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.

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..

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.

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.

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-Dupré, 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-Dupré, 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).

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.

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.

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.

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.

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.

During the sprite campaign 1998, the Institut für Meteorologie und Geophysik at the Universität 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.

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.

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.

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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.

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.

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.

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.

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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.

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.

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.

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.

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.

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.

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.

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.

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.

Simulations of intra-cloud discharges initiated by runaway air breakdown are presented. Results of new kinetic calculations (E.M.D. Symbalisty, R. Roussel-Dupré, 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.

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.

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.

Upward discharges driven by the runaway air breakdown mechanism (A. V. Gurevich, G. M. Milikh, and R. Roussel-Dupré, 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-Dupré, 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.

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.

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.

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.

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