[1] K. Papadopoulos, G. M. Milikh, P. N Guzdar, and A. S. Sharma. Gamma rays generation by lightning. EOS Supplement, 79(45):F126, November 1998.
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A new and novel mechanism for generation of gamma rays observed at equatorial latitude by the Gamma Ray Observatory is proposed. Several new interacting physics elements are involved.

-The first element is the generation of runaway electrons by thunderstorm electric field at altitudes 16 - 20 km slightly below the region where the runaway electrons become magnetized (z ~ 20) km.

-Upward leakage of the runaways above 20 km allows for the presence of helicon mode with frequency f < 10 sqrt(n_r) kHz where n_r is the local density in cm^-3 of the leaking electrons.

-ELF and VLF waves induced by lightning, reaching this region undergo self-focusing creating channels of enhanced electric field structure in the regions of runaway leakage.

-The self-focused electric fields interact resonantly with runaways inducing a new runaway discharge at altitudes between 20 - 40 km

-Gamma rays are due to beams from the runaway electrons forming at those altitudes.

A qualitative analysis will be presented. The new process can account for significant enhancement of energy deposited by lightning in the middle atmosphere with significant consequences to global warming.

[2] DA. Smith, DN. Holden, XShao, and PR. Krehbiel. Physical characteristics of compact intracloud discharges (cids). EOS Supplement, 79(45):F127, November 1998.
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Compact Intracloud Discharges (CIDs) are distinct, isolated electrical discharges that occur within intense regions of thunderstorms. Unlike other intracloud and cloud-to-ground lightning events that occur as sequential breakdown processes and often last for a large fraction of a second, CIDs are singular impulsive events that last for only a few tens of microseconds of less.

In recent years, CIDs and the thunderstorms in which the events have occurred have been studied using a variety of resources including ground-based electric field change and broadband HF arraysk, satellite instrumentation including the Blackbeard receiver and FORTE satellite, NEXTRAD weather surveillance radars, and the National Lightning Detection Network. Based on the data from these resources, CIDs have the following physical characteristics:

CIDs begin with an initial breakdown stage as indicated by RF radiation that lasts for a few microseconds. Ionization proceeds at a velocity on the order of one third of the speed of light and forms a channel that is between a couple hundred and many hundred meters in spatial extent. The channel is predominately GET abstract from AGU site..

