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a. Research Objectives
One of the most interesting new areas of research in the atmospheric sciences is the topic of visible emissions above thunderstorms. We propose to investigate some of the significant scientific questions that have been raised by the discovery of visible light emissions in the stratosphere and ionosphere above thunderstorms. Visible light manifestations extending above thunderstorms were once thought to be rare. Scattered reports of this phenomenon have appeared for many years [Boys, 1926; Malan, 1937; Wilson, 1956]. These qualitative reports have been compiled in two recent papers [Vaughan and Vonnegut, 1989; Lyons and Williams, 1994]. The introduction of video cameras and high gain photometers has allowed these phenomena to be studied quantitatively for the first time. Interest in ``sprites'' ( D. Sentman, as quoted by Lyons, [1994a]) was stimulated by an initial report of photometric and video observations of flashes in the ionosphere above active thunderstorms [ Franz et al., 1990]. A substantial number of papers has followed this report, and the phenomena are now thought to be a nearly ubiquitous manifestation of large storm systems. In fact, three or perhaps more phenomena appear to have been observed for which a number of names have been coined: red sprites, blue jets, elves and Anomalous Optical Events. The fanciful tenor of the names is the consequence of a deliberate attempt to avoid picking names that imply specific physical mechanisms.
The term red sprite refers to extended visible light emissions occurring in the mesosphere and ionosphere above active thunderstorms [Vaughan et al., 1992; Lyons and Williams, 1994; Winckler et al., 1993; Sentman and Wescott, 1993; Sentman et al., 1995; Rairden and Mende, 1995]. The sprite is a mesospheric and ionospheric phenomenon that occurs in the 40-90 km altitude range. The average maximum altitude is 88 km and the altitude of maximum brightness is 66 km. The upper portion is red, with wispy, faint blue tendrils extending down to 40 km. Recent photogrammetry has seen some tendrils as low as 30 km. Sprites have a horizontal extent of 10's of km. Sprites exist for several ms, with a maximum lifetime of 17 ms. The spectrum consists almost entirely of the first positive band of N2, with no evidence of N2+ Meinel band emissions, which strongly suggests that the emissions are excited by electrons with energies between 7.35 and 16.73 eV [ Mende et al., 1995; Hampton et al., 1996]. Evidence is accumulating that sprites are produced in association with large positive cloud-to-ground (CG) lightning strokes generated in the trailing stratiform region of large mesoscale convection complexes [ Boccippio et al., 1995; Marshall et al., 1996]. A minimum storm area of 2. x 105 km2 appears to be a necessary condition for sprite occurrence.
The term blue jet refers to blue colored emissions that appear to shoot upward from the tops of thunderstorms to altitudes of about 40 km, propagating at speeds of ~100 km/s and lasting for ~0.2 s [ Wescott et al., 1994, 1995]. Blue jets are not directly associated with any CG strokes, and appear to originate from the central tower of large storms rather than the trailing stratiform region.
The last two terms, elves and AOE's refer to transient brightening of the airglow layer in the ionosphere. These events are known as Anomalous Optical Events (AOE's) in the rocket literature and ``elves" to ground based observers [ Li et al., 1991; Boeck et al., 1992; Rodriguez et al., 1992b; Fukunishi et al., 1995, 1996]. These two phenomena may, in fact represent two distinct types of event, since AOE's appear to have a substantially longer lifetime than elves, which are a millisecond phenomenon.
