halos or the ionization and heating associated with elves. Using VLF energy from lightning strokes that occurred immediately before and after an intense lightning discharge, we can detect D region disturbances caused by the intense lightning flash. The detailed electron density changes caused by the disturbance are measured by analyzing the broadband VLF propagation changes. We also carefully investigate the causative lighting characteristics such as lightning charge moment and peak current to give additional insight into the perturbation mechanism. Our results suggest that many detectable sub-ionospheric VLF perturbations are created by high peak current lightning strokes probably associated with elves, but some appear to be generated by otherwise unexceptional strokes.