[3] PR. Krehbiel, RThomas, WRison, THamlin, JHarlin, and MDavis. Lightning mapping observations during MEaPRS in central Oklahoma. EOS Supplement, 79(45):F127, November 1998.
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A 3-dimensional lightning mapping system was operated over a county-wide area northwest of Oklahoma City in conjunction with the MEaPRS project during June of 1998. Observations were obtained of the total lightning activity in several supercell storms, a tornadic storm, and storms having a large fraction of positive cloud-to-ground lightning. A large, fast-moving storm system that propagated over the network produced normal cloud-to-ground (CG) and intracloud (IC) lightning discharges in a number of coexisting cells and occasional discharges of large horizontal extent. One hybrid IC/CG flash had an overall extent of 75 km and exhibited a bilevel structure in the main negative and upper positive charge regions of the storm. The channels in the upper positive charge region decreased to the altitude of the main negative charge as the discharge progressed into an apparent stratiform region away from the core of the storm, indicating that the positive charge region similarly decreased in altitude. A third, slightly lower level of activity was also observed that may be associated with lower positive charge. Lightning in the supercell storms was essentially continuous and had a more amorphous structure, with discharges occurring in rapid succession at different locations that filled the horizontal area of the storm every minute. Preliminary results for one predominantly positive CG storm showed that +CG discharges were associated with normal polarity IC discharges early in the storm, but that later in the storm's evolution the IC flashes appeared to be of inverted polarity, that is, between an upper negative and main positive charge region. The latter result remains to be confirmed by electrostatic field change observations. Lightning in the tornadic storms remains to be analyzed. Overall, the great plains storms are observed to be extremely active electrically.
[4] WRison, PR. Krehbiel, MDavis, THamlin, JHarlin, TBarber, and MJones. A deployable 3-dimensional lightning mapping system. EOS Supplement, 79(45):F127, November 1998.
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A lightning mapping system based on Kennedy Space Center's real-time LDAR (Lightning Detection and Ranging) system has been developed that utilizes GPS timing at a number of widely spaced locations to image lightning discharges in three spatial dimensions and time. The system was successfully deployed in two field programs in 1998 - in central Oklahoma during May and June and in central New Mexico during August and September. Lightning-produced radiation signals are received in an unused television channel (Channel 3, 60-66 MHz) and the time of arrival of impulsive events is independently measured at each station using the GPS timing signals. Overall timing accuracies of 40-50 ns rms are achieved on actual lightning events, corresponding typically to 50-150 m location errors. In the 1998 field programs, the peak VHF radiation signal was timed in successive 100 mus windows at 10 locations over a 50-km diameter area. This provided excellent location information over a 100-km diameter area and useful information out to several hundred km (with increasing errors at larger distances). An automatic, non-linear least squares solution procedure is used to locate the radiation sources and provides reliable locations when an event is observed at 6 or more stations. The system typically locates 500-1000 sources per second of lightning activity. 3600 sources were located during one horizontally extensive flash that lasted 2.5 seconds. The sources clearly show the temporal development and channel structure of lightning flashes and sometimes the dendritic structure as well. The system has been designed to be relatively low cost and is PC-based, using a custom programmable-logic PC card to digitize and time the signals. A wireless communications network (with 115 kbaud data rate for each remote station) is currently being used to control the network and will be used to automatically transmit data to a central site for real time processing and display of the locations. Electric field change and field mill sensors will also be added at each site to further characterize the lightning and the storm electrification.
[5] EMD. Symbalisty, RA. Roussel-Dupr, and VA. Yukhimuk. The transition from red sprite to columniform sprite in the context of runaway air breakdown theory. EOS Supplement, 79(45):F129, November 1998.
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The results of numerical simulations of columniform sprites, in the context of the runaway air breakdown mechanism (A. V. Gurevich, G. M. Milikh, and R. Roussel-Dupr, Phys. Lett. A, 165, 463, 1992), are presented. The results include intensity and spectra of optical emissions, and the time dependent secondary (slow) and primary (relativistic) electron concentrations. We recall that the simulations model the transient electrical environment above a large mesoscale thunderstorm complex due to a normal lightning stroke. The transition from a normal sprite discharge to a columniform sprite is proposed to be a function of the configuration of the driving electric field. In our 2-D, axi-symmetric simulations, with simple models for the driving fields, we can produce the transition by varying the strength, location, and spatial size of the initiating positive cloud to ground lightning stroke. For example, a point discharge (spatial size less than 1 km) is more likely to produce a columniform sprite than a spatially extended lightning discharge, with all other parameters being equal. We are using the recently recalculated runaway avalanche rates (E. M. D. Symbalisty,R. Roussel-Dupr, and V. Yukhimuk, IEEE Transactions on Plasma Science, scheduled to appear in October 1998 issue). We also examine the impact of different ambient electrical conductivity profiles and compare our results to observations (E. M. Wescott, D. D. Sentman, M. J. Heavner, D. L. Hampton, and W. A. Lyons, University of Alaska Preprint, 1997).
[6] WA. Lyons, TE. Nelson, ER. Williams, JA. Cramer, and TR. Turner. Changes in the electrical structure of U. S. thunderstorms ingesting smoke from the 1998 Mexican fires. EOS Supplement, 79(45):F129, November 1998.
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As well documented by both satellite imagery and surface aerosol concentration measurements, smoke from the vast 1998 El-Nino drought-related forest fires in Mexico and Central America frequently advected into the United States. Specifically, between about 7 April to 8 June, most air masses over the southern plains of the U.S. were influenced by the smoke. A strong frontal zone crossing the region between 14 - 18 May resulted in smoke being advected as far north as Ontario and eastward to New England. As documented by the National Lighting Detection Network (NLDN), thunderstorms developing within these contaminated air masses had extraordinary changes in their electrical characteristics. Specifically, large thunderstorm systems exhibited very high percentages of cloud-to-ground (CG) lightning flashes with positive polarity (+CGs). While +CGs are typically less than 10 storms, such as those with the mid-May frontal system, sustained +CG percentages of 50 to 90% during their lifetime. Moreover, the peak currents within these +CGs were much higher than normal. Throughout a two month period, storms in the southern plains averaged three times the normal frequency of +CGs and twice the average peak current. Prior research suggests +CGs have far greater potential for starting fires, disrupting electrical systems, and possibly producing larger amounts of NOx. It has been confirmed that lightning from the 1998 smoke-influenced storms produced unprecedentedly large numbers of optical sprites in the mesosphere. Similar though less pronounced +CG enhancements may have been observed in Florida sea breeze thunderstorms during that state's siege of summer wildfires. Some possible mechanisms which might explain these unexpected changes will be presented.
[7] TE. Nelson, WA. Lyons, RA. Armstrong, ER. Williams, DM. Suszcynsky, RStrabley, MTaylor, and LGardner. Some initial results from SPRITES'98. EOS Supplement, 79(45):F135, November 1998.
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SPRITES'98 was conducted at the Yucca Ridge Field Station near Fort Collins, CO from mid-May through August 1998. Using multiple remote sensing technologies, coordinated measurements of individual sprite and elve events were obtained. Both red and blue low-light imagers (LLTVs) and multi-color broad- and narrow-band photometers monitored optical emissions. Evidence of ionization within sprites obtained in previous programs was confirmed from simultaneous 4278 and 4709 nm emissions. High speed (1000 fps) images of sprites reveal new details of the temporal sequence of events. In addition, numerous high speed videos of the entire CG lightning event for both positive and negative polarity strokes provide new insights on the continuing currents and horizontal dendrite discharges. These videos were coordinated with optical measurements made with a photodiode array plus VLF and ELF transients (Q-bursts) made at MIT's Rhode Island Schumann resonance observatory. Storms ingesting smoke from Mexican fires produced unprecedentedly high percentages of +CGs, which in turn had twice the normal peak currents. A record number of sprites (nearly 400 in 208 minutes) were imaged above one such smoke-influenced storm. Sprites again were typically associated with storms with radar echoes larger than 10,000 km2, but some exceptions were noted, especially bursts of sprites in the dying phases of some supercell class thunderstorms. An unusual interaction between a meteor and a sprite was recorded. Monitoring was also coordinated with scientists from Tohoku University, New Mexico Tech, Stanford, and the University of Alaska.
[8] RStrabley, DM. Suszcynsky, RRoussel-Dupre, EMD. Symbalisty, RA. Armstrong, WA. Lyons, and TA. Nelson. Video and photometric observations of a possible metor-triggered sprite/jet event. EOS Supplement, 79(45):F135, November 1998.
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This paper presents video and photometry data of a possible meteor-triggered sprite/jet event. The data was recorded with both white-light and blue-light Xybion video cameras with millisecond time-stamping, a gps time-tagged broadband red photometer with 50 microsecond time resolution, and a VLF receiver. The event was collected on August 1, 1998 during the SPRITES '98 campaign at Yucca Ridge Field Station operated by FMA Research in Ft. Collins, CO. The event consisted of three stages, (1) the observation of a moderately bright meteor, (2) the development of a sprite in the immediate vicinity of the meteor as the meteor reached the 60 - 70 km altitude range, and (3) a slower forming ``jet'' of luminosity that appeared during the late stages of the sprite and propagated back up the ionization trail of the meteor. The event is analyzed in terms of its geometry, its association with the meteor, and the implications to existing theories for sprite and jet formation.
[9] Rick Rairden and Stephen Mende. Intensity-calibrated sprite observations from new mexico. EOS Supplement, 79(45):F135, November 1998.
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Image intensified video cameras fielded at the Langmuir Laboratory near Socorro NM recorded numerous Sprite events during the summer 1998 campaign. The Lockheed Martin/UC Berkeley effort involved field testing of a new CID camera, and ground-based trials of a NASA camera, veteran of the Space Shuttle Tethered Satellite missions. Calibrated intensity results will be presented for white light and red-filtered imagery obtained July 19-22.
[10] Simon F. Hardman, Richard L. Dowden, James B. Brundell, John L. Bahr, Zenichiro Kawasaki, and Craig J. Rodger. Sprites in Australia's Northern Territory. EOS Supplement, 79(45):F135, November 1998.
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Sprites have been observed near Darwin in Australia's Northern Territory (NT). These are the first confirmed ground observations of sprites outside America. The spatial and temporal properties of sprites seen in the NT are similar to those measured in the United States. A number of 'dancing sprites' were observed, where a succession of sprite columns form and decay, appearing to move across the sky. On one night almost all the NT dancing sprites progressed from right to left, or towards a more southerly bearing, suggesting that the direction of dancing sprite movement is determined by large scale cloud structure.
[11] Martin Fullekrg. ULF/ELF magnetic fields of sprite-associated lightning flashes. EOS Supplement, 79(45):F135, November 1998.
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During the sprite campaign 1998, the Institut fr Meteorologie und Geophysik at the Universitt Frankfurt/Germany deployed a network of 3 instruments to measure horizontal magnetic field variations of sprite-associated lightning flashes at ULF/ELF frequencies ranging from 0.01 Hz to 1 kHz. The measurement instruments are located in Santa Cruz/California, Socorro/New Mexico and Saskatoon/Canada. The network is GPS time synchronized with an accuracy of 20 usec between stations and enables the triangulation of lightning flashes with continuing current by use of time of arrival difference techniques. The slow tails of sprite-associated lightning flashes are often associated with slowly varying magnetic fields on the order of 100-200 ms which are extremely efficient in the excitation of globally observable Earth-ionosphere cavity resonances. The measurments at three different stations are interpreted as quasi-static magnetic fields of sprite-associated currents and physical properties of these currents will be derived. Some of the sprite-associated lightning flashes exhibit additional occurrences of ultra-slow tails in the Pc1 frequency range (0.2-5.0 Hz). The measurements at three different stations are used to quantify the directional dependence of ultra-slow tails along and across the magnetic field lines and an interpretion in terms of discrete excitations of ionospheric Alfven resonances will be discussed.
[12] EW. Huang, ER. Williams RA. Boldi, SJ. Heckman, WA. Lyons, TE. Nelson, MJ. Taylor, and CT. Wong. Criteria for sprites and elves based on Schumann Resonance observations. EOS Supplement, 79(45):F135, November 1998.
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Ground flashes with positive polarity associated with both sprites and elves excite the Earth's Schumann resonances to amplitudes several times greater than the background resonances. Theoretical predictions for dielectric breakdown in the mesosphere are tested using ELF methods to evaluate vertical charge moments of positive ground flashes. Comparisons of the measured time constants for lightning charge transfer with the electrostatic relaxation time at altitudes of nighttime sprite initiation (50-70 km) generally validate the electrostatic assumption of predictions made initially by C.T.R. Wilson. The measured charge moments (200-2000 C-km) are large in comparison with ordinary negative lightning, but are insufficient to account for conventional air breakdown at sprite altitudes. The measured charge moments however are sufficient to account for electron runaway breakdown, and the long avalanche length in this mechanism also accounts for the exclusive association of sprites with ground flashes of positive polarity. The association of elves with large peak currents (50-200 kA) measured by the National Lightning Detection Network in a bandpass beyond the Schumann resonance range is consistent with an EMP mechanism for these events.
[13] GSatori. El nino related variations of global lightning activity as shown by schumann resonances. EOS Supplement, 79(45):F136, November 1998.
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Schumann resonance (SR) frequencies and amplitudes have been recorded for the vertical electric component at Nagycenk Observatory in Hungary since May of 1993. The excitation source of SR is the global lightning activity which is responsive to surface air temperature. The SR frequencies are indicative to the areal extension of global lightning activity and the variations of source- receiver geometry, as well as to the speed of north-south annual migration of thunderstorm regions. All these parameters exhibit El Nino related variations as shown by SR observations. The north-south asymmetry of the land/ocean ratio plays an important role in the El Nino related redistribution of the temperature dependent global lightning activity.
[14] KSchlegel and MFullkrug. Changes of Schumann-Resonance parameters during high energy solar particle events. EOS Supplement, 79(45):F136, November 1998.
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Regular Mail
[15] RWoodard, MJ. Heavner, DR. Moudry, DD. Sentman, EM. Wescott, JS. Morrill, CSiefring, EJ. Bucsela, DL. Osborne, JT. Desroschers, HNielsen, JWinick, JKristl, THudson, LM. Peticolas, and VBesser. Spatial variation of ion and neutral emissions in sprites. EOS Supplement, 79(45):F136, November 1998.
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The EXL98 aircraft observations included both broadband cameras and narrowly filtered cameras. The paper will present a comparison between a camera with response such that it primarily measures spatial molecular nitrogen first positive (neutral) emissions and a camera filtered for 427.8 nm ionized molecular nitrogen first negative emissions. The comparison of the spatial region of neutral emissions to the spatial region of ionized emissions is a crucial issue for estimations of the total energy of sprites. A wide range of variability between the two cameras has been observed. An example of a sprite which looks almost identical in the two systems will be presented, and an example of a sprite which appears in the broadband camera but not in the 427.8 nm camera will also be presented. In addition to several examples of sprites observed in both camera systems, a detailed analysis of the responses of the two systems will be presented.
[16] DR. Moudry, MJ. Heavner, DD. Sentman, EM. Wescott, JS. Morrill, and CSiefring. Morphology of sprites. EOS Supplement, 79(45):F136, November 1998.
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The currently documented forms of sprites span the range from rather simple columniform sprites (c-sprites) in the form of vertical lines to much more complex forms. Some are popularly referred to as ``carrot-shaped'' sprites, as ``wishbone'' sprites, or as ``dancing sprites'', where one sprite is followed several ms later by another, then another. In addition, within the sprite structures, more intense balls and branches of illumination are sometimes visible. This study looks at the data gathered by University of Alaska in the summers of 1995, 1996 and 1998, and attempts to categorize sprite structures by their shape from the simplest to the most complex forms. This work builds on the preliminary morphology study done by Desrochers, EOS Supplement, 76, 46, November, 1995.
[17] DL. Osborne, JTobolski, DSentman, EWescott, JWinick, and CSiefring. The EXL98 aircraft. EOS Supplement, 79(45):F136, November 1998.
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Airborne measurements of sprites, jets, and other middle and upper atmospheric electrical effects of lightning permit diagnoses of spectral regions not accessible from the ground because of atmospheric absorption and scattering, as well as giving direct visual access to the associated thundercloud tops in certain flight profiles. The EXL98 project utilized a Gulfstream 2 business class jet aircraft to study sprites from flight altitudes of 12 to 13.5 km. The aircraft carried a suite of 8 separate co-aligned low light level television imaging systems with spectral responses covering a wide range of wavelengths 340 nm to 4.3 microns. In this paper we describe these instruments and the corresponding optical requirements for the aircraft viewing ports. The aircraft structural and power engineering and safety issues involved in creating the EXL98 aircraft are discussed, along with the challenges they presented for performing instrument integration, in a medium altitude aircraft environment, which must be compliant with Federal Aviation Administration (FAA) regulations. We describe the Global Positioning Satellite (GPS) technology that was utilized to provide both in-flight synchronization of the aircraft television systems, and synchronization of the aircraft instruments with University of Alaska ground imaging systems on Mt. Evans, CO and Jelm Mountain, WY.
[18] WM. Benesch, JS. Morrill, CSiefring, EJ. Bucsela, JH. Bowels, MJ. Heavner, DR. Moudry, DD. Sentman, EM. Wescott, DL. Osborne, JT. Desrochers, HNielsen, LM. Peticolas, JWinick, JKristl, and THudson. Overview of NUV observations during EXL 98. EOS Supplement, 79(45):F136, November 1998.
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During the recent EXL 98 mission to observe sprites and other related phenomena from aircraft, some of the video cameras were configured to make narrow band and spectral observations of emissions in the near ultra-violet (NUV) and blue spectral region( 3200 - 5000Å). The spectral features in this region are primarily due to emissions from neutral and ionized states of N_2, namely the 2PG and 1NG band systems. The need for aircraft observations arises from the effect of atmospheric attenuation in this spectral region when observations are made from lower altitudes and longer slant paths. These images and spectra will be used to estimate electron energies as well as examine sprite morphology. In this presentation, we will discuss NUV/Blue measurement philosophy and motivation, the details of the instrumental techniques and present a variety of observations made during the EXL 98 mission.
[19] BD. Green, WT. Rawlins, and ME. Fraser. Kinetics of excitation of infrared fluorescence by sprites. EOS Supplement, 79(45):F136, November 1998.
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Regular Mail
[20] CL. Siefring, PA. Bernhardt, JS. Morrill, DD. Sentman, EM. Wescott, MJ. Heavner, DL. Osborne, and EJ. Bucsela. Correlation between ambient near infrared (NIR) airglow and sprite structures. EOS Supplement, 79(45):F136, November 1998.
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An Indium-Gallium Arsinide (INGAAS) NIR camera (spectral range 900 and 1700 nm) was flown on the EXL98 missions to monitor hydroxyl airglow which gives indications of density structures in the neutral atmosphere, i.e., gravity waves. The sensitivity of the NIR camera combined with the brightness of the airglow in the NIR, and the fact that the camera was operated from an airborne platform at low elevation angles, makes these observations quite unique. These airglow observations span a much greater altitude range than is typical for ground based visible or NIR measurements. The NIR experiment was extremely successful and made the first measurements of NIR emissions from Sprites. Also of importance are cases where a clear correlation was seen between structures in the neutral density and the shape-and-form of Sprite emissions (both in the visible and the NIR). In a number of cases the terminal altitude and bright and dark bands in the Sprites where aligned with the ambient airglow. Also the characteristic tilt of the Sprites was often aligned perpendicular to gravity wave fronts. We will discuss these observations and their implications, e.g., these observations would appear to support 'classical' breakdown mechanisms as opposed to a runaway breakdown mechanism. *The work at NRL was sponsored by NASA and ONR.
[21] CS. Deehr, EM. Wescott, DD. Sentman, HC. Stenbaek-Nielsen, MJ. Heavner, DR. Moudry, CL. Siefring, JS. Morrill, and EJ. Bucsela. New evidence for ionization of blue starters and blue jets. EOS Supplement, 79(45):F136, November 1998.
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Blue jets and blue starters were first documented by 1994 aircraft observations of a severe storm near Texarkana, Arkansas. They appear to propagate upward out of the top of thunderstorms at speeds of order 100 km/sec. Blue jets propagate up to about 40 km but blue starters, which resemble blue jets, terminate abruptly after only a few km of upward travel. Single blue jets or starters have been recorded from aircraft near Kansas and Central America. During the EXL98 aircraft observations of July 1998, numerous blue starters were recorded on two evenings over severe storms in the upper Midwest. We have previously presented evidence from color TV images of blue jets and starters that suggested that the blue light must have an ionized N2 component. The 1998 observations included both white light, 427.8 nm (N2+1N) and 340 nm N2 2PG band filtered images. The 427.8 nm filter was carefully designed to exclude the N2 2PG band emissions at 426.8 nm. The 340 nm images show that the blue starters are not upward lightning flashes. The 427.8 nm data clearly demonstrate that the blue starters are ionized, and by association that blue jets are also ionized beams. The knowledge of the ionization allows a better estimate of the energy involved and effects on the upper atmosphere by these phenomena. We compare the lightning activity and hail in the 1998 storms with the previous observations and conclusions.
[22] EA. Gerken, US. Inan, CP. Barrington-Leigh, and MStanley. Results from a new telescopic imager: A survey of sprite structures. EOS Supplement, 79(45):F137, November 1998.
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Two intensified CCD video cameras were coupled with a 40 cm aperture Newtonian telescope with a Dobsonian mount to create a new, high-resolution sprite imager, called the Dobsonian Sprite Experiment (DSE). The two cameras of the DSE have fields of view of 0.92^o× 0.7^o and 12^o×9^o to allow both a broad view of each sprite and detailed imagery of one region. The experiment was fielded during July and August, 1998 at the Langmuir Laboratory for Atmospheric Research near Socorro, New Mexico. Simultaneous Very Low Frequency and Extremely Low Frequency magnetic field recordings were made at Langmuir and at Stanford, California. During this period many sprites were observed at ranges of up to 900 km. This paper reports on the morphology of observed sprites, their association with storm activity and sferic waveforms, and the variety of fine structure observed in the telescopic imager.
[23] KJ. Remick, DD. Sentman, EM. Wescott, MJ. Heavner, and DR. Moudry. Small scale strcture in sprite tendrils. EOS Supplement, 79(45):F137, November 1998.
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High resolution images of sprites reveal that they often possess a tendril structure attached to a much brighter main body at higher altitude. When they are present the tendrils originate at altitudes of about 60-65 km, extend downward to 45-50 km and exhibit a variety of structural detail and scale sizes, from wispy and barely discernible forms to narrow, straight or curved channels, or forked lightning-like fractal patterns. To date the University of Alaska video imagers have resolved this structure to a few tens of meters. In this talk we roughly categorize the various structures using a topological metric applied to images obtained during various University of Alaska ground and aircraft field campaigns, taking into account the image resolution and effective point spread function of the camera systems derived from stellar images. Results of this analysis are used to assign an effective cross section to the tendrils at given altitudes, thereby permitting improved estimates to be made of the optical emissivity of the tendrils compared to previous estimates based on the overall dimensions of sprites.
[24] AI. Sukhorukov and PStubbe. QT whistler excitation by strong lightning. EOS Supplement, 79(45):F137, November 1998.
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We propose that, in addition to the conventional quasi-longitudinal (QL) whistlers, the sprite/elves triggered lightning can produce the quasi-transversal (QT) whistlers, a mode which does not appear in the linear Earth-ionosphere transmission. The source of the excitation is the small-scale nonlinear transient current in the D-E regions, associated with fine structure of sprite/elves-discharges. An analytical excitation model is presented. The QT mode has a number of peculiar signatures, such as a large E_|| component accelerating electrons along the geomagnetic field, strong link to the lower hybrid resonance, much smaller dispersion at frequencies omega omega_LH and larger group velocities than those of the QL whistler.
[25] SJ. Lev-Tov, US. Inan, and TF. Bell. Possible mechanisms of ``early/fast'' VLF events caused by lightning. EOS Supplement, 79(45):F137, November 1998.
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Subionospheric very low frequency (VLF) waves provide a powerful remote sensing tool for detection of changes in the lower nighttime ionosphere. For example, changes in D-region electron density or temperature are manifested in changes in VLF wave amplitude and phase. One class of events in which the wave amplitude and phase change in a very unique manner is the so-called ``early/fast'' VLF events. Early/fast events were first recognized fifteen years ago and are clearly associated with lightning discharges. They are ``early'' in that they occur within 20ms of the causative lightning discharges; and they are ``fast'' in that the VLF signal change takes place over about 20ms. In recent years, some of these events have been found to be associated with sprites. Although several physical mechanisms of VLF early/fast events have been put forth, none of these has reproduced the magnitude of some of the larger signal changes observed in early/fast VLF events. In this paper we analyze new data sets of ``early/fast'' events with and without sprites. We also explore possible mechanisms for the production of the early/fast event through conductivity changes in the ionospheric region overhead the causative lightning (and, if present, sprites). Sophisticated three-dimensional subionospheric VLF propagation and scattering routines are used to quantitatively interpret effects of possible ionospheric profiles causing early/fast events.
[26] RD. Dowden, JB. Brundell, MA. Cliverd, and CJ. Rodger. Decay rates of the perturbation phasors of low and high latitude trimpis. EOS Supplement, 79(45):F137, November 1998.
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Sprite-associated amplitude and phase perturbations of subionospheric VLF transmissions (``Trimpis'') logged as a function of time can be transformed to scattered amplitude and phase versus time by simple phasor subtraction of the unperturbed phasor. The form of the scattered amplitude decay with time is related to the form of the plasma decay rate with respect to altitude. Over the range of at least 50 km to 80 km, attachment is the dominant loss term so at any altitude the plasma decays exponentially with time. Neglecting a small temperature dependence, the attachment coefficient is proportional to the square of the neutral number density and so the decay rate decreases exponentially with a scale height of 3.5 km. The reflection coefficient of sprite plasma at VLF is a strong function of plasma density only near a ``threshold'' value. Thus contributions to the total scattered VLF signal from plasma at altitudes below where the plasma density is instantaneously passing through this threshold value are negligible, while contributions from higher altitudes are still as large as they have been since plasma formation. This transition in reflection coefficient from essentially unity to essentially zero occurs over only a few km. Here we examine very strong Trimpis of high signal/noise ratio (SNR) of two types. The Type 1 Trimpis we examined were ``fast'', occurred on entirely tropical paths, from NWC (21 deg, 48' S, 114 deg, 9' E) to Darwin area (12 deg, 26' S, 130 deg, 59' E) during the lightning season and were associated with sprites. We assume that these were produced by sprite plasma. The Type 2 Trimpis were observed at Faraday and Rothera, Antarctica, on both NAA (44 deg, 39' N, 67 deg, 17' W) and NPM (21 deg, 25' N, 158 deg, 9' W). From this we deduced that the scatter (or perturbing) source for these Type 2 Trimpis was near Antarctica in a region devoid of lightning (and presumably of sprites) and so produced by electron precipitation. For both types, the rate of exponential decay decreased with time. From the form of this decay we deduced that the plasma extends down from the base of the ionosphere to at least 60 km in the case of Type 1 Trimpis and to about 75 km in the case of Type 2 Trimpis.
[27] KMiyamura, INagano, SYagitani, BBarnum, RH. Holzworth, and MC. Kelly. Wave forms of lightning-generated VLF waves: Comparison between a full-wave analysis and rocket measurements. EOS Supplement, 79(45):F137, November 1998.
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A new full-wave technique has been developed to rigorously compute space and time evolution of an ELF/VLF electromagnetic wave in the ionosphere radiated from a current source located above the ground. We can examine detailed wave forms as well as dynamic spectra of the linear electromagnetic wave fields at an arbitrary location in a horizontally stratified ionosphere. This technique has been applied to study propagation of VLF waves generated by various kinds of lightning strokes modeled with transmission line currents, especially to examine their role in generation of luminous emissions above lightning such as Sprites and Elves. By using the full-wave technique in this study we investigate in detail space and time evolution of lightning-generated VLF electromagnetic fields in the ionosphere, to discuss their linear propagation characteristics. The computed VLF wave forms are directly compared with those actually measured over thunderclouds by the Thunderstorm-III rocket launched from Wallops Island. This enables us to estimate possible magnitude and shape of lightning strokes which generated the VLF waves measured onboard the rocket. Quantitative examination of amplitudes and phases of those wave forms would even clarify nonlinear response of the ionosphere to an extremely intense VLF wave generated by a strong lightning stroke.
[28] GVeronis, VP. Pasko, and US. Inan. Characteristics of mesospheric optical flashes (elves) produced by lightning discharges. EOS Supplement, 79(45):F137, November 1998.
[ bib ]