The most immediate question that one must ask in studying sprites, jets and elves, after one has determined that they exist, is the question of the excitation mechanism for the emissions. The visible emissions associated with tropospheric lightning are produced thermally, by Joule heating of the lightning channel by the currents passing along it. The case of sprites may be very different. Since the air pressure at the 40-90 km altitude where these flashes are observed is very low, there are other mechanisms for producing visible light in addition to thermal excitation by DC current. Models that have been proposed to explain sprite emission include heating by electromagnetic pulses from lightning [ Inan et al., 1991], which can heat electrons enough to cause significant secondary ionization [ Taranenko et al., 1992, 1993; Rodriguez et al., 1992a; Rowland et al., 1994; Inan et al., 1993, 1996b]. In addition, the radiation electric field that is emitted upward toward the ionosphere by a CG stroke can be much larger than it is nearer the ground, owing to the large propagation speed of the current pulse wavefront in a CG stroke and the fact that the electromagnetic potentials of a moving charge (the Liénard-Wiechert potentials) are functions of retarded time (that is, include the effect of the finite speed of light) [ Krider, 1992, 1994; Milikh et al., 1994]. Thus, the radiation electric field that is emitted upward toward the ionosphere by a cloud-to ground (CG) lightning stroke could well be large enough to produce an ac breakdown discharge in the ionosphere above a thunderstorm without transferring any DC current from the storm. Direct excitation by large quasi-electrostatic fields is another model that has been proposed, which predicts very different EM field signatures at altitudes above cloud tops [Wilson, 1920, 1925; Pasko et al., 1995, 1996b; Winckler et al., 1996; Boccippio et al., 1995; Marshall et al., 1996]. Finally, several authors have suggested that runaway electron processes may be producing the emissions [ Bell et al., 1995; Winckler et al., 1996; Taranenko and Roussel-Dupre, 1995]. Models for blue jets have suggested that they are types of streamer discharge [Vitello et al., 1995] that form above highly charged thunderclouds and propagate upward [ Pasko et al., 1996a; Sukhorukov et al., 1996]. It is interesting to note that one of these models is of a positive streamer [Pasko et al., 1996a], while the other is of a negative streamer [Sukhorukov et al., 1996]. The EMP models cited just above are also possible models of elves (see below).
The second question that arises concerns the amount of current that these flashes are delivering to the global circuit. It must be stressed that we do not, as yet, know if any current is being carried by these flashes, which is why Sentman introduced the term ``sprite" in preference to the term CS lightning. Since these events are relatively rare phenomena, no direct field and current observations by aircraft, balloon or rocket have yet been made. This question has important economic consequences. If substantial currents are involved, sprites may pose a significant hazard to civil aviation or spacecraft operations. Measurements of VLF scattering by sprites indicate that the sprite produces a significant ionization enhancement [Dowden et al., 1996]. Thus, it is possible for sprites to carry significant current. The positive streamer jet model explicitly predicts that jets contribute an appreciable amount of current to the global circuit [ Pasko et al., 1996a]. Farrell and Desch  have calculated the expected radio frequency (RF) emission spectrum from these flashes, provided they carry a current of 30 kA. They find that the signal will be 50 dB lower in the very low frequency (VLF, 3-30 kHz) than the signal from a 30 kA CG stroke. However, below 300 Hz, in the ultra low frequency (ULF, < 30 Hz) and extremely low frequency (ELF, 30 to 300 Hz) bands, the expected signal will be tens of dB stronger [see also Hale and Baginsky, 1987; Hale, 1992, 1994; Fraser-Smith, 1993; Ma et al., 1994]. Comparison of the electromagnetic signal observed in the ELF band during a sprite by a balloon-borne observatory with these models will be a significant contribution to answering these questions.
Ac perturbations of the geoelectric field have been observed in the mesosphere at frequencies below the Schumann resonance fundamental, between 0.8 and 2.6 Hz [D'Angelo et al., 1983; Bering and Benbrook, 1995]. These signals do not appear to be propagating modes of the Earth-ionosphere waveguide. The present best guess interpretation is that these signals arise as a consequence of excitation of the ionospheric Alfvén resonator, presumably by lightning transients or very possibly by sprites [Bering and Benbrook, 1995].
