AE11A-0278
The spectral dependence of TGFs on source distance
AE11A-0279
Statistical comparisons of RHESSI Terrestrial Gamma-ray Flashes and WWLLN-detected lightning
We use the RHESSI database of terrestrial gamma-ray flashes (TGFs) and the WWLLN database of radio atmospherics from lightning in the years 2003-2008 to study the relation between TGFs and nearby thunderstorms, expanding upon work presented at the 2005 CEDAR workshop by one of us (EHL). We compare TGF detection times with the detection times of lightning from nearby storms to constrain the phase of storm development in which TGFs occur. We search for systematic displacements in the East/West and North/South directions between TGFs and the nearby thunderstorms. North/South asymmetry might arise if TGF electrons are produced at high enough altitudes that they are beamed along the magnetic field. Using all nearby lightning detected within minutes of the TGF allows us to use the full database of hundreds of RHESSI TGFs, rather than just the small number that are matched with a specific WWLLN atmospheric.
AE11A-0280
Analysis of a subset of BATSE TGFs that do not suffer from instrumental limitations
Due to the high intensity at the peaks of terrestrial gamma flashes (TGF) very fast electronics are needed to detect all the photons. As the space borne instruments that have detected TGFs so far were designed for other purposes, none of them have had such fast electronics. In a non-paralyzable system this can be corrected for, but for a paralyzable system it is inmost cases impossible to obtain an estimate of the incident count rates. However, if the count rates are below a certain level, even data from a paralyzable system can be analysed with confidence. In this study we have therefore singled out weak TGFs measured by BATSE, which do not suffer from paralyzation effects. From simulations we have found that the time delays of the softer TGF photons are pure effects of Compton scattering in the atmosphere. Thus, the time delay increases as the TGF production occurs at lower altitude and as the observational nadir angle increases. Time delays seen in the selected subset of TGFs confirm this.
AE11A-0281
Cosmic-ray source of runaway electrons in thundercloud electric field.
To increase an accuracy of numerical simulations of the high-altitude and high-energy electric phenomena in thunderstorm atmosphere basing on the electric breakdown combining the cosmic-ray effects and relativistic runaway electron (RE) avalanches it is necessary to have accurate source of seed REs produced by cosmic rays. We calculated the source using the Monte-Carlo technique. Actually a number of secondary electrons was calculated with energies above the runaway threshold depending on the field overvoltage relative to the minimum of the drag force affecting electrons. The cosmic radiation propagation through the atmosphere was simulated based on simplified model of the nuclear cascade: a cosmic proton was assumed to completely lose its energy in the first interaction with air nuclei, producing 15 pions with equal energies. The flux of primary radiation was divided into 20 angular groups containing equal number of particles. In view of the lack of the experimental data on the RE source it is impossible to directly estimate the accuracy of the obtained source. Therefore the model reliability was verified by comparing results of secondary radiation calculations with known experimental altitude variations in the secondary cosmic rays and their spectra. The source was calculated as the specific generation rate of the secondary electrons by cosmic radiation dependent on electric field overvoltage and the altitude above the Earth's surface. It is recommended as a source of relativistic runaway electron avalanches in numerical simulations of electric discharges in atmosphere controlled by REs in thunderstorm fields and their emissions: optical, gamma and neutrons. The source already was used to simulate the high-altitude discharge and its emissions. In particular, the calculated gamma-ray pulses (photon numbers and spectra, pulse duration) agree with detected terrestrial gamma-ray flashes (TGFs). Conclusions of the published analyses (Cummer and Lyons, 2005; Cummer et al, 2005; Willams et al., 2006) of experimental data are confirmed that TGFs are connected with typical thunderclouds and lightning discharges causing small dipole moment variations and TGFs are not correlated with Sprites.
