AE23A-0888
Distribution and seasonal variation of global lightning activities observed by ISUAL experiment
Lightnings serve as an important charge transporter between cloud and ground, and emit sferics those propagate in the ground-ionosphere cavity. The question on how global lightning activity might change in the future as a result of the global warming has attracted great attention. During the three-year observation of ISUAL/FORMOSAT-2, besides upper atmospheric transient luminous events more than 60,000 lightnings which exceeded the trigger threshold were recorded. In this presentation, the distribution, occurrence rate, and seasonal variation of these lightnings at local time between 22:30 and 23:00 will be reported, and compared with that of the OTD/LIS experiment (Christian, et al., 2003). Our results may suggest that intensive lightnings are possible more efficient to generate elves, and the production efficient is assessed in this presentation.
AE23A-0889
Estimating radiated energy of lightning strokes detected by the World Wide Lightning Location Network
The World Wide Lightning Location Network (WWLLN) provides real-time lightning locations globally by measuring the very low frequency (VLF) radiation emanating from lightning discharges. The location accuracy and efficiency of the WWLLN have been estimated for certain regions of the globe. The next upgrade to the WWLLN is to provide an estimate of the energy radiated by each WWLLN-detected lightning stroke. Each WWLLN receiving station measures the integrated VLF energy at the antenna. Using this integrated energy value at each station and by accounting for ionospheric attenuation on the stroke-to-receiver path, we calculate an estimated amount of energy radiated at the location of the lightning stroke. We then find the correlation between radiated energy and peak current by comparing lightning strokes detected by both WWLLN and by the Los Alamos Sferic Array. Estimates of stroke strength from lightning strokes on a global basis will be useful in explaining the global distribution of transient luminous events such as sprites and elves, as well as many other scientific questions.
AE23A-0890
Differences in the Optical Characteristics of Continental US Ground and Cloud Flashes as Observed From Space
Continental US lightning flashes observed by the Optical Transient Detector (OTD) are categorized according to flash type (ground or cloud flash) using US National Lightning Detection NetworkTM (NLDN) data. The statistics of the ground and cloud flash optical parameters (e.g., radiance, area, duration, number of optical groups, and number of optical events) are inter-compared. On average, the ground flash cloud-top emissions are more radiant, illuminate a larger area, are longer lasting, and have more optical groups and optical events than those cloud-top emissions associated with cloud flashes. Given these differences, it is suggested that the methods of Bayesian Inference could be used to help discriminate between ground and cloud flashes. The ability to discriminate flash type on-orbit is highly desired since such information would help researchers and operational decision makers better assess the intensification, evolutionary state, and severe weather potential of thunderstorms. This work supports risk reduction activities presently underway for the future launch of the GOES- R Geostationary Lightning Mapper (GLM).
AE23A-0891
Upward Electrical Discharges From Thunderstorms
Thunderclouds commonly produce intracloud lightning as a result of the balanced content of the main cloud charges, and cloud-to-ground lightning as a result of charge imbalances developing in the storm. Thunderclouds also produce upward lightning discharges in the form of blue jets and gigantic jets. The present study is based on a combination of recent observations of upward discharges and computational studies of storm electrodynamics, supported by numerical simulations of the discharges using a stochastic lightning model. The results indicate that the mechanisms by which blue and gigantic jet discharges escape from clouds are similar to those of cloud-to-ground lightning.
AE23A-0892
On Possibility to use VLF Transmitter Signals Received on Satellite and Ground for Global Diagnostics of Ionospheric Perturbations Associated With Seismicity and Volcano Eruption
We present two methods of the global ionosphere diagnostics using VLF signal received on a board of satellite in association with two cases of strong seismic activation. The method of reception zone changes reveals evident effect before and during great Sumatra earthquake with long-time duration order of one month. The result leads to conclusion on size of perturbation area order of several thousands kilometers. Disadvantage of this method is in difficulty to separate preseismic and postseismic effects. In contrast, the difference method allows to do so and shows the appearance of preseismic effect in several days for seismic activation near Japan. However for reliability of such analysis we need in regular satellite data and in the check by the ground reception of subionospheric VLF signal. It is not obvious that both effects are excited by the same generation mechanism at the ground. So, these methods look as complementary.
