AE44A-01 INVITED
Two Types of Energetic Radiation Observed in Winter Thunderstorms
The dose rate of the gamma-rays increases in association with the activities of the thunderstorm. They were observed on the ground in the winter season of Japan. To investigate the time profile of the radiations during the winter thunderstorms, four sets of the radiation detectors were prepared which consist of the long proportional counters. These detectors have different characteristics of the response for the energy of the incident particles by mounting different thick shielding covers. Those results were compared with the results measured at the same time by the environmental radiation monitors set up around a nuclear power facility. Electric field was also measured by using a field mill. As a result, the following two types of the radiation enhancements have been found during the winter thunderstorm activities; the gradual variation of photon intensity with energy of a few MeV, and the burst type of the radiation that is attributed to the injection of high energy photons with the energy over 10MeV.
AE44A-02
Observations of Non-thermal Gamma-Ray Emissions from Winter Thunderclouds
High-energy radiation bursts associated with lightning phenomena, including non-thermal x/γ rays, have been recently observed by satellites and ground-based observatories. In particular, radiation-monitoring posts, arranged in power plants located on the coastal area of Sea of Japan, frequently have observed such bursts associated with winter thunderstorms. However, those bursts observed at the coastal area have remained much less understood. Installed at Kashiwazaki-Kariwa power plant on the coastal area, our new radiation-detection system detected a remarkable radiation enhancement associated with strong thunderstorms on 2007 January 6 (UT). The burst lasts for a ~40seconds prior to lightning discharges, the energy spectrum extending up to 10 MeV. Our results strongly suggests that the burst consists of bremsstrahlung photons from electrons accelerated beyond 10 MeV in thunderclouds. Given the results, we will present one emission model and estimate the energetics of the radiation.
AE44A-03 INVITED
On the possibility of accelerating electrons to X-ray energies in the electric fields created during the meeting of positive and negative streamer fronts in laboratory electrical discharges
In laboratory sparks X-rays are observed in two bursts, one occurring about 1 microsecond before the onset of the current and other taking place at the time of onset of the current [1, 2]. The location of the first burst in the discharge process suggests that the first burst of X-rays observed is probably generated by streamers. To investigate this further, streamers are modeled as finitely conducting resistive channels with potential gradients, 450 kV/m for positive and 1 MV/m for negative. A streamer has a radius of about 50 micrometers and in a streamer moving in the stability field (slightly above the potential gradient) the ion concentration at the head is such that the radius of the active region in which the electric field exceeds the breakdown value is about 200 micrometers. As the background field increases beyond the stability field the ion concentration at the head continues to increase until the streamer branches. At present, the exact streamer charge in the head when the streamer branching takes place is not known but it could be several times the typical charge. In the calculations presented here it is assumed that the streamer will branch when the head charge is about five times that of a streamer moving in stability field. We have evaluated the electric field configuration in space for a streamer moving in stability field, for a streamer just about to branch and for the encounter of two streamers of opposite polarity just about to branch. The resulting field configuration is utilized together with the energy dependent frictional force on electrons, as presented by Moss et al. [3], to evaluate the maximum energy an electron will receive in accelerating in the electric field of the streamers. The results of the calculation show that an encounter between two streamers of opposite polarity just about to branch can provide sufficient energy for the electrons to become runaways in 1.1 MV/m field, the average value measured in the gap just before breakdown. The photographs of sparks generated in the same gap configuration provide evidence that in these discharges the negative streamer front generated by the cathode is met by an oppositely moving streamer front somewhere in the vicinity of the mid gap making it possible for the runaways to gain more than 100 keV in crossing rest of the gap. Moreover, the calculations show that the field enhancement associated with the encounter of two streamers takes place when the streamers are within about few millimeters from each other. Since the speed of streamers range from about 20 cm/us to about 100 cm/us, the duration of the high electric field lasts only for a few nanoseconds and the duration of the X-ray burst generated by such an encounter cannot be longer than this time. This is in agreement with the experimental observations. [1] Dwyer et al., X-ray bursts produced by laboratory sparks in air, Geophys. Res. Lett., 32, L20809, doi:10.1029/2005GL024027, 2005. [2] Rahman et al., X-rays emitted by 80 cm long sparks in air, submitted to Geophys. Res. Lett., 2007. [3] Moss et al., Monte Carlo model for analysis of thermal runaway electrons in streamer tips in transient luminous events and streamer zones of lightning leaders, J. Geophys. Res., vol. 111, AA02307, doi:10.1029/2005JA011350, 2006.
AE44A-04 INVITED
Analysis of Los Alamos Sferic Array Observations of Lightning Discharge Events Correlated with Terrestrial Gamma-ray Flashes
Since April 2004, Los Alamos National Laboratory has operated a new and upgraded sferic sensor array, the Los Alamos Sferic Array (LASA), for lightning study in Florida and over the Great Plains. In July 2006, additional sensors were deployed to Key West, the Bahamas, and the Cayman Islands to better cover the Caribbean region. During this time period, LASA has observed a number of lightning discharge events that were closely associated with terrestrial gamma-ray flashes (TGFs) detected by the RHESSI satellite. Unlike some other existing lightning detection networks, LASA records full radio waveforms produced by lightning, in a broad frequency range of 0.16 500 KHz. The time waveform allows us to study the discharge properties in a great detail and, most importantly, enables the determination of the discharge height for some of the TGF-related events. In this presentation the four-year LASA observations of TGF events will be summarized. The associated charge moment change, peak current, discharge altitude and location (as referenced to RHESSI's position) will be discussed.
AE44A-05 INVITED
What could cause terrestrial gamma-ray flashes?