AE11A-0282
Thunderstorm top characteristics of RHESSI identified terrestrial gamma-ray flashes
Recent RHESSI observations suggest that thunderstorms may be the source of terrestrial gamma-ray flashes (TGFs). However, precisely where a TGF occurs within or above the thunderclouds is not know. Furthermore, it is not understood what conditions result in the production of TGFs. In this presentation, we investigate the properties of the thunderstorms that likely produced the TGF. Specifically, RHESSI-identified TGFs that are associated with localized sets of WWLLN lightning flashes are used to isolate specific convective systems within the larger RHESSI footprint. Cloud top thunderstorm characteristics associated with TGFs are examined using an array of satellite platforms. When available, the GOES Imager, MODIS, AVHRR, and TRMM data are used to retrieve cloud top temperature, height, etc. of the TGF-producing convection.
AE11A-0283
Properties of the x-ray emission from rocket-triggered and natural lightning as measured by the Thunderstorm Energetic Radiation Array
The Thunderstorm Energetic Radiation Array (TERA) is located at the University of Florida/Florida Tech International Center for Lightning Research and Testing (ICLRT) at Camp Blanding, FL. The array covers the ~1 km2 ICLRT facility and includes 45, 7.6 cm diameter NaI/photomultiplier tube (PMT) detectors. From 2005 through 2007, TERA has recorded tens of successful rocket triggered lightning and many natural cloud-to-ground (CG) flashes. The observed flashes have shown signature of energetic radiation on most of the TERA detectors extending out to a distance of 500 m from the lightning channel. In this study, we present an analysis of the x-ray emission of two large events, a rocket triggered event (UF0707 on July 31, 2007) and a natural CG flash (MSE0704 on July 16, 2007). As reported for previous events, the x-ray emission was observed to occur during the dart leader phase of the rocket triggered lightning and the stepped leader phase of natural lightning, just prior to the time of the return stroke. We investigated the nature of the source energetic electrons, including their directionality, energy spectrum and luminosity, using a detailed Monte Carlo simulation of the bremsstrahlung production and propagation of x-rays. We will discuss the implication of these results in understanding the potential mechanisms for producing energetic radiation in lightning.
AE11A-0284
Spectroscopy of the BATSE TGFs with the LADs
Terrestrial gamma-ray flashes (TGFs) are millisecond-long bursts of high-energy x-rays and gamma-rays associated with lightning that have been observed by the Burst and Transient Source Experiment (BATSE) onboard the Compton Gamma Ray Observatory (CGRO) satellite and by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) satellite. A detailed spectroscopic analysis of the RHESSI data suggests that the TGFs observed by RHESSI originate from deep in the atmosphere and sometimes may be so intense that they saturate the RHESSI electronics. It has previously been shown that the BATSE electronics suffer from deadtime in the readout electronics during the peak intensity of the TGF. We study the ability of Monte Carlo simulations of relativistic runaway and atmospheric scattering to constrain the altitude of the TGFs using data from BATSE's Large Area Detectors (LADs) once the effects of deadtime are included in the analysis.
AE11A-0285
The high-energy radiation dose received aboard aircraft exposed to a terrestrial gamma- ray flash
Terrestrial gamma-ray flashes (TGF) are large bursts of high energy radiation observed from space that originate from our atmosphere. These millisecond long flashes of gamma-rays are often so bright that they saturate detectors, even from 600 km away. Several independent observations suggest that terrestrial gamma-ray flashes originate from thunderstorms deep within the atmosphere, near the altitudes where commercial aircraft fly. Based upon the flux of gamma-rays observed by the RHESSI spacecraft, detailed gamma-ray propagation models show that at least 1.0E17 energetic, multi-MeV electrons, are typically produced at the source. This large number of energetic electrons could potentially be a hazard for aircraft passengers, pilots and electronics. Using theoretical and observational estimates of the size of the TGF source region, we calculate the high-energy radiation dose from the energetic electrons and the gamma-rays for an aircraft exposed to the TGF from a close range. Finally, we shall discuss upcoming observations that will help constrain this radiation risk from TGFs.
AE11A-0286
Terrestrial gamma-ray flash electron observation with SAMPEX
Terrestrial gamma-ray flash (TGF) emissions are predicted to be accompanied by energetic electron emissions. These electrons have been suggested to explain several anomalous TGF observations by the RHESSI and BATSE spacecraft and additionally provide a new window for study of TGFs as they carry additional information about the source through their pitch angle and arrival time distributions and confinement by the geomagnetic field. This information could prove very useful for analysis of TGF source mechanisms. We present results of simulations of electron and photon emissions and describe the implications of these results for inferences about the TGF source. We also present new results of a search of recent data from the SAMPEX satellite for these electrons in conjunction with a detailed search of very-low frequency (VLF) radio data for possible coincident lightning. Possible inferences from these observations and their implications for TGF source altitude, photon emission directional distribution, and meteorological connections will be discussed.