AE23A-0893
Modeling the Slow-Tail of Atmospheric Waves to Approximate the Distance of Propagation
A lightning strike emits an electromagnetic wave known as an atmospheric or sferic, which propagates through the earth-ionosphere waveguide. Sferics can be recorded by extremely low and very low frequency, ELF and VLF, receiver systems. The recorded signal is composed of two segments, a pulse containing VLF frequencies, followed by a slow-tail, containing the ELF components. The slow-tail is essentially a single cycle wave, which is delayed with respect to the rest of the sferic due to the dispersive nature of the ionosphere. The recorded time- domain slow-tail varies with the lightning strike's current moment, and the waveguide's media characteristics. It is possible to approximate the location of the lightning source with measurements of the sferic. Many methods require measurements from multiple stations, however the goal of this work is to approximate the distance a sferic propagated with a single station. J.R. Wait developed a mode theory where propagating ELF radio are characterized by the first mode. The research reported here uses the first mode equations to model a slow-tail that propagated a certain distance. We include a comparison to measurements on slow-tails observed at widely variable distances from their causative lightning, and analyze the accuracy of our model. Using the inverse of this method along with sferics from known locations, we approximate the form of the current moment at the source and use an average of this waveform to improve our slow-tail model. With an accurate computed slow-tail we can approximate the distance of propagation by fitting the computed waveform to the observed slow-tail. An analysis is given of the effectiveness of this method. As expected, since this method uses data from only one station, the estimation error from this method are larger than those of the traditional multiple station estimation method. However, in most instances our method was accurate to within hundreds of kilometers. With such accuracy, this method can be used to predict abnormalities in the conditions of the ionosphere along the path of propagation. In addition, it is a powerful tool that can be used to validate multiple station estimates and determine any errors due to single station faults.
AE23A-0894
The similarities between the hallucinations associated with the partial epileptic seizures of the occipital lobe and ball lightning observations
Ball Lightning was seen and described since antiquity and recorded in many places. Ball lightning is usually observed during thunderstorms but large number of ball lightning observations is also reported during fine weather without any connection to thunderstorms or lightning. However, so far no one has managed to generate them in the laboratory. It is photographed very rarely and in many cases the authenticity of them is questionable. It is possible that many different phenomena are grouped together and categorized simply as ball lightning. Indeed, the visual hallucinations associated with simple partial epileptic seizures, during which the patient remains conscious, may also be categorized by a patient unaware of his or her condition as ball lightning observation. Such visual hallucinations may occur as a result of an epileptic seizure in the occipital, temporo-occipital or temporal lobes of the cerebrum [1,2,3]. In some cases the hallucination is perceived as a coloured ball moving horizontally from the periphery to the centre of the vision. The ball may appear to be rotating or spinning. The colour of the ball can be red, yellow, blue or green. Sometimes, the ball may appear to have a solid structure surrounded by a thin glow or in other cases the ball appears to generate spark like phenomena. When the ball is moving towards the centre of the vision it may increase its intensity and when it reaches the centre it can ‘explode' illuminating the whole field of vision. During the hallucinations the vision is obscured only in the area occupied by the apparent object. The hallucinations may last for 5 to 30 seconds and rarely up to a minute. Occipital seizures may spread into other regions of the brain giving auditory, olfactory and sensory sensations. These sensations could be buzzing sounds, the smell of burning rubber, pain with thermal perception especially in the arms and the face, and numbness and tingling sensation. In some cases a person may experience only one seizure during lifetime and may not be aware of the reason for the experience. Being of good health otherwise, the person may categorize the experience as a ball lightning encounter. If, as described above, the seizure spread into other regions of the brain the resulting experience may appear as electrical effects (the smell, heat sensation, tingling feeling etc.) of ball lightning. Epileptic seizures are a common and important medical problem, with about one in eleven persons experiencing at least one seizure at some point. Thus some of the ball lightning encounters presented in the literature could very well be associated with the experiences of persons who had an epileptic seizure with visual hallucinations. [1] Blom, S. et al., Epilepsy, Neurology, Edited by S-M Aquilonius and J. Fagius, Liber, 2000. [2] Panayiotopoulos, C. P., J. Neorl. Neurosurg. Psychiatry, 66, 536-540, 1999. [3] Bien et al, Brain,123, 244-253, 2000.