A new two-dimensional cylindrically symmetric electromagnetic model of the lightning-ionosphere interaction includes effects of both the lightning radiated electromagnetic pulses (EMP) and the quasi-electrostatic (QE) fields, thus allowing effective studies of lightning-ionosphere interactions on time scales ranging from several mus to tens of ms. The temporal and spatial evolution of the electric field, lower ionospheric electron density and optical emissions calculated with the new model are used to theoretically investigate the effects of the lightning return stroke current waveform (i.e., the current rise and fall time scales) and of the observational geometry on the optical signals observed with a photometer. For typical lightning discharges of ~100mus duration the ionospheric response is dominated by the EMP induced heating leading to the highly transient and laterally expanding optical flashes known as elves. The optical signal characteristics are found to be highly sensitive to both the observational geometry and the current waveform. The onset delay with respect to the lightning discharge, the duration and the peak magnitude of optical emissions are highly dependent on the elevation and azimuth angles of field of view of individual photometric pixels. The shape of the optical signal clearly reflects the source current waveform. For a waveshape with relatively slow rise time (~50mus) a double pulse shape of the photometric signal is observed.

[29] RA. Roussel-Dupre, EMD. Symbalisty, and VYukhimuk. Initiation of intra-cloud discharges by runaway air breakdown. EOS Supplement, 79(45):F137, November 1998.
[ bib ]

Simulations of intra-cloud discharges initiated by runaway air breakdown are presented. Results of new kinetic calculations (E.M.D. Symbalisty, R. Roussel-Dupr, and V. Yukhimuk, IEEE Transactions on Plasma Science, scheduled to appear in October, 1998 issue) are used to characterize the relativistic electron distribution function. Both conventional and runaway breakdown are included in the simulations; however, the discharge is initiated by a runaway streamer that develops when the electric field strength exceeds one-third the threshold for conventional breakdown. Optical, radio-frequency, and gamma emissions are computed and compared with existing data. Other details of the discharge including typical temporal and spatial scales and the development of steady-state saturation electric fields will be discussed in the context of observations.
[30] RJL. Grard. The role of turbulence and convection in the electric charging of thunderstorm clouds. EOS Supplement, 79(45):F138, November 1998.
[ bib ]

The electric current which circulates downwards in the Earth atmosphere results from the motion of positive and negative ions drifting in opposite directions under the influence of a predominantly vertical potential gradient. It is observed experimentally that a body, such as a balloon or a gondola, moving upwards against the electric field collects an excess of positive ions; conversely, a falling body such as a water drop, acquires a negative charge. This mechanism just requires that the velocity of the body exceed that of the drifting small ions, which is less than a few cm/s at altitudes under 30 km in a fair weather atmosphere. In a similar way, parcels of hot and cold air ascending or descending in a cloud are selectively charged. Some ions are captured by aerosol particles and form large ions which, due to their extremely reduced mobility, are subsequently transported with the draft against the force applied by the electric field. Charge separation increases the ambient electric field and current density, which in turn enhances the charging process. This phenomenon takes place in spite of the reduced conductivity which prevails within clouds, due to the recombination of small positive and negative ions on aerosol particles. The proposed mechanism is particularly efficient during the growth phase of a thundertorm cloud, where the air motion results essentially from rising eddies. It is shown that, under certain conditions, the electric field may grow exponentially from a few 10 V/m to a few 100 kV/m in a period of about 20 minutes and that a typical thunderstorm cloud can generate a current of the order of 1 A.
[31] NG. Lehtinen, US. Inan, and TF. Bell. Effects of upward driven runaway electrons in the conjugate hemisphere: Conjugate Sprites? EOS Supplement, 79(45):F138, November 1998.
[ bib ]