AE11A-0287
Charge moment change of lightning discharge associated with terrestrial gamma-ray flash
Terrestrial Gamma-ray Flashes (TGFs) are gamma-ray emission phenomena associated with lightning discharge. TGFs have a duration time of a few msec and an energy range of a few 10 keV-20 MeV. TGFs were firstly discovered by BATSE on CGRO satellite in 1994. RHESSI satellite observed over 170 TGFs per year from 2002 to 2005. The orbit of RHESSI is circular with an inclination of 38 degrees at an altitude of 600 km. So far the characteristics of lightning associated with TGFs have been investigated. However, the relationship between the TGF occurrence and the charge moment change of parent lightning discharge has not been examined in global scale. Tohoku University has been measuring ELF magnetic field in the frequency range of 1-100 Hz at 4 sites located globally, including Onagawa in Japan, Esrange in Sweden and Syowa in Antarctica. From these data, we can estimate the charge moment change, location, and polarity of lightning around the world. We derived the charge moment change of lightning discharge generating TGFs from the ELF waveforms and the timings of TGFs listed by RHESSI observation. 174 TGF events were observed by RHESSI in 2004. We analyzed 141 events whose ELF data at 3 stations are available. We estimated lightning location based on the time of arrival method. If the rise timing of lightning ELF pulse, called ELF sferics, is estimated within +/- 15 msec of the timing of TGFs observed by RHESSI and the estimated location of lightning discharge is within a range of 1500 km from the sub-satellite point, we regard it as parent lightning of TGFs. Detection Limit of the charge moment change with global uniformity was 175 C-km. We analyzed 141 events observed in 2004 and identified 9 TGFs with ELF sferics. We consider the other 132 TGF events are related to smaller lightning discharge whose charge change moment change are less than the detection limit of 175 C-km. The minimum charge moment change in 9 events is 176 C-km, and the maximum is 465 C-km. We derived the occurrence probability of TGFs, dividing TGFs occurrence number by lightning occurrence number for each range of the charge moment change. Relative occurrence probabilities of TGFs in the ranges of 175-350 C-km and 350-525 C-km to that in the range of 0-175 C-km is 14 % and 4 %, respectively. This fact suggests that the occurrence probability of TGFs is higher for lightning discharge with smaller charge moment change.
AE11A-0288
Evidence of negative leaders which precede fast rise ICC pulses of upward
During winter thunderstorm season in Japan, a lightning observation campaign was conducted with using a VHF broadband digital interferometer (DITF), a capacitive antenna, and Rogowski coils to study the charge transfer mechanism associated with ICC pulses of upward lightning. All the detection systems recorded one upward negative lightning stroke hitting a lightning protection tower. The upward lightning consists of only the Initial Stage (IS) with one upward positive leader and six ICC pulses. The six ICC pulses are sub-classified clearly into two types according to current pulse shapes. The type 1 ICC pulses have a higher geometric mean (GM) current peak of 17 kA and a shorter GM 10-90% risetime of 8.9 μs, while the type 2 ICC pulses have a lower GM current peak of 0.34 kA and longer GM 10-90% risetime of 55 μs. The type 1 ICC pulses have the preceding negative leaders connecting to the channel of the continuing current, while the type 2 ICC pulses have no clear preceding negative leader. These negative leaders prior to the type 1 ICC pulses probably caused the current increases of the ICC pulses, which means that the negative leaders created the channels for the ICC pulses. The height of the space charge transferred by one of the type 1 ICC pulses was estimated about 700 m above sea level at most. This observation result is the first evidence to show explicitly the existence of the negative leaders prior to the fast rise ICC pulse. Furthermore, the result shows that space charge could exist at a low attitude such as 700 m above sea level. This fact is one of the reasons why upward lightning occurs even from rather low structures during winter thunderstorm season in Japan.