AE23A-0895
Electromagnetical, Visual and Meteorological Analyses of two Gigantic Jets Observed Over Missouri, USA
An amateur astronomer operating a Watec 100N camera in Broken Arrow, Oklahoma discovered 2 gigantic jets (GJ) rising out of a thunderstorm over SW Missouri on 20 August 2007, at 9:24:04 and 9:25:48 UTC. They are the first observed to occur over the continental United States and this far poleward (37.2°N). The azimuths of the GJs corresponded with two different cores of a slow-moving multicell storm, spaced apart by 50 km. At distances of 239 and 204 km from the camera, the GJs reached heights of 94 and 83 km, with bright transition zones between 45-61 and 50-64 km. The jets only became visible to the camera at/near completion of the pathway to the ionosphere, followed by a glow moving upward through the transition zone over 250-300 ms (at 1.6-5 104 m/s). The storm cluster was aged 4 hours by the time of the GJs. The events occurred over separate NEXRAD radar echo tops reaching 15 km, with evidence of new strong updrafts from GOES IR imagery and radar hail signatures. Within 5 minutes before each event, flash rates of positive cloud-to-ground flashes increased and dominated over negative by a factor of 3-4 for a few minutes (but low rates), but without quick rises or lulls. Weak NLDN- detected flashes (intracloud) correlated to both events in time and space. One sprite was observed 50 minutes after the events. Initial inspection of ELF-VLF transients recorded by receivers operated by Duke and Stanford universities indicated signals close to the approximate event times, but multi-site analysis has yet to confirm the geographical origin and the likelihood of any possible association.
AE23A-0896
Sprite Halos and Associated Lightning Characteristics over South America
As part of the Brazil Sprites II balloon campaign ground-based measurements were conducted from the Southern Space Observatory (SSO) near Santa Maria, Brazil (29.4 S, 53.8 W) during the summer 2006. Remarkably, over 580 transient luminous events (TLEs) were imaged from two large thunderstorms located over north-eastern Argentina on February 22-23 and March 3- 4 2006. The optical observations were made using two intensified CCD cameras arranged to view the low elevation sky with overlapping fields of view to capture the TLE information from the extensive storm systems. This poster focuses on the optical characteristics over 200 halos and sprite-halos with their associated polarity and charge moments as determined by an ELF-VLF lightning sensor also deployed at the SSO. The halo events from these two storms were overwhelmingly associated with positive lightning discharges, however at least 7 TLE's were associated with negative lightning strokes. In contrast, recent satellite measurements of halos over land-sea interfaces suggest that they are mainly associated with negative cloud-to-ground discharges. Coincident halo data with the World Wide Lightning Location Network (WWLLN) have also enabled a detailed investigation of the altitude and apparent optical diameters of the halos and sprite halos.
AE23A-0897
An enhancement of the ionospheric sporadic E layer in response to negative polarity cloud-to- ground lightning
Lightning data from 2005 to 2007, collected using a Boltek Storm Tracker system installed at Chilton, UK, was used to investigate the mean response of the ionospheric sporadic-E layer to lightning strokes in a superposed epoch study. Unlike similar previous studies, the lightning detector used in the current study is capable of discriminating between positive and negative lightning strokes and between cloud-to-ground and inter-cloud lightning strokes. Superposed epoch studies carried out separately using these subsets of lightning strokes as trigger events revealed that the dominant cause of the observed ionospheric enhancement in the Es layer results from negative cloud-to-ground lightning events.
AE23A-0898
Sprite and Lightning Infrasound Measurements during the 2005 Eurosprite Campaign
Liszka [2004] first assumed that infrasound have a specific signature in their spectrogram (a chirp) when a sprite occurs. During the Eurosprite 2003 campaign, Farges et al. [2005] correlated such kind of infrasound with camera observation of sprite. They used the data of the Flers infrasound station located in the North-West of France. The correlation between the time of an infrasound due to sprite and the sprite time is done by calculating the propagation time with a ray-tracing model. A clear relation between the infrasound duration and the sprite size has been found. For the Eurosprite 2005 campaign, in addition to the permanent infrasound station of Flers, a supplementary station has been set up in St Just at ~400 km southward away from Flers. The two main objectives for the infrasound campaign were: i) to measure the same sprite infrasound with two stations and to possibly locate it and ii) to evaluate the attenuation with distance of the infrasound produced by lightning. The expected result of this second objective was the confirmation that the chirp signature is due to the sprite alone and not to the sprite and its parent lightning. The conditions allowing the observation of sprites have been investigated. For instance, numerous infrasounds of sprites were detected during a large storm on September 26th. But the location of this sprite-producing storm (southward relatively to the two sensors and over the Mediterranean Sea) limited the capability to detect them with both stations and then to locate a sprite from joined infrasound measurements. Studies on the characteristics of infrasound of lightning have been done, particularly on September 10th, when a storm moved North-Eastward from the Bay of Biscay, passing just over the St Just station. At the same time, no lightning activity was recorded at less than 200 km away from Flers station. The amplitude of infrasound produced by lightning is higher than the noise only when lightning are located at distances lower than 50 km from the station. When lightning are close to station, the infrasound amplitude reach few Pa and a dynamic of 30 dB (in comparison to the noise level) is then found. Simultaneously, nothing has been detected in the Flers station data. These joined measurements show clearly that infrasound produced by lightning could be only measured at short distance from the source. The chirp signature is then definitely a property of sprite infrasound. Spectrum of isolated lightning is also examined. A flat response between 0.3 and 1 Hz is found when the lightning distance is less than 20 km from the station. As Liszka shown at the previous AGU Fall meeting [2006], we found also some infrasound which can document the measurement of infrasound due to intracloud discharge. At last, from the infrasound signals detected at St Just, we try to evaluate the charge amount discharged during a return-stroke as it was theoretically assumed by Dessler [1973] and Few [1985].