Runaway acceleration of relativistic electrons driven by mesospheric quasi-static electric fields following large positive cloud-to-ground lightning discharges has been put forth as a possible physical mechanism for sprites and terrestrial gamma-ray flashes. The intense and upward propagating runaway electron beams are ultimately trapped along the Earth's magnetic field lines and travel to the geomagnetically conjugate region. At that point, the runaway electron beam encounters the Earth's atmosphere and is backscattered, producing light, ionization and x-rays, very much like a beam of precipitating auroral electrons. We calculate the characteristics (energy and pitch angle distribution) of the runaway electron beam as a function of the intensity of the parent lightning and the geomagnetic latitude. We discuss the pitch-angle scattering of the electrons due to beam-plasma interaction during their propagation along the geomagnetic field line. The backscattering of electrons, as well as optical and x-ray emissions in the conjugate region is estimated using the Monte Carlo approach.
[32] VYukhimuk, RRoussel-Dupre, and ESymbalisty. X-ray and radio pulses produced by red sprites, simulation results. EOS Supplement, 79(45):F138, November 1998.
[ bib ]

Upward discharges driven by the runaway air breakdown mechanism (A. V. Gurevich, G. M. Milikh, and R. Roussel-Dupr, Phys. Lett. A, 165, 463, 1992) can generate significant x-ray emissions as well as radio emissions. The results of numerical simulations of the x-ray, radio and optical emissions from Red Sprite discharge are presented. The sprite simulations are done using the recently recalculated runaway avalanche rates (E. M. D. Symbalisty, R. Roussel-Dupr, and V. Yukhimuk, IEEE Transactions on Plasma Science, scheduled to appear in October 1998 issue). The x-ray emissions are generated as a result of relativistic beam deceleration by air, their time evolution and angle distribution are calculated taking into account the attenuation of x-rays in the atmosphere. The radio emissions are produced as a result of the relativistic avalanche growth and may consist of one or two pulses. The first pulse precedes the onset of the high altitude optical emissions and is associated with the beam formation at lower altitude   18-30 km, the source of the second pulse approximately coincides in space and time with high altitude optical transient. The time separation between two pulses is in range 150 - 800 microseconds, but can appear to be different for observers because of the spatial separation of their sources. The results are compared with observations.
[33] BH. Barnum, RH. Holzworth, and MC. Kelley. Simultaneous ground and rocket based optical measurements of lightning flashes. EOS Supplement, 79(45):F138, November 1998.
[ bib ]

During the Thunderstorm III rocket flight from Wallops Island VA, during the night of September 2, 1995, simultaneous rocket and ground based measurements were made of Cloud to Ground (CG) and Intracloud (IC) lightning flashes. The results of this study show that the CG triggered IC events are much brighter at the rocket than the CG events. This is due to the source altitude and the incloud light scattering. The higher altitude of the IC lightning channels produce much brighter optical signals at the rocket. The associated IC events usually occur within 20 milliseconds of the initial CG stroke, and initiate from near the top of the CG lightning channel, as shown by the Wallops Island LDAR VHF system. Because of the close timing between the CG and IC strokes, rocket or satellite measurements can easily mistake the brighter stroke for the actual CG lightning. The CG and IC strokes each initiate an upgoing whistler wave in the ionosphere, but the CG stroke typically has much more power at lower VLF and ELF frequencies. The VLF and ELF power spectra give a way to discern the nature of the lightning source for space based optical sensors. Other results of this study show that there is no correlation between the NLDN measured peak currents and CG stroke optical brightness for over 100 NLDN CG cases at Wallops Island.
[34] DA. Morss. SPARKE (Spherical Propagating Atmospheric Radiative Kinetic Emission): Fireball in the sky? EOS Supplement, 79(45):F138, November 1998.
[ bib ]

Sprite imaging using low light, image intensification video cameras in the Omaha, Nebraska area on 22 August 1996 revealed a phenomena that was unexplained when reported at the Dec 1996 AGU Fall meeting in San Francisco. At that time, a number of probable causes had already been ruled out, e.g., space debris re-entry, meteorites, aircraft, helicopters, and insects. Estimates of range and speed were speculated at that time based upon the probable distances from and direction to the nearest thunderstorms on that evening. Assuming a thunderstorm-related initiation, it was estimated that SPARKE seemed to have originated some 750 km away, and traveled with a speed of approximately 3000 km/sec. Subsequent investigation of sources and data associated with atmospheric ``lights'' included the general area of ball lightning. While most of the reported sightings and probable mechanisms of origin did not seem applicable to SPARKE, the reports of microwave simulation warranted further investigation. Consequently, tests were conducted using a small microwave oven with a carbon source placed within the cavity. The phenomena seen in the oven led to video capture of the activity using the multispectral image intensification video cameras. It then became a task of examining videos of this observed phenomena at very slow speed to find similarities (albeit at much reduced power and speed) to SPARKE. Given that up-scaling the power of the microwave and sizes of the carbon sources is not outside the bounds of reality, the phenomena morphology observed during the microwave experiments may similarly be possible in nature. We thus conclude that it is entirely conceivable that the combination of events necessary to generate SPARKE can be explained with natural events associated with thunderstorm lightning. Details of the experiments and images of the associations will be presented at the conference.
[35] Juan A. Valdivia and Gennady M. Milikh. Gamma ray spectra due to thunderstorms. EOS Supplement, 79(45):F138, November 1998.
[ bib ]

Recent observations of high energy photons (x-rays, gamma-rays),of atmospheric origin, at different altitudes have revealed some interesting phenomena. Measurements include balloons [Eack et al., 1997] as well as satellite observations [Fishman et al., 1994]. These observations may provided us with the possibility of discovering the underlying physical process in the generation of the gamma rays. It is generally accepted that these gamma rays are related to runaway electrons produced by thunderstorms, but there are a number of possible mechanism capable of producing such runaways (Quasi-static fields, electromagnetic pulses, whislers, etc). The objective of this report is to analyze the spatial and temporal evolution of the spectrum of high energy photons produced at different heights and by different initial source distributions. This model, that include photon diffusion (Compton scattering) and photo ionization, can be used to find the location of the runaway source. The different models of the generation of the runaway electrons are used to compute the source spectrum of the high energy photons, which are then compared with observations using the photon propagation model described above. Fishman et al., Science, 264, 1313, 1994.
[36] KB. Eack, DSuszcynsky, WBeasley, WD. Rust, RRoussel-Dupre, and ESymbalisty. A high time-resolution balloon-borned x-ray detector. EOS Supplement, 79(45):F138, November 1998.
[ bib ]

We have designed and flown a balloon-borne x-ray detector with high time resolution to look for the existence of runaway breakdown processes in sprites. A secondary goal is to obtain additional data on x-ray emissions previously observed from inside the thunderstorm, but with greater time resolution. The x-ray spectrometer samples 3 energy bins at a rate of 15kHz. In addition to the x-ray detector, an electric field change instrument was also incorporated into the package and data acquisition system to examine the hypothesis that an electrostatic field change above a thunderstorm complex is associated with sprites. Although this instrument was designed with specific sensors, the data acquisition and telemetry systems are both much more generic allowing other sensors be used with the existing payload design.
[37] MW. Kirkland, DM. Suszcynsky, SO. Knox, RC. Franz, JGuillen, JGreen, and RE. Spalding. Observations of lightning from space using the FORTE photodiode detector. EOS Supplement, 79(45):F142, November 1998.
[ bib ]

We present the initial results obtained from observations of optical lightning activity as observed by the Photodiode Detector (PDD) onboard the 800 km-circular, low-earth orbiting Fast On-orbit Recording of Transient Events (FORT) satellite. The PDD is a silicon photodiode sensitive to wavelengths from 0.4 to 1.1 microns, with a peak in the responsivity at 0.85 microns, and having an 80 field-of-view corresponding to a footprint diameter of approximately 1200 km on the Earth's surface. This footprint matches that of the Lightning Location Sensor and the 3-dB footprint of the primary antenna connected to the RF system. In the most frequently employed trigger mode, the PDD captures a 1.9-millisecond time history of transient optical waveforms with 15-microsecond resolution. While optical reflections from spacecraft or ocean surfaces do result in the detection of false events, these false events are easily discriminated from optical events attributed to lightning activity. We examine the PDD data collected since launch and compare our observations to ground-based and aircraft-based observations of optical lightning emissions. We present examples of the data and infer source optical energies and additional propagation path lengths due to clouds.
[38] Stephen O. Knox, Abram R. Jacobson, Paul Argo, and Robert Franz. Correlation of FORTE satellite radio-frequency lightning observations with NLDN stroke reports. EOS Supplement, 79(45):F142, November 1998.
[ bib ]

The FORT satellite has completed a six-month (April - September 1998) focused campaign observing lightning over North America, in order to study the correlation (if any) between radio-frequency emissions and very-low-frequency sferics from lightning. FORT is in a 70* inclination, circular low-Earth orbit and makes several passes per day within radio sight of North America. FORT records discrete records of very-high-frequency (VHF) lightning signatures. A record typically lasts 400 microsec and is triggered by a multichannel coincidence trigger. FORT carries a GPS clock and subsecond counter allowing 1-ms absolute timing of registered waveforms. On the order of ten-thousand such waveforms can be captured and downloaded per day. We are particularly interested in the time relationship between FORT VHF records and National Lightning Detection Network (NLDN) stroke times. We correct the FORT time-stamp back to the putative source (indicated by NLDN stroke report rendered with typically <50-microsec uncertainty), and construct a histogram of NLDN - corrected FORT timestamps. We had expected this histogram to reveal a preponderance of FORT triggers preceding (by milliseconds to tens of milliseconds) a cloud-ground stroke. Instead, we find that the dominant (and perhaps only) correlation is prompt. That is, to within our methodological uncertainty of 100 microsec, when a VHF burst seen by FORT is detectably coincident with an associated NLDN-reported stroke, then the temporal coincidence is indistinguishable from being prompt. We will discuss these issues as well as what we mean by detectably coincident. We will also show the characteristics of the promptly-coincident VHF bursts and show to what extent they differ from VHF bursts which are not associated with sferics.
[39] Paul E. Argo, Abram Jacobson, Matt Kirkland, and Robert Massey. Initial comparisons of optical and radio frequency satellite observations of thunderstorms with ground base sferics. EOS Supplement, 79(45):F142, November 1998.
[ bib ]

The FORTE satellite, launched 29 August 1997, carries radio-frequency-receivers for the study of lightning, as well as fast photodiode radiometer and CCD imaging detectors. In the first year of operations we have detected more than 2 million radio-frequency events, and more than one million optical events we believe to be associated with lightning. Over the continental United States we have used both a Los Alamos based sferics array and the National Lightning Detection Network data to provide ground truths for our measurements. The results are reminiscent of the ancient story about the blind men feeling the elephant. This is in part due to the fact that each of the detection methods is sensitive to emissions created during different parts of the entire lightning process. We will discuss this, and use the multi-view perspective to help understand more of the storm emission processes.
[40] Kyle C. Wiens, Robert S. Massey, Xuan-Min Shao, Marc H. Eberle, and Kenneth B. Eack. The los alamos electric-field-change sensor array. EOS Supplement, 79(45):F142, November 1998.
[ bib ]