AE11A-0289
Time-synchronized High-speed Video, Electric and Magnetic Fields, and Currents From Triggered Lightning
Measurement and analysis of salient aspects of classical rocket-and-wire-triggered lightning at the International Center for Lightning Research and Testing at Camp Blanding, Florida, have been published, for example, by Olsen et al (2006), by Rakov et al. (2003), by Wang et al. (1999), and by Lalande et al. (1998). Here we add to and attempt to unify those studies by presenting the first high-speed video images (10,000 and 20,000 frames per second) precisely time synchronized with electric and magnetic field (dc to 3 MHz), and current (dc to 16 MHz) measurements extending from the time of rocket launch through the whole triggered flash. We focus special attention on two particular events in the triggered-lightning chronology: (1) the transition, occurring a few seconds after the rocket launch, between (a) the first phase of a triggered event when the rising triggering wire shields increasingly with time the electric field measured at ground while the wire carries negligible electrical current and (b) the next phase involving the launch of an upward positive leader (UPL) from the wire top with resultant significant current flow in the wire; and (2) the transition between (a) a UPL in an intact wire and (b) the explosion of the wire, the resultant current decrease to near zero, and the subsequent large current pulse known as the initial current variation (ICV). We also present the first video images of upward leaders being launched from the rocket-triggering structure in response to downward dart leaders preceding return strokes. Finally, we discuss the apparent differences between the high-speed video images and related data from streak photography on film (e.g., Olsen et al. 2006) and from ALPS (e.g., Wang et al. 1999).
AE11A-0290
Analysis and Correlation of High-Speed Video and Lightning Mapping Array Results For Two Lightning Flashes Triggered in Electric Fields of Opposite Polarity
Two rocket-triggered lightning flashes were captured on high-speed video (shot from approximately 1.5 kilometers away). These triggered flashes were also observed with Langmuir Laboratory's Lightning Mapping Array (LMA), a 3-D VHF time-of-arrival system, providing a chance to correlate high-speed video with LMA data and representing some of the first rocket-triggered flashes to be observed by the LMA. The flashes were triggered on August 6 and August 7, 2008, by rockets launched from South Baldy (approximately 3300 meters above sea-level) at Langmuir Laboratory. The electric field at ground was negative (fair-weather) polarity for one flash and positive (foul-weather) polarity for the other. The field changes produced by the two flashes were of opposite polarities as well. The high-speed video showed the wire illuminating from the bottom upward for the flash triggered with a positive electric field at ground and from the top downward for the flash triggered with a negative electric field at ground. For both flashes, the high-speed video showed upward branching. However, the flash triggered with a positive electric field at ground exhibited downward stepping preceded by upward streamers, while the flash triggered with a negative electric field at ground exhibited upward stepping and no visible streamers.
AE11A-0291
Flash-level comparisons between detections of Narrow Bipolar Events at VLF/LF and VHF frequencies
Narrow Bipolar Events (NBEs) were originally defined by characteristics of the electric field waveforms observed from the parent lightning discharge when viewed from the very low frequency (VLF) to low frequency (LF) bands (0-500 kHz). NBEs continue to be a phenomenon of great interest, especially because their powerful radio emissions are detected with high efficiency from orbit. The central, and so far unanswered, question relating to NBEs is what lightning process is responsible for producing such a unique electric field change signature. Past studies have shown they are produced by a form of intracloud discharge. However, it is not yet known whether NBEs are the result of a profoundly different physical process than is involved in other forms of intracloud lightning or are just the tail end of a distribution of all intracloud lightning. It was recently shown that all events located and identified as being NBE by a ground-based VLF/LF system (the Los Alamos Sferic Array, LASA) produce high-powered VHF signals detected on-orbit, but the inverse was not the case. In this study, we examine the flash-level context of NBEs from the perspective of joint ground-based observations at different frequencies. The LASA system will be used for investigating the VLF/LF signatures in conjunction with the Oklahoma Lightning Mapping Array (LMA). From the VHF LMA system, we will re-investigate the question of NBE classification (can they be uniquely defined by their VHF signatures as well as their VLF/LF?) and the temporal and spatial context between VHF sources and VLF/LF waveform onset. We also will use the mapping system to infer storm charge structure so we can determine the storm context in which NBEs are produced.