AE23A-0899
Mechanism of infrasound radiation from sprites
The possibility of infrasound generation by sprite discharges has been discussed by Bedard et al. [Eos Trans. AGU, 80 (46), Fall Meet. Suppl., F227, A51B-18, 1999]. Specific chirp-like infrasound signatures possibly related to sprite discharges have been identified by Liszka [J. Low Freq. Noise Vibr. Active Control, 23, 85, 2004], and their phenomenology further discussed by Liszka and Hobara [JASTP, 68, 1179, 2006]. Farges et al. [GRL, 32, L13824, 2005] provided first direct correlation of the chirp-like signatures in 0.1-9 Hz range with optical observations of sprites. The sprite correlated infrasound signatures are characterized by arrival of low frequencies before high frequencies, pressure amplitudes on the order 0.01-0.1 Pa at horizontal distances ~eq400 km, and durations of several tens of seconds, which are well correlated with physical horizontal dimensions of sprites observed optically [Farges et al., 2005]. The strength of observed infrasound emissions may appear to contradict available analysis of air heating in sprite streamers giving temperature changes Δ T/T~0.2-2% [Pasko et al., GRL, 25, 2123, 1998], an accurate analysis of energy budget of sprite discharges indicating Δ T≤ 0.5 K [Sentman et al., JASTP, 65, 537, 2003], and limited information on sprite rotational temperature indicating consistency of observed molecular nitrogen band emissions with rotational temperatures in the range 220-230 K (not exceeding 300 K) [Green et al., GRL, 23, 2161, 1996; Morrill et al., JASTP, 60, 811, 1998; Bucsela et al., 65, 583, 2003; Kanmae et al., GRL, 34, L07810, 2007]. In this talk an analysis is presented demonstrating that observed sprite infrasound signatures are consistent with weak air heating in streamer channels in sprites on the order of several degrees K, and with known atmospheric absorption properties of infrasound [Blanc, Ann. Geophys., 3, 673, 1985; Sutherland and Bass, J. Acoust. Soc. Am., 115, 1012, 2004, and references therein].
AE23A-0900
Sprite-generated Infrasonic Chirps
Sprite-generated infrasonic chirps were recorded at infrasonic arrays belonging to the present Swedish-Finish Infrasound Network during the past 13 years. The present study is concentrated on morphology of the phenomenon, in particular on its temporal and spatial distributions. The occurrence of chirps reveals a 3.2-year variation, which is in phase with the corresponding component of solar activity. Daily- and annual variations indicate that the phenomenon does not necessarily coincide with the thunderstorm activity. In addition to a conventional wavelet analysis the chirps are analyzed using a set of 6 wavelet filters covering the frequency band 0.5 to 6 Hz. A representation of the chirp in the angle-of-arrival – frequency domain describes both the source and the influence of the atmosphere on the chirp signal. It provides the information about the horizontal extent of the source. The angle-of-arrival – frequency representation may be used for categorization of the phenomenon. Also the horizontal trace velocity of the chirp signal is studied in the frequency domain. It provides the information about the angle of incidence of the ray at different frequencies and thus about the vertical extent of the source.