We have deployed an array of electric-field-change sensors, located in New Mexico, to help identify the lightning processes responsible for the VHF RF signals detected by the FORTE satellite. The sensors are located at Los Alamos, Socorro, Roswell, and Tucumcari. Each station consists of a flat-plate antenna followed by an integrating amplifier and a line driver, whose output is recorded by a 1 MS/s 12-bit digitizer in a PC. The time constant of the integrator is 1 ms, and waveforms are recorded for 8 or 16 ms, with 50 samples, respectively. The trigger circuit is bipolar, and the trigger levels can be controlled by the PC. A GPS receiver and timing card within the PC allow time-tagging of the trigger time to two microsecond accuracy. When taking 8-ms records, the system can trigger on and record events separated by as little as 30 ms for periods of several seconds. The hard disk has a capacity of 2 GB, which is well over 100,000 events. The PCs run the Linux operating system, and data and control links to Los Alamos are via the internet. We currently download waveform data for events that are reported (within a 1.1 ms coincidence time) by three or more stations. Event locations are computed with a simple time-of-arrival algorithm, and are found to agree reasonably well with locations of events observed by the National Lightning Detection Network (NLDN). The array has been operational since mid-May 1998, with very little down-time. On active days, stations may record 30,000 or more events, with well over 5000 events reported by three or more stations. Data from the array will be reported in this and other papers in this session.
[41] Xuan-Min Shao, David A. Smith, Kenneth B. Eack, Robert S. Massey, and Kyle C. Wiens. Observations of compact intracloud discharges (CIDs). EOS Supplement, 79(45):F143, November 1998.
[ bib ]

Data from a number of resources were used to study thunderstorms and thunderstorm radio emissions that occurred in the southwestern US between May and September of 1998. These resources included the Los Alamos sferic array, instrumentation on-board the FORT satellite, the National Lightning Detection Network, and pertinent NEXRAD weather surveillance radars. The two former resources are described in detail in other papers of this session. A primary goal of our campaign was to study compact intracloud discharges (CIDs), unique and isolated thunderstorm events that have been identified as sources of narrow positive and narrow negative bipolar electric field change pulses (NPBPs and NNBPs), subionospheric pulse pairs (SIPPs), and transionospheric pulse pairs (TIPPs). CIDs are powerful, singular discharges that occur in regions of high electric field strength and high charge density. Their physical characteristics are described in detail in another paper of this session. Many hundreds of CIDs were observed by the sferic array during the summer of 1998 and were located using methods of differential time of arrival. Distances from the discharges to stations of the array ranged from just a few kilometers to many hundreds of kilometers. Source heights were determined using relative delays between CID groundwave signals and reflections from the earth and ionosphere. In some cases, FORT data were also used to determine 3-D source locations. CID locations have been compared to those of other thunderstorm electrical discharges and to images of radar reflectivity. The events occur in localized regions of thunderstorms that also produce regular intracloud and cloud-to-ground lightning. Radar reflectivity data show that these localized regions correspond to thunderstorm cores where reflectivities in excess of 40 dBZ exist. Further analysis of data from this summer should provide insight into the following questions: Do CIDs of opposite polarity occur at different altitudes within thunderstorms? Do CIDs occur only in regions of high reflectivity? Do all regions of high reflectivity produce CIDs? What are the temporal and spatial relationships between CIDs and other intracloud and cloud-to-ground thunderstorm electrical discharges? Can phases of thunderstorm or different storm types be identified as being more or less likely to produce the events?
[42] EPhilip Krider, Launa M. Maier, Martin J. Murphy, and Donal W. Schibert. The onset of electrification in florida thunderstorms. EOS Supplement, 79(45):F143, November 1998.
[ bib ]

The NASA Kennedy Space Center (KSC) and USAF Eastern Range (ER) currently operate a network of 31 electric field mills (the LPLWS system), 7 VHF pulse time-of-arrival receivers (the LDAR system), and a 5-station network of gated, wideband magnetic direction finders (the CGLSS system) to identify possible lightning hazards to launches and to a variety of ground operations. The field mill data can be used to detect the onset of cloud electrification and also to determine the locations of any lightning-caused changes in the thundercloud charge distribution. Previous reports have shown that intracloud (IC) flashes tend to produce two clusters of LDAR sources in convective storms, one typically at 7-8 km altitude and the other at 9-10 km. The field mill data show that the upper cluster corresponds to a region of positive charge and the lower cluster to a region of negative charge. The LDAR sources in cloud-to-ground (CG) flashes tend to begin at or just below the lower edge of the negative region and propagate downward toward a lower positive charge region that is near the 0C level. During the onset of electrification, the lower positive charge center usually appears first in the electric field records that are close to isolated cells, i.e., the electric field is directed downward. Later, the field reverses polarity, and the effects of the negative charge at higher altitude dominate the field pattern under the storm. The implication of these results for thunderstorm electrification and lightning warnings will be discussed.
[43] DR. MacGorman, MStolzenburg, WD. Rust, TMarshall, PKrehbiel, RThomas, WRison, WBeasley, KEack, and MLockwood. Electric field profiles and lightning in storms with frequent positive cloud-to-ground lightning. EOS Supplement, 79(45):F143, November 1998.
[ bib ]

Eight balloon soundings of the electric field have been obtained in severe storms in which the majority of ground flashes lowered positive charge to ground, instead of the more usual negative charge. These storms occurred in Oklahoma, Texas, and Kansas on four different days. Two soundings were obtained in the strong convection of different isolated supercell storms on one day, and one was obtained completely through the anvil of yet another isolated storm on a second day. Two soundings were obtained in the convective line of each of two severe squall lines that produced mostly positive ground flashes. The remaining sounding was in the transition region between the convective line and stratiform region of one of these squall lines. Preliminary analyses of the soundings are compared with published soundings from storms that produced primarily negative ground flashes. Cloud-to-ground lightning data from the U.S. National Lightning Detection Network have been analyzed for one of the isolated storms. The evolution of positive ground flash rates relative to both radar-derived storm parameters and severe weather was similar to that observed by previous studies of isolated severe storms having frequent positive ground flashes. Lightning also was mapped in three dimensions for one of the isolated storms in which a sounding was obtained and for several other severe storms that occurred in central Oklahoma during field operations. Initial analysis of mapping data for some positive ground flashes shows that they were bilevel hybrid flashes, each flash having two layers of extensive horizontal intracloud structure. The layers are inferred to correspond respectively to upper positive charge and lower negative charge in the thunderstorm charge distribution. Later in the storm, bilevel intracloud flashes appeared to occur in an inverted charge distribution, with upper channels propagating through negative charge and lower channels propagating through positive charge. Cloud flash rates in Oklahoma storms typically were much larger than ground flash rates, particularly during periods when the storms produced severe weather.
[44] Ronald J. Thomas, William Rison, Timothy Hamlin, Jeremiah Harlin, and Paul R. Krehbiel. 3-dimensional lightning observations in central new mexico. EOS Supplement, 79(45):F143, November 1998.
[ bib ]

Following its initial operation in Oklahoma, Tech's 3-dimensional lightning mapping system was set up in August of 1998 over a 60-km diameter area around Langmuir Laboratory and the New Mexico Tech campus in central New Mexico. The results show that lightning activity in New Mexico storms has a simpler structure and smaller extent than in the larger Great Plains storms. Lightning discharges are also much less frequent in a storm, compared to the nearly continous activity observed in Oklahoma. Intracloud lightning often has only a vertical channel but in more developed storms shows a clear bilevel structure, undoubtedly associated with the main negative and upper positive charge regions of the storm. These results agree with previous observations both in New Mexico and in Florida. A new finding is that cloud-to-ground discharges sometimes show considerable activity in a third region below the negative charge level, presumably associated with lower positive charge. The overall lightning activity in the first storm to be looked at (the storm of August 18) exhibited a trilevel structure, consistent with the tripole model of storm electrification, and appears to map out the basic charge structure of the storm. Similar structures have been observed in the more complex Oklahoma storms. A complete analysis of the August 18 observations is expected to show the evolution of the lightning activity throughout the life cycle of the storm and will be compared to dual-polarization radar observations of storm structure. The relation between the lightning channel levels and the storm charge regions will be investigated at Langmuir Laboratory in the summer of 1999 using balloon-borne electrostatic field measurements by Marshall, Stolzenburg, and Rust.
[45] Masashi Hayakawa, Alexander P. Nickolaenko, Irina G. Kudintseva, Stanislav V. Myand, and Leonid M. Rabinowicz. Natural sub-ionospheric ELF pulses in time domain. EOS Supplement, 79(45):F143, November 1998.
[ bib ]

There are problems when constructing the time domain solution for the natural electromagnetic signals of the extremely low frequency band. Such pulses originate from the powerful lightning strokes and propagate in the spherical Earth-ionosphere guide for long distances. Since the wave attenuation in the lower part of ELF band is small, the pulses multiply circle the Earth's circumference giving rise to the Schumann resonance phenomenon. Recent studies indicate on possible link between the red sprites and the Q-burst pulses observed in the Schumann resonance band (from a few to some tens Hertz frequencies) or with the slow tail atmospherics. The standard way of obtaining such a solution is following. The mode is used in the frequency domain. The zonal harmonic series representation is applied, which is an expansion of the fields over the Legendre polynomials. Then, the fast Fourier transform is applied to obtain the time domain solution. There are two areas on the sphere where the solution in the frequency domain becomes divergent. These are the vicinities of the source and its antipode, where the radio wave focusing occurs. To solve the problem everywhere in the spherical Earth-ionosphere cavity, we construct the solution directly in the time domain. For this purpose we make an analytical Fourier transformation of each term in the zonal harmonic series representation and then compute the time domain fields by direct summing of the series obtained analytically. The main result of the report is following. (i) One fails to describe the field at the point where the source was regardless the time when computing the field in the frequency domain first and then applying a FFT algorithm. The time domain representation obtained allows computing the electromagnetic pulse everywhere in the Earth-ionosphere cavity for t > 0, including those multiply crossed the Earth's circumference.
[46] DD. Sentman. The effects of lightning on the middle and upper atmospheres: Snapshot of a rapidly evolving discipline. EOS Supplement, 79(45):F164, November 1998.
[ bib ]

Interest in the electrical effects of tropospheric lightning on the middle and upper atmospheres took a sudden leap in 1989 with the serendipitous video capture of a cloud-to-stratosphere discharge by R. Franz and colleagues. Shortly thereafter similar signatures were found by W. Boeck and colleagues in space shuttle video tapes. Since these seminal reports appeared, activity in this new field has exploded as other researchers scrambled to determine the nature and extent of this new phenomenon. Beginning in 1993 and continuing to the present, ground and aircraft based video studies have amassed thousands of images of these middle atmospheric apparitions, now generally referred to as sprites, whose luminous flash within the mesosphere, at altitudes of 50-90 km, coincides with a large underlying positive cloud-to-ground lightning discharge. Spectral measurements have identified the emitting atmospheric species of red sprites (N2), and optical signatures of accompanying ionospheric heating have been detected (ELVES). A new, even more elusive and apparently unrelated electrical phenomenon known as a blue jet was also discovered to spurt optically upward directly from the top of a thunderstorm to altitudes of about 40 km (130,000 ft). Measurements of electromagnetic emissions have identified several distinctive signatures associated with sprites: ULF impulses, Q-bursts in the ELF, and slow tails in the VLF frequency bands. Ground-based VLF studies have shown that a certain class of gamma ray bursts observed from earth orbit originate in thunderstorms, and are possibly part of the same process that produces sprites and related phenomena. The focus of current ground and aircraft experiments is to extend optical remote sensing into the ultraviolet and infrared regimes as a way to determine the energy budget of sprites/ELVES/jets, and to study their wide band electromagnetic emission properties. Several satellite initiatives are underway to perform topside studies of these events unimpeded by the terrestrial atmosphere. This talk will present an overview of past and present observations in the rapidly evolving discipline of the effects of lightning on the middle and upper atmospheres, and point out several areas of possible significance of these processes in the larger geophysical system.
[47] Umran S. Inan, Christopher P. Barrington-Leigh, Elizabeth A. Gerken, and Timothy F. Bell. Telescopic imaging of fine structure in sprites. EOS Supplement, 79(45):F164, November 1998.
[ bib ]