AE11A-0292
New Experimental Data on Lightning Events Producing Intense VHF Radiation Bursts
Cloud lightning discharges that produce both (1) single bipolar electric field pulses (so-called Narrow Bipolar Pulses or NBPs) having typical full widths of 10 to 20 μs and (2) intense HF-VHF radiation bursts (much larger than those from any other cloud-to-ground or "normal" cloud discharge process) are referred to as Compact Intracloud Discharges or Energetic Intracloud Events. In 2007 and 2008, we measured wideband electric fields, electric field derivatives (dE/dt), magnetic field derivatives (dB/dt), and narrowband VHF radiation bursts (at 36 MHz) produced by over 100 such events that occurred during 20 thunderstorms in Gainesville, Florida. There were also many storms that generated no "energetic" lightning events. The majority of intense VHF radiation producers occurred in isolation (within a few hundred milliseconds) from any other lightning process, but some were found to occur prior to, during, or following cloud-to-ground or "normal" cloud lightning discharges. The rate of occurrence of NBPs is rather low, although we observed two sequences of two NBPs accompanied by intense VHF bursts that were separated by only some tens of milliseconds. Some NBPs were followed by small electrostatic field changes. When the measuring system was triggered on strong VHF signal, it was found that some corresponding wideband electric field waveforms were not typical NBPs. This implies that the process giving rise to the strong VHF burst may produce a variety of wideband electric field signatures. On the other hand, NBP-like wideband electric field signatures are often produced by thunderclouds without accompanying strong VHF signals. It follows that the use of characteristic wideband NBP signature as the only data selection criterion may result in both rejection of some intense VHF radiation producers and acceptance of some pulses that are not accompanied by significant VHF bursts. We found that either dE/dt or dB/dt signature can be used as a reliable data selection criterion. Inferences made from the acquired experimental data on the process giving rise to NBPs that are accompanied by intense VHF bursts will be presented.
AE11A-0293
Coordinated Optical/VLF Lightning Observations
We examine the coordinated optical and VLF emissions from lightning detected by the Los Alamos Sferic Array (LASA) during the summer of 2005. The sensor station located in Norman, OK that summer was equipped with an optical photodiode detector in addition to the VLF radio receiver usual to the LASA stations. We apply signal-to-noise ratio thresholds and require that the optical signal arrived within a specific temporal window about the VLF detection to claim coincidence, thus enabling a statistical assessment of the variations in optical emissions from various types of lightning discharges. We differentiate among lightning discharge types on the basis of the VLF waveform. Examination of 35,272 events finds that 43% of negative polarity cloud-to-ground discharges (CGs) have a coincident optical detection, while only 23% of negative polarity in-cloud discharges (ICs) show optical coincidence. Further, positive polarity VLF events are substantially less likely to show optical coincidence: 9% of positive ICs and 10% of positive CGs were optically detected. There is essentially no correlation observed, overall, between the intensity of the optical and VLF emissions from a given event, regardless of event type. Light & Jacobson (JGR, v107, p4756, 2002) found evidence that the type of discharge referred to as a "narrow bipolar event" (NBE) lacks corresponding optical emission, but the question remains open of whether they are completely dark or whether they do generate optical emissions which were simply below the detection threshold for that particular satellite-based data set. These ground-based data show that VLF NBEs are at least a factor of two to four less likely to show optical counterparts than other, more common types of lightning discharge. We attempt to set an upper limit to their optical emission in this work.
AE11A-0294
Energy estimates for lightning flashes
The energy released by a lightning flash is related to ΔQ, the charge transferred by the flash, and the electric potential, V(z), in the storm due to the thundercloud charges. The energy of the flash can be estimated directly from the product, (ΔQ)(ΔV), where ΔV is the voltage difference that the lightning charge is transferred between. Using in-cloud lightning electric field change data from balloons, ground-based field change data, and Lightning Mapping Array data, we estimated ΔQ of several IC and CG lightning flashes from four thunderstorms that occurred in 1999 above a mountaintop in New Mexico. From the balloon soundings of electric field, we also deduce the vertical V profile though these storms. In this presentation, we will show how we combine these measurements to obtain energy estimates for 13 IC and 5 CG flashes. More general calculations of mature stage lightning energies are made using the average potential differences between the extrema of potential wells for an entire storm or group of storms along with the average charge transferred by the flashes within those storms.
