AE21A-01 INVITED
Sub-Orbital Measurements of Energetic Radiation Near Thunderstorms: A Review
Since C.T.R. Wilson first proposed that the electric field inside a thunderstorm might accelerate free electrons to very high energies, many measurements have been made to verify his hypothesis. These measurements have been made from the ground, from airborne platforms, and most recently from satellites. This paper will review measurements made by airborne instrumentation (e.g. aircraft, balloon, rocket, etc.) and look at the particular scientific questions that can be and are being addressed.
AE21A-02 INVITED
Observations of Two Terrestrial Gamma-Ray Flashes (TGFs) over a Wide Energy Range with the Fermi Gamma-ray Burst Monitor
The Fermi Gamma-ray Space Telescope was launched into low-Earth orbit on June 11 of this year, carrying the Gamma-ray Burst Monitor (GBM), an omnidirectional instrument capable of detecting Terrestrial Gamma- ray Flashes (TGFs) with high sensitivity. The GBM detector array is a set of 14 scintillation detectors arranged around the spacecraft and pointed in different directions, primarily intended for the observation of gamma-ray bursts (GRBs). Compared to the earlier BATSE and RHESSI instruments, which gave us our current understanding of TGFs, GBM has better absolute timing accuracy, is capable of measuring larger fluxes without saturating, and measures over a wider energy range. These capabilities make it an ideal instrument to answer two of the major remaining questions about TGFs: whether they are the cause or an effect of the associated lightning, and their maximum possible brightness and energy content. The first TGFs from GBM were observed on 7 August and 28 August, 2008. Their measured energy spectrum extends from ~10 keV to ~15 MeV, over three decades in energy. It is observed that the spectrum below ~100 keV shows characteristics of being highly absorbed and scattered from higher energies, and the radiation begins to fall rapidly above ~10 MeV, consistent with previous observations with RHESSI.
AE21A-03 INVITED
The AGILE-MCAL instrument as TGF monitor
The Mini-Calorimeter detector on-board the AGILE satellite was designed for astrophysical observations in the gamma-ray field, including cosmological Gamma-Ray-Burst (GRB). Thanks to its flexible on-board trigger logic for transient event, since the beginning of its operation, in April 2007, MCAL was also able to detect very fast transient phenomena in the energy range from 0.3 up to several MeV. These exhibit the characteristic of Terrestrial Gamma-ray Flashes (TGF). The candidate MCAL TGF events have a duration that can last from some hundred of μsec. to some millisec and reaches up to MeV in energy. For each triggered TGF its photon-by-photon history is recorded, with every photon described with energy, position on the detector plane and time (2 μsec. resolution) data. The on-ground data analysis strategy, based on more sophisticated algorithm than on-board logic, allows to discriminate between cosmological GRB and TGF and then to supply the spectral fluxes and the incoming area on the heart, within a 2000 km circle, of the observed TGF. MCAL can be then be used to increase the statistic of observed TGFs in the gamma ray domain and eventually to correlate them with observation in other energy ranges. The MCAL TGF observation strategy, as well as a short catalogue of detects events will be described and discussed.