AE23A-0901
On the Lightning Induced Transient Emissions (LITEs) in the OH Airglow Region with ISUAL Observations
Elves and sprites are the two recently observed lightning induced transient emissions (LITEs) in the mesosphere. Observations of elves by ISUAL broadband filter onboard the FORMOSAT-II satellite have sometimes shown a transient brightness at or below the OH airglow altitude in the event of a lightning flash. It is generally accepted that electrons heated by lightning EMPs impact N2 species to produce the optical emissions. Given that the sudden brightness oftentimes occurred at the OH airglow altitude and that OH species require only 3 to 4 eV to produce optical emissions, it is very possible that OH species can be another light-emitting species interacting with the EMP-accelerated electrons. Due to too much overlap of OH and N2 spectrum within the bandwidth of the broadband filter, it was not possible to determine which species are responsible for the observed enhancements. In response to the need to find the causes for such enhancements, ISUAL conducted a 9-day campaign in January 2007, with some observations devoted exclusively to capturing LITEs with the 630 nm filter for such an investigation. We will present the analysis of the observations from the CCD camera, spectrophotometer, and array photometer, and will discuss the implications of the results.
AE23A-0902
Modeling of Optical Emissions From Red Sprites After Benchmarking With Laboratory Experiments
We have benchmarked our optical model (POEM, the Physics Based Optical Emission Model) with various laboratory air fluorescence experiments after making significant improvements to the model with regards to such issues as collisional quenching, thus increasing our level of confidence in the model. We have transferred features from our benchmarked air fluorescence model to a sprite modeling version of the POEM code. We will show results from a fully 2-D electromagnetic model known as UNIMAX (the Unified Maxwell code) and the improved POEM optical model to demonstrate the level of agreement between the simulations and severla optical measurements (camera, photometer, and spectral measurements) of sprites, including those we believe to involve the phenomena of runaway electron breakdown. We model optical emissions across the spectral range from 300 – 1000 nm. We will distinguish which measurements and model results are indicative of conventional breakdown and which are indicative of runaway breakdown.
AE23A-0903
Production of Nitrogen Oxides by Laboratory Simulated Transient Luminous Events
Restoration of the polar stratospheric ozone layer has occurred at rates below those originally expected following reductions in chlorofluorocarbon (CFC) usage. Additional reactions affecting ozone depletion now must also be considered. This research examines nitrogen oxides (NOx) produced in the middle atmosphere by transient luminous events (TLEs), with NOx production in this layer contributing to the loss of stratospheric ozone. In particular, NOx produced by sprites in the mesosphere would be transported to the polar stratosphere via the global meridional circulation and downward diffusion. A pressure-controlled vacuum chamber was used to simulate middle atmosphere pressures, while a power supply and in-chamber electrodes were used to simulate TLEs in the pressure controlled environment. Chemiluminescence NOx analyzers were used to sample NOx produced by the chamber discharges- originally a Monitor Labs Model 8440E, later a Thermo Environment Model 42. Total NOx production for each discharge as well as NOx per ampere of current and NOx per Joule of discharge energy were plotted. Absolute NOx production was greatest for discharge environments with upper tropospheric pressures (100-380 torr), while NOx/J was greatest for discharge environments with stratospheric pressures (around 10 torr). The different production efficiencies in NOx/J as a function of pressure pointed to three different production regimes, each with its own reaction mechanisms: one for tropospheric pressures, one for stratospheric pressures, and one for upper stratospheric to mesospheric pressures (no greater than 1 torr).
AE23A-0904
Use of Narrow Bipolar Event sferics to probe lower-ionosphere disturbances
We are developing a method to probe the lower ionospheric D-region with multi-gigawatt LF radiation from Narrow Bipolar Event lightning discharges. The goal is to attain a useful lower-ionospheric "sounding", akin to an ionosonde, but taking advantage of natural wideband radio emissions whose power greatly exceeds any man- made transmitter. We are developing a full-wave model from which to retrieve D-region structural information via comparison of the model with data. We have already exercised this tool against D-region disturbances from solar flare transients. The ultimate goal is to exercise the tool against more localized disturbances caused by lightning EMP, thunderstorm quasi-electrostatic heating, and lightning-instigated energetic particle precipitation. This poster will report on progress to date.