Recent theoretical work [Pasko et al., GRL, 25, p. 2123, 1998] has indicated that the sprite optical luminosity as observed in regular video may be constituted by a superposition of a large number of highly structured streamers having fine structure with scale sizes of few meters. To look for the presence of such fine structure, we use a 40-cm wide, ~2-m tall Newtonian telescope with a field-of-view of 0.92^o× 0.7^o (~10-m video resolution at a range of 500-km), boresighted with a wide field-of-view (12^o×9^o) video camera. The rotatable and tiltable mounting base of the telescope provides easy elevation and azimuth control, while aiming of the telescope is facilitated by the known phenomenology of sprites, i.e., that they often tend to occur at approximately the same location for tens of minutes. Both the wide- and narrow-fields-of-view are recorded with image intensified CCD video cameras, with simultaneous recording of the broadband very low frequency sferic signal from causative lightning flashes. The first observations with the telescopic imaging system were conducted from the Langmuir Laboratory annex, located near the 11,000 ft high South Baldy peak, providing clear views of thunderstorms in Kansas, Colorado, Texas, and the north-west coast of Mexico. Preliminary data from the telescope show the presence of a fascinating complex of fine structures within the body of the sprite, clearly indicating the presence of columnar channels of luminosity with lateral extent <10-m. In one case, on July 13, 1998, observing a storm ata range of ~580 km, many tens of multiply-oriented (but mostly vertical) streamer-like structures were simultaneously observed within the field of view, with lateral scales ranging from <10-m to up to 100-m.
[48] HFukunishi, YTakahashi, YWatanabe, AUchida, MSato, and WA. Lyons. Frequent occurrences of elves discovered by array photometer observations. EOS Supplement, 79(45):F164, November 1998.
[ bib ]

Fukunishi et al.[1996] found that Elves occur just after the onset of sferics but preceding to sprites. However, during the SPRITES'98 campaign carried out at Yucca Ridge Field Station, Colorado in July and August 1998, we discovered that elves occur very frequently without sprites. For example, we identified 52 elves events without sprites in 6 hours during the night on August 1-2, 1998. Since the duration of elves is about 1 ms, it is difficult to identify them on video images except for unusually bright events. However, the identification of occurrences of elves is easy by using array photometer data since elves always show time delays with decreasing elevation angles, but such time delays are not observed for cloud flashes. We used two sets of 16-channel array photometers set up in vertical and horizontal, respectively. Each array photometer has individual fields-of-view of 0.7 x 11 degrees and the total field-of view of 11 x 11 degrees and the sampling time of each channel is 50 microseconds. Several important features were found. First, elves are excited not only by positive cloud-to-ground lightning discharges (CGs) but also by negative CGs. Second, the durations of optical flashes measured by the vertical array photometer are 200-500 microseconds for individual channels and the onset of flashes delays with decreasing elevation (time delay from top to bottom is 300-600 microseconds). Third, the durations of optical flashes measured by the horizontal array photometer are 300-900 microseconds and the onset of optical flashes is simultaneous with the almost same luminosity variations at all channels. The observed features of elves are consistent with a model proposed by Inan et al. [1996] in which the optical emissions are produced as a result of heating by electromagnetic pulses (EMPs) from lightning discharges. The frequent occurrences of elves suggest that lightning-induced EMPs are playing an important role for heating of the lower ionosphere.
[49] Christopher P. Barrington-Leigh, Umran S. Inan, and Elizabeth A. Gerken. Temporal and spatial structuve of sprites and elves: Photometric and CCD observations. EOS Supplement, 79(45):F164, November 1998.
[ bib ]

Data acquired with a high-speed photometric array (the Fly's Eye) combined with high-resolution CCD imaging and Very Low Frequency sferic recordings allow the precise measurement of the onset delay, development, and duration of red sprites and elves. The two complimentary optical instruments provide the first simultaneous documentation of the spatial fine structure and submillisecond temporal characteristics of sprites and elves. Elves are unambiguously identified by means of their rapid lateral expansion as observed with the Fly's Eye and are clearly produced by both positive and negative cloud-to-ground flashes. Bright sprites are found to have typical durations of 30-100 ms, and the altitude dependence of their observed structure and optical properties is discussed. Elves and sprites are detected in both blue and red photometers, and preliminary analysis indicates that the ratio of blue and red emission intensities is consistent with that expected for air breakdown at high altitudes.
[50] DD. Sentman, EM. Wescott, JWinick, CSiefring, JMorrill, DBaker, PBernhardt, MHeavner, DMoudry, DOsborne, JDesrochers, LPeticolas, and VBesser. The EXL98 sprites campaign. EOS Supplement, 79(45):F164, November 1998.
[ bib ]

The EXL98 (Energetics of Upper Atmospheric Excitation by Lightning, 1998) project was a joint effort by the University of Alaska, Air Force Research Laboratory, and Naval Research Laboratory to study the energy budget of sprites, jets and ELVES using jet aircraft and ground remote sensing methods. The EXL98 field campaign was conducted from Colorado and Wyoming during July, 1998, with aircraft flights targeted on nocturnal thunderstorm systems in the Middle and Upper Plains states. Simultaneous sprite observations were obtained from the airborne imaging platform carrying a suite of eight low light level television (LLTV) camera systems, and at two ground locations equipped with LLTV, a spectrograph, and high speed photometers. Campaign and flight mission planning were carried out at the Laboratory for Atmospheric and Space Physics (LASP) at the University of Colorado. The measurements focused on low light level imaging across a wide range of wavelengths, including the near ultraviolet (NUV), visible, near infrared (NIR), and medium infrared (MIR). This talk will review the science rationale for the EXL98 project, discuss aircraft and ground resources utilized during the campaign, and present preliminary results.
[51] Jeffrey H. Bowles, Carl L. Siefring, Jeff S. Morrill, Paul A. Bernahrdt, Davis D. Sentman, Dana R. Moudry, Eric J. Bucsela, Daniel L. Osborne, Eugene M. Wescott, and Matthew J. Heavner. Do sprites sometimes connect to the cloud tops? EOS Supplement, 79(45):F164, November 1998.
[ bib ]

In this paper we present exciting results from the EXL98 mission which indicate that Sprites may occasionally connect from the ionosphere all the way down to the cloud tops. During the EXL98 mission one of the major goals was to obtain Near-UltraViolet (NUV) and Near-InfraRed (NIR) measurements of Sprites. Because of atmospheric absorption and scattering, to fulfill these goals it was necessary to fly the plane close to the storms (100-200km). As a result we obtained many images which view from the horizon (cloud tops) into the tendril portion of the Sprites. The NRL NUV camera, running without a filter, observed two cases where it appeared that a secondary breakdown started from near the horizon and propagated upward toward the remnants of a Sprite. The UAF narrow field visible camera was often saturated by scattered light from the lightning flash in this area. However, on a number of occasions this camera showed 'fingers' extending from the horizon to the bottom of the Sprites as it came out of saturation. One of these cases corresponds to the observations made with the NUV camera of a possible secondary breakdown following the Sprite by several video frames. The evidence for a 'direct' connection from a Sprite to the cloud tops is not conclusive, but the possibility is extremely important. We hope these results will spur further experimental and theoretical investigations into this possibility. *The work at NRL was sponsored by NASA and ONR.

[52] Veronika Besser, Dana R. Moudry, Matt J. Heavner, Davis D. Sentman, Eugene M. Wescott, Jeff S. Morrill, Carl Siefring, Daniel L. Osborne, and Don L. Hampton. Case study of a sprite. EOS Supplement, 79(45):F165, November 1998.
[ bib ]

On July 22, 1998, at 04:57:43 UT, a sprite over central Iowa was imaged from the EXL98 aircraft from a very close range. The sprite was captured in six cameras: a wide field (approx. 60^o field of view (FOV)) monochrome camera, and five narrow field cameras (approx. 16^o FOV): a monochrome camera, an intensified color camera, a camera filtered at 427.8 nm (FWHM 1.6 nm), a near-ultraviolet (340 nm) camera, and a near-infrared (0.8-1.7 mum) camera. The entire sprite was captured in the wide field camera, the other cameras imaged only the tendrils.
In addition, approximately 100 ms after the onset of the sprite, spike or finger-like features were seen propagating upward from the cloudtops towards the lower portions of the tendrils of the sprite, stopping short of reaching them. This is the first time this feature has been reported.
[53] Victor P. Pasko. Theoretical models of sprites. EOS Supplement, 79(45):F165, November 1998.
[ bib ]

Spectacular luminous optical emissions appearing in clear air above thunderstorms and known as sprites provide dramatic evidence of strong electrodynamic coupling between tropospheric lightning discharges and the mesosphere/lower ionosphere. In this talk we will provide a review of existing interaction and coupling mechanisms proposed for the explanation of experimentally observed features of sprites (i.e., their spatial structure, optical spectra and time dynamics). We will discuss the conventional as well as the relativistic runaway breakdown processes stimulated at mesospheric altitudes by large quasi-electroctatic field transients following intense positive cloud to ground lightning discharges, and the role of these two breakdown mechanisms in sprite production. We will also discuss the properties of sprites on a variety of spatial scales, including their fine spatial structure (~1 m), as well as their contribution to the large scale atmospheric electric circuit (scales >~100 km).
[54] Richard F. Fernsler and Harvey L. Rowland. Theory and experiment for sprites and elves. EOS Supplement, 79(45):F165, November 1998.
[ bib ]

Analytic models were previously given [1] for elves and red sprites produced in the upper atmosphere, and recent experiments provide support for the model predictions. The underlying criterion is gas breakdown from the electric fields induced by lightning discharges, and results were given for both vertical and horizontal discharges. The radiation field from a lightning return stroke was found to be capable of producing flat, thin, short-lived elves just below the ionosphere, provided the return-stroke current exceeds  50 kA. Similarly, the quasi-static field from a continuing current was found to be capable of producing red sprites provided the charge moment (charge times height of charge) exceeds  150 Coulomb-km. In addition, red sprites are predicted to elongate and filament as they propagate down in altitude via enhancement of the field at the tip. Similar enhancement does not occur for the radiation fields producing elves. In this talk the two models are briefly reviewed and then compared with experiment. Work supported by NASA and ONR. 1. J. Geophys. Res. 101, 29653 (1996).
[55] Russell A. Armstrong, David M. Suszcynsky, Robert Strabley, Walter A. Lyons, and Thomas Nelson. Simultaneous multi-color photometric and video recording of Sprites and their parent lightning mechanisms and energy deposition. EOS Supplement, 79(45):F165, November 1998.
[ bib ]