AE21A-04 INVITED
Numerical simulation of terrestrial gamma-ray flashes detected in the near space for moderate variations of thundercloud dipole moment
In the framework of the mechanism of the breakdown driven by runaway electrons (REs) new simulations were executed of the high-altitude atmospheric discharges triggered by intracloud lightning flashes, as the most common globally, especially in tropics, where the sources of the terrestrial gamma-ray flashes (TGFs) detected aboard CGRO (Fishman et al, 1994) and RHESSI (Smith et al, 2005) were observed. Typical cloud charge configurations and small variations of the vertical dipole moment, 60-70 Ckm, were simulated. Computations were executed using 2D fluid code with the multi-group description of the RE kinetics in self- consistent electric field and new cosmic ray source of seed REs. The kinetics of low-energy secondary and background electrons, positive and negative ions was simulated by drift equations allowing for ionization, recombination and electron attachment. The brightness of optical emissions in different nitrogen systems was calculated. The RE Bremsstrahlung transport to the near space was simulated by MC code ELIZA allowing for all kinds of photon, electron and positron interactions using evaluated libraries of cross-sections. The ascending RE flux was accepted to start at some altitude (source). The earlier calculated dependencies were used for the source RE and Bremsstrahlung angular and energy distributions and for the Bremsstrahlung emission rates on the "overvoltage". At the orbit altitude, 500 km, dependencies of the gamma-photon specific current (1/(s×electron)) and the angular distribution (1/ster.) on the source altitude were calculated. The calculated maxima of the RE concentration and the air fluorescence brightness above the cloud appeared at altitudes as low as 11 km. The brightness is less than 100 kilorayleigh. At the altitudes common for the Sprites the calculated brightness does not exceed 50 rayleigh. The calculated gamma-photon numbers at the detectors are rather close to the detected TGF photon numbers. Thus, conclusions of the published analyses (Cummer and Lyons, 2005; Cummer et al, 2005; Willams et al., 2006) of the representative set of experimental data are confirmed that TGFs are connected with typical thundercloud charge configurations and intracloud lightning discharges causing small dipole moment variations and TGFs are not correlated with observed Sprites connected with large moment changes.
AE21A-05
On the Mechanism of X-ray Generation in Dart Leaders of Lightning Flashes
Radiation with energies 30 - 250 keV during the dart leader phase of rocket-triggered lightning was reported by Dwyer et al. [1]. However, the mechanism of X-ray generation by dart leaders is unknown at present. Recently, Cooray et al. [2] developed physical and mathematical techniques necessary to calculate the electric field associated with the tip of the dart leaders. The results obtained in that study show that as the dart leader tip passes a given point on the defunct return stroke channel, the electric field increases within a fraction of a microsecond to values several times higher than the electric field necessary for the conventional electrical breakdown in low density air comprising the channel. After reaching the peak, the electric field decays within a few microseconds to a low but steady value. For a typical dart leader (peak current about 1 kA and 10% - 90% rise time of about 0.5 μs) moving at 107 m/s, the peak electric field may reach up to about 3 × 106 V/m. The temperature of the defunct return stroke channel during the passage of the dart leader could lie in the range of 2000 - 5000 K [3]. Since the channel is at atmospheric pressure, the electric field necessary to produce conventional electrical breakdown in the low dense air of the defunct return stroke channel is about (4.5 - 1.8) × 105 V/m. We have utilized the electric field associated with the tip of a typical dart leader together with the energy dependent frictional force on electrons, as presented by Moss et al. [4], to evaluate the maximum energy an electron will receive in accelerating in this electric field. In the calculation, we have taken into account the fact that the electric field is changing both in space and time and that the gas in the defunct return stroke channel is at atmospheric pressure and at elevated temperature (i.e. reduced gas density). The results of the calculation show that if the temperature of the defunct channel is larger than about 3500 K, electrons could be accelerated to energies in the MeV range, whereas the electron energies reduces drastically with decreasing temperature and around 3000 K the energy gain is no more than a few tens of electron volts. This critical temperature decreases with increasing dart leader current. The results, valid for a typical dart leader moving at 107 m/s, show that the electric field at the tip of dart leaders is capable of accelerating electrons to MeV energy range through cold runaway mechanism. [1] Dwyer et al., Measurements of x-ray emission from rocket- triggered lightning, Geophys. Res. Lett., 31, L05118, doi:10.1029/2003GL018770, 2004. [2] Cooray, V., M. Becerra and V. Rakov, On the electric field at the tip of dart leaders in lightning flashes, Proc. 28th International Conference on Lightning Protection, pp. 339 – 344, Kanazawa, Japan, 2006. [3] Uman, M. and R. E. Voshall, Time interval between lightning strokes and the initiation of dart leaders, J. Geophys. Res., vol. 73, pp. 497 – 506, 1968. [4] 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.