AE23A-0905
Charge Transfer Characteristics and Initiation Mechanisms of Long Delayed Sprites
Simultaneous measurements of high altitude optical emissions and the magnetic field produced by sprite-associated lightning discharges enable a close examination of the link between low altitude lightning process and high altitude sprite process. In this work, we report results of the coordinated analysis of high speed (1000--10000 frames per second) sprite video and wideband (0.1 Hz to 30 kHz) magnetic field measurements made simultaneously at the Yucca Ridge Field Station and Duke University during the June through August 2005 campaign period. During the observation period, the high speed camera detected 83 sprite events in 67 TLE sequences, which are caused by the same number of +CGs. 46% of these sprite events are delayed more than 10 ms after the lightning return stroke. With the estimated lightning source current moment waveform, we computed the continuing current amplitude and total charge transfer characteristics of the long delayed sprites (>10 ms delay). Our calculation shows the total charge moment change of the long delayed sprites can vary from several hundred C km to more than ten thousand C km. All the long delayed sprites are related with intense continuing current bigger than 2 kA. This continuing current provides about 50% to 90% of the total charge transfer. However, a bigger continuing current does not necessarily mean a shorter time delay. This indicates that other processes also involved in the sprite initiation for long delayed sprites. In our observations, the sferic burst, a high frequency noise caused by intra-cloud activity, is always accompanied by a slow intensification in the lightning source current before the time of sprite initiation. Thus we used the lightning source current as an input and employed a 2-D FDTD model to numerically simulate the electric field at different altitudes and compare it with the breakdown field. Including the effect of the electron mobility dependence on electric field, the simulation results showed that the slow intensification itself plays an important role in sprite initiation. For events analyzed, the predicted altitude from the FDTD simulation agree well with the measurement from high speed video images. Both the measurements and the simulations indicate that sprites with long time delay tend to initiate at a lower altitude comparing to short delayed sprites.
AE23A-0906
VLF EM signatures recorded by DEMETER at the time of ISUAL TLEs
DEMETER is an ionospheric micro-satellite launched on a polar orbit at an altitude of 710 km in June 2004. Its main scientific objectives are to study the ionospheric perturbations in relation with seismic and anthropogenic activities. Therefore, its scientific payload allows to measure electromagnetic waves and plasma parameters all around the Earth except in the auroral zones. There are two modes: a survey mode where only spectra with low frequency (~ 20 Hz) and time (2 s.) resolution are available and a burst where waveforms up to 20 kHz are recorded. The ISUAL (Imager of Sprites and Upper Atmospheric Lightning) experiment on FORMOSAT 2 (the former ROCSAT 2) determines spatial, temporal and spectral properties of lightning induced upper atmospheric TLEs (Transient Luminous Events): sprites, elves, blue jets, etc. FORMOSAT 2 has been launched in May 2004 onto a Sun-synchronous orbit located at 890 kilometers. The two satellites cover the same time period but they are not synchronized and simultaneous observations are only possible when DEMETER is at two specific locations beyond the coast of Chili and beyond the east coast of Australia. At these two locations the two satellites are magnetically conjugated (FORMOSAT 2 being in the Northern hemisphere). This paper will show the characteristics of some events simultaneously observed by the two satellites (Electromagnetic emissions and TLEs).
AE23A-0907
Daytime Early VLF Perturbations Exhibiting Long Recoveries and Wide-angle Scattering
Recent observations of perturbations on Very Low Frequency (VLF) transmitter signals at remote sites show evidence of direct coupling of lightning to the ionosphere in the form of Early VLF events during daylight hours and exhibit different characteristics than their nighttime counterparts. The daytime events are coincident with recorded NLDN lightning flashes and separately recorded radio atmospherics, and are so far observed in two distinct situations. The first set of observations are perturbations on relatively short paths where the distance between the transmitter and receiver is ~400 km, with the causative lightning (and hence the likely ionospheric disturbance) within 20-50 km of the receiver. The second type of daytime Early VLF events occur on long-distance paths (>1000 km), with the causative lightning several hundred kilometers distant from the receiving site. The identified events exhibit highly directional scattering patterns including wide-angle scattering of up to 180 degrees indicating either a hard or a highly structured/periodic scatterer. In contrast to nighttime Early VLF events in which VLF backscatter occurs only very rarely (<5%), three of the six identified daytime disturbances coincident with NLDN-identified lightning flashes exhibit wide-angle scattering. Most of the daytime Early VLF events also exhibit unusually long recoveries (>9 minutes). These newly identified events imply the existence of a new type of scattering source which is more likely to be observable during daylight hours. It is also possible that the nature of the underlying ionospheric disturbances that cause Early VLF events are similar during daytime or nighttime and that the different scattering properties (i.e., forward scattering at night versus wide angle during daytime) are due to the different ambient characteristics of the Earth-ionosphere waveguide (e.g., much lower VLF reflection height) during daytime.