We have obtained new multi-color photometry data simultaneously with blue, red and high-resolution broad-band video imagery of sprite events which indicates that more than one mechanism can cause sprites. The time-resolved emissions from the nitrogen first-positive, second positive and first negative systems indicate different time histories. We have found that the relative time histories are not unique from event-to-event but generally fall into two (or perhaps three) categories. We find that the time histories can be interpreted to yield both the ionization level and the energy accommodation. Data has been obtained which confirms ionization in sprites varying from very strongly ionized to relatively weakly ionized. The simultaneous red and blue video images associated with the sprite events indicates that columnar sprites exhibit different energetic profiles than carrots. We will present the systematic analysis of the time-histories of the photometric emission behavior of the sprite events, correlate them with the corresponding imagery and discuss implications for the electrodynamic modeling of the phenomena.
[56] Mike J. Taylor and Larry C. Gardner. Simultaneous red and blue imaging of sprites during the yucca ridge 1998 campain. EOS Supplement, 79(45):F165, November 1998.
[ bib ]

As part of the Sprite-98 campaign conducted at Yucca Ridge, CO two intensified Image Isocon cameras and a bare CCD imager were used to investigate the spectral morphology of Sprites. The CCD and one Isocon camera were filtered to observe the nitrogen first positive band emission at 665 nm while the second Isocon camera was used to detect any blue emissions originating at  400 nm. A fourth, Xyibon, camera was unfiltered and used to provide reference image data. All four cameras were co-aligned and mounted on a motorized alt-azimuth tripod providing accurate pointing information. Joint observations were conducted during a one month period from 8 July to 5 August, and despite the limited storm conditions Sprites were imaged successfully on several nights in the red emission but only occasionally were they detected simultaneously at blue wavelengths. An initial comparative analysis of the red and blue Sprite signatures and their associated lightning discharges will be presented together with an unusual observation of light apparently propagating up from an isolated Sprite and re-illuminating a meteor trail.
[57] MJ. Heavner, DR. Moudry, DD. Sentman, EM. Wescott, JS. Morrill, CSiefring, EJ. Bucsela, DL. Osborne, JT. Desrochers, HStenbaek-Nielsen, JWinick, JKristl, THudson, LM. Peticolas, and VBesser. Ionization in sprites. EOS Supplement, 79(45):F165, November 1998.
[ bib ]

The EXL98 sprite observing campaign of July 1998 involved multiple cameras across the wavelength range 300 nm to 5 microns. A filter at 427.8 nm was used on one camera to detect emissions from the first negative group of molecular nitrogen, which would be a direct observation of the amount of ionization. The filter was carefully selected to reject 426.8 nm light and any other second positive (non-ionized) molecular nitrogen emissions which have made previous blue filtered observations of sprites difficult to interpret with regards to the ionization question. The paper will present observations made with the filtered camera and simultaneous EXL98 aircraft images as evidence for the observation of ionization. The evidence for ionization will be discussed in the context of previous observational campaigns.
[58] Keith M. Groves, John M. Quinn, ERussell Shinn, Peter Ning, and Matthew R. Cox. An upper limit on ionization in sprites. EOS Supplement, 79(45):F165, November 1998.
[ bib ]

During the past few years, much progress has been made towards characterizing the morphology of red sprites and understanding the source mechanisms leading to their occurrence. A combination of optical measurements and theoretical calculations have been applied to determine the levels of ionization produced in sprite discharges, but no definitive results have yet been produced. During July-August 1996 we collected radio frequency (RF) measurements of lightning-induced atmospheric effects in northeastern Colorado using a 28 MHz coherent backscater radar. Such a radar serves as a powerful diagnostic of electron density. A CCD-intensified video system was co-located with the radar to verify the occurrence of sprites within the radar beam. The combined data sets were compared with CG lightning events as recorded by the National Lightning Detection Network to correlate RF signatures with individual lightning strikes. More than 200 sprite events were seen simultaneously with the radar and the optical system; numerous more events were observed with the radar in the absence of good optical conditions. No echoes associated with sprite occurrence were observed. An analysis of the radar sensitivity and a consideration of various scattering models enable one to place an upper bound on the levels of ionization present in red sprites. The resulting ionization values are strongly dependent on the assumed structure and extent of the scattering region. High resolution optical measurements and theoretical analyses are needed to constrain the scattering calculations further. Additionally, a number of uncorrelated radar echoes were observed during the campaign, sometimes in conjunction with thunderstorm activity. The significance of these echoes and their origin will be discussed.
[59] Gary R. Swenson and Rick L. Rairden. What is the source of the sprite seed electrons? EOS Supplement, 79(45):F165, November 1998.
[ bib ]

It is clear from spectral measurements of the N2 1PG vibrational distribution originatin from Sprites, that the energy distribution of electrons responsible, had a mean of  1 eV. This energy is orders of magnitude colder than auroral secondary electrons. This cold source cannot be explained by normally produced secondaries from electron induced ionization of local molecules whose ionization thresholds are typically >15 eV. A mesospheric source of low energy electrons can originate from negative ions, where electron release energies are typically .5-1.5 eV. It will be shown that measured brightnesses of Sprite cores are consistent with climatological average negative ion densities available. The remaining question is to explain the souce of 'seed' electrons, which when exposed to the broad electric field, accelerate to remove the electrons from the parent molecules.
[60] Bryon David Green, Terry Rawlins, and Russ Armstrong. Molecular excitation as a probe of sprite mechanisms. EOS Supplement, 79(45):F175, November 1998.
[ bib ]

REGULAR MAIL
[61] Jeremy R. Winick, Joseph A. Kristl, Thomas Hudson, Davis D. Sentman, Eugene M. Wescott, Laila Jeong, Carl Siefring, and Daniel Osborne. Do sprites produce infrared emission? preliminary results from EXL98. EOS Supplement, 79(45):F175, November 1998.
[ bib ]

We present the preliminary results of measurements taken by MWIR filtered video cameras from high altitude aircraft flights during the EXL98 campaign. AFRL and USU Stewart Radiance Laboratory provided video cameras filtered for the 2.8 mum and 4.3 mum bands. These bands are enhanced in aurora as a result of emission from chemiluminscently produced NO(v) (Deltav=2 at 2.7-3.0 mum) and CO_2 (Deltav=1 at 4.3 mum) produced from near-resonanat vibrational energy transfer from N_2(v). These processes are initiated primarily by secondary electrons. In sprites we anticipate most electrons are heated by discharge electric field as opposed to being secondaries from ionization processes. Observations of copious N_2 first positive emission in sprites indicates that electrons of greater than 7 eV are present. Since the cross section for production of N_2(v) is very large for electrons of 2-5 eV,enhancement of CO_2 4.3mum is likely. Formation of NO(v>=2) depends upon higher energy electrons that dissociate and/or ionize N_2, and its formation in sprites is less certain. However, NO(v>=2) emission is a useful indicator of NO formation and can tell us whether sprites have any photochemical impact in the region of the mesosphere where the NO density is near its minimum. In light of these observations we will examine the mplications for IR radiance for space-based limb viewing geometry in which case the background emission, atmospheric transmission, and field-of-view limitations are less severe. We will also discuss the similarities and differences between aurora mechanisms and likely sprite mechanisms.
[62] Paul A. Bernhardt, Carl L. Siefring, Jeff S. Morrill, Davis D. Sentman, Eugene M. Wescott, Matthew J. Heavner, Daniel L. Osborne, and Eric J. Bucsela. Near-infrared (NIR) measurements during the EXL98 campaign. EOS Supplement, 79(45):F175, November 1998.
[ bib ]

A NIR digital/video camera operating in the spectral range between 900 and 1700 nm was flown on the EXL98 missions. This camera had two functions. First, it was used to monitor nighttime airglow which gives indications of density structures in the neutral atmosphere. The camera uses a relatively new technology Indium-Gallium Arsinide (INGAAS) focal plane array which is sensitive in the NIR. It offers many advantages over similar measurements in the visible because of its sensitivity and the relative brightness of the airglow in the NIR. In fact, from the EXL98 airborne platform it was possible to monitor airglow modulations at the full 30 frame/sec video rate. Second, was to search for excitation of N2 first positive emissions in the NIR by Sprites. The NIR experiment was extremely successful - observing both the first Sprite related NIR emissions and the first correlation between structures in the neutral density and visible and NIR Sprite emissions. In addition, a number of cases were observed where features in the background airglow appeared to be related to structures in the thunderstorm clouds. In this paper we will present an overview of these measurements. *The work at NRL was sponsored by NASA and ONR.
[63] EJ. Bucsela, JS. Morrill, CSiefring, MJ. Heavner, DR. Moudry, DD. Sentman, EM. Wescott, DL. Osborne, and WM. Benesch. Estimating electron energies in sprites from 1NG/2PG intensity ratios. EOS Supplement, 79(45):F175, November 1998.
[ bib ]

As part of the EXL 98 mission which observed sprites from an aircraft, video images were taken with narrow passband filters at 4278Å and 3407Å . These passbands correspond to the N_2
+ 1NG (1,0) and N2 2PG (0,0) bands, respectively. These filters were designed specifically to have very narrow passbands in order to exclude nearby contaminating emissions. This set of image data will be used to generate intensity ratios of the above spectral features, specifically 1NG(1,0)/2PG(0,0) which can, in turn, be related to the population ratio of the N2
^+(B) and N_2(C) states. By making assumptions about the electron distribution we can estimate the electron energies implied by the observed intensity ratios. In this presentation, we will discuss the details of both the method of estimating energies and the observing techniques. Preliminary results from the EXL 98 mission will be discussed.
[64] Yukihiro Takahashi, Yoshiaki Watanabe, Akihiro Uchida, Masaaki Sera, Mitsuteru Sato, and Hiroshi Fukunishi. Energy distributions of electrons exciting sprites and elves inferred from the fast array photometer observations. EOS Supplement, 79(45):F175, November 1998.
[ bib ]

Recent spectral observations and numerical model calculations have suggested that the 1st and 2nd positive bands of N_2 are dominant emissions in sprites and elves, although it has been also reported that the sprite emission contains the 1st negative band of nitrogen molecule ions (N_2
+). The intensity ratio of 1st and 2nd positive bands of N2 gives information on the energy distribution of electrons which excite nitrogen molecules through collision processes. In order to investigate the spectral characteristics of sprites and elves we employed two multi-anode array photometers (MAPs) in the SPRITES '98 campaign. One of the MAPs with a shrap cut filter measures the 1st positive band in the range 560-850 nm, while the other with a blue filter measures the 2nd positive band in the range 350-450 nm. Each photometer has 16 channels with individual fields-of-view of 0.7 ×11 and samples data at a rate of 20 kHz. During the campaign, we observed 22 sprites and analyzed the ratio of 1st and 2nd positive bands in detail for 7 events of them. It was found that the 2nd positive band of N2 was remarkably enhanced at the initial phase of columnar sprites. We will discuss the temporal and spatial changes of electron energy distributions quantitavely for both sprites and elves based on these MAP data.
[65] Edgar A. Bering, James R. Benbrook, Eugene M. Wescott, Davis D. Sentman, Hans C. Stenbaek-Nielsen, and Walt A. Lyons. The SPRITES 99 balloon campaign. EOS Supplement, 79(45):F176, November 1998.
[ bib ]