AE21A-06 INVITED
The role of extensive cosmic-ray air showers in lightning initiation
Over the years, many researchers have suggested that runaway electron avalanches may play an important role in thunderstorm physics and lightning initiation. For instance, it has been proposed that runaway electron avalanches seeded by extensive cosmic-ray air showers could result in enough ionization to initiate lightning. This hypothesis, which attempts to address the long-standing mystery of how lightning initiates in the relatively low electric fields observed inside thunderstorms, has gained a great deal of attention, both from the scientific community and from the popular press. Unfortunately, so far, there is little observational evidence and only limited theoretical work to support this idea. In this presentation, the possible roles that runaway electron avalanches and extensive cosmic-ray air showers play in thunderstorm and lightning processes will be discussed, including new theoretical and modeling results and planned UF-FIT experiments to be performed to address these topics.
AE21A-07
High Resolution Mapping of the Final Stepped leader Phase from dE/dt TOA Measurements
A time of arrival network composed of eight wideband (DC-20 MHz) electric field derivative (dE/dt) antennas and eight co-located NaI(T1) scintillation detectors is deployed at the UF-FIT ICLRT to study the final part of the leader phase in cloud-to-ground lightning. It can resolve lightning processes within several hundred meters of ground with 2-3 m of horizontal uncertainty and typically less than 10 m of altitude uncertainty for sources 50 m or higher above ground. We discuss here the locations of dE/dt stepped-leader and other pulses from a negative first stoke that occurred on 2 June 2006 . A dominant region of stepped-leader activity (dE/dt pulses) was identified 500 us prior to the return-stroke about 350 m above ground. The leader descended toward ground, exhibiting extensive branching. Ninety dE/dt pulses were located during this period. At 70 us before the return stroke, the downward leader had descended to an altitude of roughly 120 m. After this time, the leader descended as four distinct channels to an altitude between 50 and 75 m. Approximately 40 pulses were located . The final dE/dt pulse of the stepped-leader phase corresponded to the final step in the leader branch nearest a tree line. Several us later, a "burst" of four dE/dt pulses, termed here a "leader burst" to distinguish it from the characteristic leader step dE/dt pulses, was observed at all stations, corresponding to the rapid advancement of the downward leader from near the main leader towards the tree line. During the leader burst, the leader channel descended over 30 m in altitude and propagated a horizontal distance of about 60 m in less than a microsecond. The termination of this burst, at an altitude about 10 m, coincided with the start of the initial rising portion of the dE/dt (and E-field) waveform, the so called "slow front" of the return stroke. Two pulses occurring during the slow front were located very near the termination of the leader burst. The locations determined for the slow-front pulses were within 10 m horizontally of a pine tree, about 7 m tall, which was a casualty of this flash. The leader burst and all eleven major leader steps occurring during the 70 us prior to the return stroke were proficient x ray producers. The slow-front pulses and the fast transition were also coincident with x rays although these were much smaller in amplitude (numbers and/or energy) and observed by only a few of the eight stations.
AE21A-08
High-Speed Video Observations of Upward Leaders from Tall Towers
High-speed (7,200 frames per sec) video observations of upward leaders from several tall towers, of heights greater than 150 m in Rapid City, South Dakota, revealed a variety of processes associated with the development of these upward leaders which were previously unknown from studies of rocket-triggered lightning. Confirmed by the NLDN data, and also by immediately preceding video images, all upward leaders were triggered either by the approaching negative leaders of intracloud or negative cloud-to-ground flashes, or by the return strokes of positive cloud-to-ground flashes. This indicates the positive polarity of the upward leaders. Following the progression of the branched upward positive leaders, recoil leaders retraced parts of the decayed channels of the forked structure toward the stems of the positive leaders. Some of these recoil leaders were extremely powerful, based on saturating bright channel luminosity. A few upward leaders developed as single channels without visible branches. In those cases recoil leaders manifested themselves as pulsing channel luminosities. On a few occasions, the positive upward leaders were followed, after the visible current cut-off from the ground, by a series of dart leader-return strokes of a negative cloud-to-ground flash, which is a development well established by the studies of rocket-triggered lightning.