AE23A-0909
Origin Of The Pancake-Shaped And The Donut-Shaped Elves
Among the known species of upper atmospheric transient luminous events, elve is the most abundant in the ISUAL recorded events. The correlation between elves and their causative lightning is an interesting problem needed to be addressed. We selected 760 elves with distinct morphology from the 4000-plus elves recorded by ISUAL since June 2004. From these elves, the occurrence ratio between the pancake-shape and the donut- shape elves is deduced and the value is around unity. This ratio was unexpected since the known occurrence ratio between the cloud discharges (IC) and the cloud-to ground flashes (CG) is known to be around three to one. To explore the root cause(s) behind this discrepancy, a 2-dimensional EMP model with horizontal lightning components are studied numerically. The results show that the initiating current for the pancake elves from ICs is substantially higher than that for the donut elves from CGs. The morphological origin of these two distinct types of elves will also be discussed.
AE23A-0910
Estimated intensity of the EMP from lightning discharges necessary for elves initiation based on balloon experiment
Transient optical phenomena in the mesosphere and lower ionosphere called transient luminous events (TLEs) have been investigated extensively since the first discovery in 1989. In the lower ionosphere, elves are generated by the electromagnetic pulses (EMPs) radiated from the intense lightning current. On the ground-based observation, cameras can not always identify the occurrence of elves because elves emission is sometimes reduced significantly by the atmosphere and blocked by clouds. Therefore, it has been difficult to determine the threshold of intensity of EMPs necessary for initiation of elves. We simultaneously carried out optical and sferics measurements for TLEs and lightning discharges using a high altitude balloon launched at Sanriku Balloon Center on the night of August 25 / 26 in 2006. We fixed four CCD cameras on the gondola, each of which had horizontal FOV of ~100 degree. They cover 360 degree in horizontal direction and imaged the TLEs without atmospheric extinction nor blocking by clouds. The frame rate is 30 fps. We installed three dipole antennas at the gondola, which received the vertical and horizontal electric fields radiated from lightning discharges. The frequency range of the VLF receiver is 1-25 kHz. We also make use of VLF sferics data obtained by ground-based antennas located at Tohoku University in Sendai. We picked up six elves from the image data set obtained by the CCD cameras, and examined the maximum amplitudes of the vertical electric field for 22 lightning discharge events including the six elves events observed both at the balloon and at Sendai. It is found that the maximum amplitudes of the vertical electric field in the five elves events are much larger than those in the other lightning events. We estimate the intensity of the radiated electric field necessary for elves. About one elves event, we donft see intense vertical electric field in the balloon data.
AE23A-0911
The relationship between sprite luminous intensity and electrical property of parent lightning discharges using FORMOSAT-2/ISUAL data
Sprite, transient luminous phenomena induced by tropospheric lightning discharge in the mesosphere, has been extensively investigated for almost two decades. However, due to the atmospheric absorption and scattering, it is difficult to estimate its exact luminous intensity with ground base observation. Recently, FORMOSAT-2/ISUAL observed many sprite optical emissions without such atmospheric effects. The data enable us to discuss their luminosity and the parent lightning characteristics quantitatively. In this paper, we firstly report the relationships between them based on FORMOSAT-2/ISUAL and ELF network data. In order to estimate sprite luminosity, we use ISUAL/array photometer data. The array photometer consists of two photometers measuring two wavelength ranges, that is, 360-470 nm (blue) and 520-750 nm (red), with sixteen channels arrayed in vertical each. We can estimate the electron energy of sprite from the ratio between blue and red color intensities. Some channels with sprite are contaminated by scattering emission from cloud-to–ground and/or cloud-to-cloud lightning discharges. From contaminated data we subtract lightning emission using lightning channel data. Then, we can estimate corrected sprite luminosity integrated temporally and spatially. In addition, we estimate its parent lightning charge moment from ELF data. The result of comparison between luminosity of carrot type sprite and the charge moment shows that they have positive correlation, indicating that charge moment is one of the essential parameters that determine sprite luminosity. We also analyzed the ratio between blue and red emissions and found that there exists also positive correlation between the ratio and charge moment of parent lightning. This result seems consistent with quasi- electrostatic model assuming the electric field over thunderstorm is proportional to the lightning charge moment and that it determines sprite occurrence and its luminosity. Furthermore, we classified sprites into three types, that is, halo, carrot, and column, and discuss their relative luminous intensity and their relationship to the characteristics of parent lightning.