There are several competing models for the production of sprites, jets and elves. It has become clear it is not possible to select between these models using only ground-based data, owing to the fact that the ground shorts out the field signatures of interest. Consequently, a balloon campaign was proposed and funded. The campaign, which was originally scheduled for July, 1998, has been rescheduled for July and August of 1999. The 1999 Sprite Balloon Campaign will attempt to conduct six high altitude balloon flights from Ottumwa, Iowa, during the dark of the moon in the summer of 1999. Nominal campaign dates include July 6-21, and August 4-19, with lower priority fall-backs of June 7-21 and Sept. 3-17. The balloons will be launched at sunset, float at a nominally constant altitude of 32 km and drift westward at a predicted speed of 40 knots. Cutdown will occur after sunrise in central Colorado. Balloon tracking will be made available in real-time on redundant Web sites. The balloon payloads will be instrumented with dual three axis electric field detectors, three axis fluxgate and induction magnetometers, X-ray scintillation counter, Geiger-Mueller tube, upward looking high-speed photometer, vertical current density ammeter, conductivity measurements, and an ambient temperature thermometer. A multiply redundant telemetry scheme will give us five orders of magnitude of dynamic range in field amplitude sensitivity. The use of event triggered on-board memory will allow us to sample 8 quantities at a digitization rate of at least 50 kHz per channel during sprite events. The efficiency of the on-board event trigger will be checked by using 4 broad-band analog waveform channels. Ground observations may include low light level TV observations from as many as three sites: tentatively, WIRO, on Jelm Mtn., Wyoming, Mt Evans, Colorado, and Yucca Ridge, Colorado. At least one of these sites will also have a fast photometer. Other topics that will be covered include: why the program was postponed and the trigger criteria for the on-board fast memory.
[66] Jyh-Long Chern, Lou-Chuang Lee, Kuan-Ren Chen, Rue-Ron Hsu, Han-Taong Su, and Chin-Chun Tsai. ISUAL project: Observations of red sprites on Taiwan's ROCSAT-2. EOS Supplement, 79(45):F176, November 1998.
[ bib ]

A satellite observation mission of red sprites has been planned on ROCSAT-2, which is to be launched in June 2002. ROCSAT-2, the second scientific satellite of Taiwan, is a low-earth orbit sun-synchronous satellite, with an altitude 891km and inclination angle 98.9. The scientific payload, ISUAL (Image of Sprite; Upper Atmospheric Lightning), will contain a two-dimensional CCD imager, 6 narrow-band photometers, and 2 photometer arrays. The ISUAL imager is a limb viewing instrument pointing in a side direction, which is perpendicular to the satellite moving direction and away from the sun. The main objectives are (1) to obtain the global distribution of red sprites, (2) to determine the spatio-temporal dynamics of red sprites, (3) to identify the UV bands, (4) the degree of ionization in the sprite emission region; (5) to observe airglows and auroras. International collaborations have been established and will be further encouraged.
[67] Lee H. Marshall. An overview of electromagnetic measurements relating to sprites, elves and jets. EOS Supplement, 79(45):F176, November 1998.
[ bib ]

During the past several years, numerous measurements have been made of lightning- related middle and upper atmosphere (30-100km) optical phenomenon such as sprites, elves, and jets. The relationship of these events to parent lightning strokes and storms suggests energy coupling processes upward from the lower atmosphere. The large geometry and time-scale of sprites and particularly jets, and comparatively short time-scale of elves suggest energy coupling processes that could be quite different from each other and whose duration may vary over several orders of magnitude. For these reasons, electromagnetic measurements have been made, in conjunction with optical observations, extending from the sub-hertz ULF range, through the widely studied ELF and VLF ranges, the LF and HF ranges and up into the VHF and UHF ranges. This paper will present an overview of the electromagnetic measurements relating to sprites, elves, and jets. Both electric and magnetic field measurements over a wide range of frequencies and distances will be discussed. Also, apparent differences between the theoretical explanations and the empirical observations will be discussed, and the areas greatly in need of additional observation will be outlined.
[68] Timothy F. Bell, Chris Barrington-Leigh, Elizabeth A. Gerken, and Umran S. Inan. Continuing currents, charge transfer, and optical emissions associated with red sprites observed in thunderstorms in Mexico. EOS Supplement, 79(45):F176, November 1998.
[ bib ]

Red Sprites consist of large scale, transient, luminous , predominently red structures in the mesosphere which appear above thunderclouds following positive cloud-to-ground lightning discharges. Over the past few years Sprites have been extensively studied and documented in the midwestern US, where they are very common during the summer months. Although it is well known that Sprites occur in storms at other locations, such as Florida, Australia, the Amazon basin, and Africa, much less is known of the characteristics of these Sprites. During the summer of 1998, Stanford University operated a number of instruments at the Langmuir Laboratory in New Mexico in order to observe Sprites that occurred within   900 km of the laboratory. On a number of evenings vigorous Sprite activity was observed in thunderstorms in northern Mexico. In the present paper we present optical images of the Mexican Sprites in conjuction with calculations of the continuing current and positive charge transfer associated with each Sprite, and discuss the differences between Sprites in the midwestern US and northern Mexico.
[69] E. M. Wescott, D. D. Sentman, H. C. Senback-Nielsen, M. J. Heavner, D. R. Moudry, U. S. Inan, T. F. Bell, and M. Stanley. `columniform' sprites: Their optical characteristics, ELF and VLF signatures, and relationship to lightning. EOS Supplement, 79(45):F176, November 1998.
[ bib ]

This paper summarizes observations since 1995 of a distinctive form of sprites associated with positive CG flashes in mesoscale thunderstorms, and discusses physical mechanisms for their features. We have examined the relationships between the visible forms, the causative lightning current, and the ELF and VLF waveforms. The sprites are characterized by long vertical columns about 10 km long, less than 1 km in diameter, often with several black bands near the bottom. Some show faint tendrils extending above and below the column, with distinctive features. Three dimensional triangulation of what we define as a columniform sprite (c-sprite) event on the evening of June 19, 1995 showed that the individual elements had an average terminal altitude of 86.7 km and an average bottom of 76.2 km. The sprite columns are nearly vertical in video imagery, however some show obvious curvature. On some evenings c-sprites are the dominant form of sprite activity above thunderstorms: On the night of July 17, 1998 we observed 21 examples over a severe storm in New Mexico, some associated with ELVES. Previous work involving comparison of c-sprite forms vs National Lightning Detection Network (NLDN) positive cloud-to-ground current shows a progression from simple, thin vertical forms to brighter and more complicated forms with increasing current. Investigators from New Mexico Tech using high speed TV systems (> 1000 fps) have found that all forms of sprites begin as c-sprites in the first ms, and evolve into the many other forms of sprites. The question that we address in this paper is: What physical processes start c-sprites, and what determines how far they will develop into other more complicated forms?
[70] Steven C. Reising, Umran S. Inan, Timothy F. Bell, Yukhiro Takahashi, and Maasaki Sera. Further evidence of electrical current in sprites using measurements of ELF radio atmospherics with simultaneous high time-resolution multi-anode array photometer observations. EOS Supplement, 79(45):F176, November 1998.
[ bib ]

The magnetic field signature of extremely low frequency (ELF, here defined as 15-1500 Hz) radio atmospherics (``sferics'' for short), originating in positive cloud-to-ground (CG) discharges in a Kansas and Texas panhandle thunderstorm and measured ~500 km away at Yucca Ridge Field Station, Colorado, consists predominantly of a positive-going pulse of ~1 ms width. Of the ELF sferics which were launched by sprite-producing positive CG flashes, roughly 20% exhibit a second ELF pulse, which has been shown to be simultaneous with Sprite luminosity by comparison with high time resolution (~30 mus) photometer measurements [Cummer et al., GRL, 25, 1281, 1998]. The explanation of the second ELF pulse as radiation from electrical currents in sprites is consistent with recent results of theory and simulation [Pasko et al., GRL, in press, 1998]. We present new results of simultaneous ELF measurements of radio atmospherics and multi-anode array photometer (MAP) measurements which demonstrate that the second ELF pulse and sprite luminosity occur simultaneously and that the delay between the causative positive CG and the sprite varies between 1 and 70 ms. In addition, our data set indicates a clear distinction between ``columnar'' sprites, with a delay of <3 ms between CG and sprite, and ``carrot'' sprites, with a delay of >4 ms between CG and sprite. Elves are often observed preceding the occurrence of column sprites. The MAP data measure vertical structure in sprites with ~10 km altitude resolution and provide time series of optical intensity with ~50 mus resolution. Comparison of MAP data with the second ELF pulse allows a preliminary assessment of the altitude distribution of electrical currents in sprites.
[71] L. C. Hale, L. H. Marshall, C. L. Croskey, and J. D. Mitchell. The roles of mesospheric aerosol in sprite-related phenomena. EOS Supplement, 79(45):F176, November 1998.
[ bib ]

[72] Steven A. Cummer, Mark Stanely, Paul Krehbiel, and Marx Brook. Simultaneous ELF-radiating currents and submillisecond sprite development. EOS Supplement, 79(45):F177, November 1998.
[ bib ]

A critical parameter in current sprite theories is the charge transfer in the source lightning discharge. This charge transfer is related to the mesospheric electric field thought to be the driving force in sprite production. A direct comparison of theory and observation requires simultaneous observations of sprite optical emissions and lightning charge transfer. Such a study has proved difficult because of fast time resolution required to image sprite dynamics. Studies of simultaneous ELF radio atmospherics and single-channel photometer data have shown that a peak of ELF radiation is often time correlated with bulk sprite brightness [Cummer et al., GRL, 25, 1281, 1998; Brook et al., EOS, 78, F82, 1997; Bell et al., EOS, 78, F70, 1997] which has been interpreted as the radiation from electric current in the sprite itself. But it remains to be seen how this ELF-radiating sprite current relates to detailed sprite characteristics. To answer both of these questions, we use an ELF propagation model to extract radiating currents from simultaneous ELF radio atmospherics. Combining this with submillisecond time resolution sprite images, we quantitatively determine for individual events the threshold of lightning charge transfer for sprite initiation and the relationship of charge transfer and sprite development. In the same manner, we investigate the relationship between optical sprite characteristics and sprite current flow. These results will be interpreted in the context of existing theories of sprite production.
[73] Mark A. Stanley, Marx Brook, Steve A. Cummer, Chris P. Barrington-Leigh, and Elizabeth A. Gerken. Broadband detection and characterization of day-time sprites and of negative cgs which initiated sprites. EOS Supplement, 79(45):F177, November 1998.
[ bib ]

The optical detection of sprites during the day-time has been impractical due to their low intrinsic brightness. However, the recent discovery of a unique ELF signature due to sprites [Brook et al., EOS, 78, F82, 1997; Bell et al., EOS, 78, F70, 1997; Cummer et al., GRL, 25, 1281, 1998] has made it possible to search for their presence during the day. Several sprite field changes were detected during daylight hours on August 14, 1998 at Langmuir Laboratory, NM. Each of the field changes was delayed from an energetic positive CG by anywhere from 11 to 15 milliseconds. The positive CGs were located by the National Lightning Detection Network (NLDN) within a large storm system at least 2 hours prior to sunset. The current moment for the parent discharge and sprite will be extracted from the waveforms and will be compared with night-time positive CGs and sprites. It will be shown that these sprites were more energetic than those which occur at night. On the night of August 29, 1998, small columniform sprites were detected on video coincident with some energetic negative CG elves. Based on joint photometry and video measurements, the sprites were initiated shortly after the negative CGs and persisted for several milliseconds. The source current moment for the negatives will be determined and compared with that of sprite-producing positive CGs.
[74] J. Valdivia. Intracloud lightning and red sprites. EOS Supplement, 79(45):FNNN, November 1998.
[ bib ]

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