Despite over a decade of research, the sources of Terrestrial Gamma-ray Flashes (TGFs) remain a mystery. Not only are the exact source locations unknown, the basic mechanism for producing the energetic electrons that emit the high-energy photons is still under active debate. When TGFs were first reported using data from the Burst and Transient Source Experiment (BATSE) on NASA's Compton Gamma-ray Observatory (CGRO), it was almost immediately suggested that the source was associated with sprites or other high altitude (>30 km) phenomena. However, new spectral measurements of TGFs by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) satellite as well as detailed modeling of the gamma-ray propagation through the atmosphere indicate that the source altitudes are, in fact, much lower, between 15 and 21 km. Such altitudes are within the range of thunderstorm tops but are much too low for sprites. Because gamma-rays experience large attenuation in the atmosphere, such low source altitudes imply that the number of runaway electrons at the source must be very large, in excess of 10 to the power 16, which could produce measurable currents and changes in the electric charge moment. In this talk, possible source mechanisms of TGFs will be discussed, including runaway electron production through relativistic runaway electron avalanches acting on the cosmic-ray background and on extensive air showers, runaway electron production by the relativistic feedback mechanism, and runaway electron emission from lightning leaders and/or streamers.
AE44A-06
A Reanalysis of the Time Evolution and Spectroscopy of the BATSE TGFs
Terrestrial Gamma-Ray Flashes (TGFs) were discovered by the Large Area Detectors (LADs) of the Burst And Transient Source Experiment (BATSE) onboard the Compton Gamma Ray Obervatory (CGRO). BATSE saw a total of 70 TGFs during its 10 year lifetime. A new study of TGFs by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) has increased the number of TGFs observed (750 and counting) and the detail of the spectroscopic analysis of TGFs. We present a reanalysis of the BATSE data in light of the large statistical sample of the RHESSI events. Specifically, we show that the BATSE LADs are suffering from deadtime during the peak of the BATSE LAD events, which could have a substantial impact upon the types of TGFs that triggered BATSE. We now believe that the long, soft tail found in most BATSE events can be understood as a combination of the expected effects of Compton scattering in the atmosphere -- in agreement with a new analysis of the RHESSI data -- plus effects associated with the instrumental deadtime. In addition, we examine the prospects for extracting significant information about the source altitude of TGFs from the coarse spectroscopic data provided by the LADs given the uncertainty in the distance of BATSE from the TGF source region.
AE44A-07
Balloon-borne vector electric field measurements
Electric field-change measuring systems have been employed on the ground beneath thunderstorms for many decades to deduce the movement of electrical charge during lightning flashes. We began flying electric field change instruments on balloons in 2004 to measure changes in electric vectors inside thunderstorms. Here we report on results from a new version of the earlier instrument with a faster sampling rate (70,000 per second), which was flown in the summer of 2007. During one detected lightning flash, when the instrument was 6.0 km above sea level and ascending, the vertical component Ez of the electric field was almost completely undisturbed by the flash, while concurrently the horizontal components Ex and Ey were widely differing functions of time with large amplitudes. These results suggest that charges in different horizontal directions at the altitude of the balloon were affected at different times during the flash.
AE44A-08
Hydrometeor Environments Near Lightning Centroids and the Impact on Initial Breakdown
One of the unanswered questions in lightning physics is the initiation of lightning. Studies have consistently shown that ambient fields in thunderstorms are too small to cause the dielectric breakdown of air. Two theories have emerged to explain this discrepancy: runaway breakdown and hydrometeor breakdown. In each theory, the ambient field is enhanced locally to the dielectric breakdown field. Early studies investigated the possible influence of various types of hydrometeors (i.e. supercooled water drops, ice crystals, graupel) on enhancing the ambient electric field and initiating breakdown. More recent studies have embraced runaway breakdown in which free electrons are accelerated by the ambient electric field. These electrons collide with atmospheric molecules, producing a number of slow electrons. This increases the conductivity in the atmosphere, producing enhanced electric fields. To date there has been no conclusive data that convincingly favors either the hydrometeor or runaway breakdown mechanism. In this presentation, a new study is introduced using multi-parameter radar and VHF lightning mapping array data. Hydrometeor environments are analyzed near lightning initiation centroids for several types of storms near Huntsville, AL, USA.
AE44A-09
Measurements of Wideband Electric Fields and Their Derivatives in Conjunction With HF and VHF Radiation Produced by Lightning Discharges
In summer 2007, the electric field measurement station on the University of Florida campus in Gainesville, Florida, was expanded to include electric field derivative (dE/dt), high frequency (HF) and very high frequency (VHF) electric field measurements. The wideband electric field measurement system had a useful frequency bandwidth of 16 Hz to 10 MHz. The upper frequency bandwidth of the dE/dt measurement system was 17 MHz. The HF and VHF measurement systems had their center frequencies at 5 MHz (with a bandwidth of 4.7 MHz to 5.4 MHz) and 36 MHz (with a bandwidth of 34 MHz to 38 MHz), respectively. The sampling rate was 100 MHz. Over 1500 four-channel records of lightning discharges on 6 thunderstorm days were acquired. Narrow Bipolar Pulses (NBPs) and pronounced preliminary breakdown pulse trains (both followed and not followed by strokes to ground) were observed among other lightning events. The dataset also includes a few positive cloud-to-ground discharges. Examples of correlated wideband electric field, dE/dt, HF and VHF radiation records will be presented and discussed. Some lightning events produced dE/dt and VHF signatures that were characteristic of NBPs, but were not accompanied by a characteristic NBP wideband electric field waveform. At least one lightning flash (three strokes to ground followed by a cloud discharge) was also recorded at Camp Blanding, about 45 km away from Gainesville.