AE23A-0912
Development of the sprite and lightning imaging system onboard the SPRITE-SAT
SPRITE-SAT is now being developed in-house by the Tohoku University team, which will be launched in August 2008. The total weight of the satellite would be less than 50 kg, including 4-5 kg science mission payload. There are two scientific objectives in this micro-satellite mission: the first is to identify the generation mechanisms of sprites by investigating their horizontal structures, and the second is to identify the generation mechanisms of TGFs by investigating their source location and relationship to lightning discharges. Lightning and Sprite Imager- 1 and 2 (LSI-1 and 2) are CMOS cameras with 512 x 512 pixels and the pixel size of 25 um, which pointed at nadir to take images of the horizontal structures of lightning and sprites. In order to detect lightning emissions, we equip LSI-1 with a broadband filter between 740 and 830 nm. We also equip LSI-2 with a rather narrow band- pass filter centered at 762 nm. The optics and the detector array altogether yield an effective field of view (FOV) of 35 deg, giving the pixel resolution of less than 660 m from the altitude of 660 km. Wide Field CCD imager (WFC) is a CCD camera with 659 x 494 pixels and the pixel size of 7.4 um, which takes images of lightning discharges inducing TGFs. WFC is also pointed at nadir and is equipped fish-eye lens (FOV is larger than180 deg). The outputs of all cameras are digitized by 10 bit A/D conversion. One instrumental case contains LSIs and WFC and the total weights is 630 g. In order to detect TGFs, terrestrial gamma-ray counter (TGC) which consists of CsI scintillator is installed at the satellite. TGC can detect gamma-rays in the energy range from 30keV to a few MeV. This satellite also equips a VLF antenna which receives VLF radiations from lightning discharges. At the presentation, we will show the specifications of the instruments and the status of the satellite development more in detail.
AE23A-0913
Very Small Meteors -- A comparison of ablation, deceleration and detection theory with observations
In order to determine the mass and velocity distributions of the large flux of very small meteors (< 10 microns) from radar scattering data, it is necessary to model the plasma that is created as these meteors burn up in the upper atmosphere. Although an extensive body of large-aperture radar scattering data has been accumulated in recent years, the essential physics that relates the measured radar cross-section to meteor mass and velocity has only been partially formulated in the limit of long-wavelength scattering from nominally spherical plasmas. In this work we present a comprehensive model of meteor ablation and plasma formation around the meteor head in order to better understand the radar scattering cross-sections. Plasma profiles are determined from first- principle fluid or Monte-Carlo simulations including collisions and collective fields. The results indicate that the plasma parameters vary widely with altitude, as expected, and hence with the initial meteor mass. Collective effects can play an important role in determining the plasma properties. The implications of this plasma model on radar scattering will be discussed and the results compared with existing data.
AE23A-0914 INVITED
Origin and Evolution of 'Sprites': Cause and Mechanism
Recent observations of cloud-to-ionosphere discharges have led to a variety of appearance besides lightning to aurora transition. One of them is a modified form of eBall Lightningf, so-called gSpritesh termed by ionospheric physicists. This paper aims to explain a sequence of processes to manifestation of this phenomenon on the basis of eElectrohydrodynamics in Dusty and Dirty Plasmasf with novel physical concepts of eElectrical Cusp-Reconnectionf and eCritical Ionization Velocitiesf. Most of cloud-to- ionosphere discharges is initiated by electric reconnection in principle with global lightning activity. Their bipolar nature with much higher occurrence rate to positive cloud-to-ground flashes than to negative c!oud-to-ground flashes indicates that the cloud shape and charge distribution is thought to be inclined and extended rather horizontally with a sequence of cusped charge distributions or horizontal double layers. Such a horizontal extent of cloud clusters is possibly due to stratospheric jet streams on the topside of clouds in a way similar to the cloud and charge distributions in a coastal region of the Sea of Japan during winter, although the scale is much 1arger. This enables us to apply a scenario similar to triggered lightning for winter thunderstorms, based on the EHD concepts, electric reconnection and critical ionization velocities. We are now concerned with a leader ascending from a topside cloud that is the same as usual cases of the cloud-to-ground discharge in appearance and shape until some altitude. As it approaches the ionosphere, however, the atmospheric pressure goes less and less. Accordingly, its appearance should be gradually changing. As a result, there may be a chance for it to be forced to stop. A possible scheme with a whole process for 'sprites' formation is just like that. In view of the fact that 'sprites' are observed in much higher correlation with positive cloud-to-ground flashes than negative cloud-to- ground flashes directly underneath the thunderclouds, its upward leader is regarded as a positive leader. When this positive leader is ascending with decreasing gas pressure, its velocity is always limited up to its critical velocity or less, since it transfers to some charged identity with electrostatic energy. In this way, possible formation of ball and/or bead lightning, termed as `Sprites' could be expected. This is a formation mechanism of column- or bead-like ba11 and/or bead lightning, namely the so-called `Sprites'.