AE11A-0295
Distribution of the Magnitude of the Slow-Tail of Sferics
A lightning strike emits an electromagnetic wave known as an atmospheric, or sferic, which can propagate away to long distances through the earth-ionosphere waveguide. Sferics can be recorded by both extremely- low frequency (ELF; f < 3 kHz) and very-low frequency (VLF; 3 kHz ≤ f < 30 kHz) receiver systems. The recorded signal is composed, in general, of two segments: a pulse containing VLF frequencies, followed by what is known as a slow-tail, containing the ELF components. The slow-tail is essentially a single cycle wave, which is delayed with respect to the the VLF part of the sferic. Certain sferics have relatively large slow-tails while others have no visible slow-tail. The research presented here analyzes the statistics of the magnitude of slow-tails. We measure the energy of the slow-tail signal and normalize it by dividing by the energy of the VLF portion of the sferic. By taking measurements from many sferics, we can obtain the distribution of the magnitude of slow-tails. A number of different factors can then be taken into account to see how they affect the distribution, such as the distance the sferic propagated before being recorded, the intensity of the causal lightning strike, the location of the strike, and the ionospheric conditions over the path. Given this distribution, we can characterize the average "normal" slow-tail and compute an average slow-tail for any given path over which the sferic propagated. The slow-tail can also be used to calculate a current moment for the causal lightning strike, as well as obtain an approximate distance between the strike and the recording station. In this way, by obtaining a better understanding of the variation of the magnitudes of the slow-tails, we can obtain more information about their properties and refine the computations which use slow- tail measurements as their input.
AE11A-0296
Role of lightning discharge in the global electric circuit
Overall picture of global electrical circuit has been thought as the current system including the solid Earth, atmosphere, ionosphere, and magnetosphere. However, the atmosphere has been dealt as main region in the previous studies. Recently, Transient Luminous Events (TLEs), such as sprites and blue jets, were discovered in the middle and upper atmosphere. Existence of such TLEs obviously indicates the electrical connection between the atmosphere and ionosphere. Currently, it is necessary to establish the new circuit model considering the atmosphere-ionosphere coupling. As a first step to the new circuit model, we try to expand the 3-D model circuit with the estimation of charge transportation by global lightning activities. We are able to estimate it with ELF electromagnetic wave measurement network. Tohoku Univ. group had installed search-coil magnetometers at four sites in the world to observe ELF waves, which has been obtaining the long-term data. Since the attenuation in the ELF range is quite small, the wave can propagate for long distance. Therefore, the detection of Q-burst with the several stations allow us to estimate lightening positions and charge moment changes with arrival time and bearing. Despite of low attenuation, ELF wave is under the influence of propagation loss. Therefore, it is difficult to detect Q-burst with the uniform rate. Consequently, the new algorism was developed to compensate the effect of propagation in Tohoku Univ. group. One of the purposes in this presentation is to discuss the algorism newly developed, which detects Q-burst with the uniform rate. Results with above algorism would be useful to consider the comparison between lightning activities and satellite data for the global cloud and precipitation activities.
AE11A-0297
A Search for Bremsstrahlung From Preliminary Breakdown Using A Lanthanum Bromide Detector
The process of lightning initiation, called preliminary breakdown, is as of yet not properly understood. We present information on a detector system designed to provide more information on the process of preliminary breakdown by the observation of bremsstrahlung caused by lightning initiation. The detector system was deployed for a test campaign to South Baldy Peak in New Mexico on August 7, 2008. The detector system is composed of a 3x3 sodium iodide scintillator and a 1.5x1.5 lanthanum bromide scintillator. The output of these detectors was digitized to preserve maximum information about the observed x-rays, and GPS synched timing information was recorded. Preliminary results will be presented as well as plans for improvements on the system for a future run planned for the summer of 2009.