The best understood effect of the global circuit on the magnetosphere is the precipitation of trapped energetic electrons by interaction with the VLF radiation from lightning strokes [Bering et al., 1980b; Voss et al., 1984]. Recently, a number of other equally interesting effects of lightning strokes on the ionosphere have been investigated. These phenomena include the so-called ``early" Trimpi effects (see below), the occurrence of fast, large amplitude electric field transients, particle acceleration in addition to precipitation, enhancements in plasma density and temperature, enhanced airglow emission and gamma ray emission. Many, if not all, of these phenomena may be part of the processes that we call sprites, jets, elves or AOE's. These various phenomena may, in fact, all be manifestations of the same phenomenon, or they may be indicative of three or perhaps four different types of event. As noted above, transient brightening of the airglow emission layer (termed anomalous optical event (AOE) in the rocket literature) have been observed [Li et al., 1991; Boeck et al., 1992; Rodriguez et al., 1992b]. Similar, albeit much shorter events have been seen from the ground and are termed elves [Fukunishi et al., 1995, 1996]. These events appear to be quite distinct from sprites or jets. There are no reports of a sprite occurring in connection with an AOE. These events have also been associated with ``Trimpi" effects that occur too rapidly after the causative lightning stroke to have been produced by the precipitation of energetic trapped particles [Helliwell et al., 1973; Armstrong, 1983; Inan et al., 1988; Rodriguez et al., 1992b; Inan et al., 1993].
A group at Stanford University has developed a comprehensive model of the early or fast ``Trimpi" effect and elve phenomena. This model starts by calculating the heating in the ionosphere that will be produced by an upward propagating pulse of electromagnetic radiation from lightning [Inan et al., 1991], which can heat electrons enough to cause significant secondary ionization. The model has been used to calculate optical signatures [Taranenko et al., 1992], and D region density and temperature enhancements [ Rodriguez et al., 1992a; Taranenko et al., 1993; Rowland et al., 1994]. Preliminary efforts at observational verification are underway [ Inan et al., 1993]. Alternative models have been proposed. Very large amplitude electric fields from lightning strokes have been observed to penetrate the ionosphere 1 to 2 ms faster than whistler mode waves of the same frequency [Kelley et al., 1990]. These fields have a significant component parallel to the Earth's magnetic field. Based on these observations, Burke  has suggested that AOE's and early Trimpi's are the result of a rapid lowering of the mirror height of trapped energetic electrons by this parallel field pulse. This competing model has not yet received the degree of development that the Stanford model has. One of the unresolved problems is that the apparent velocity of these large fields was larger than any known propagating wave mode and is not understood. One hypothesis suggests that these signals arise as the result of non-linear parametric decay of the intense whistler mode wave pulse into lower hybrid waves and at least one other mode (a whistler is a left hand polarized electromagnetic wave with a frequency less than the local electron cyclotron frequency) [Kelley et al., 1990]. These lower hybrid waves are, in turn, expected to produce an upwelling of suprathermal ions above thunderstorms [Bell et al., 1993]. The parallel electric field pulse has also been observed to accelerate ionospheric electrons upward to energies on the order of 1 keV [Burke et al., 1992].
Gamma ray flashes associated with bremsstrahlung from MeV electrons are sometimes observed in space above thunderstorms [Fishman et al., 1994; Inan et al., 1996a]. Noting that significant distance is required to accelerate electrons to MeV energies at the electric field levels observed above lightning strokes, Fishman et al. have attributed these gamma ray events to sprites because the emitting volume of the sprite is observed to have a horizontal extent of 10's of km. This hypothesis is controversial, since there is a well developed alternative model. The very large amplitude electric fields mentioned in the previous paragraph have a significant component parallel to the Earth's magnetic field. Based on these observations, Burke  has suggested that AOE's are the result of a rapid lowering of the mirror height of trapped energetic electrons by this parallel field pulse. Fishman et al. do not appear to have considered the possibility that the source of the bremsstrahlung could have been trapped radiation belt electrons that have been precipitated via Burke's  mechanism of mirror height lowering. One of the difficulties with the Burke mechanism is the fact that some of the terrestrial origin gamma ray bursts appear to come from the near vicinity of the equator, where there is no trapped energetic population to be precipitated [ Inan et al., 1996a]. Direct, simultaneous observations of sprites and gamma rays by a stratospheric balloon payload are required to resolve this question. Until a dedicated satellite is launched to study this question, a balloon experiment is the only available method for making the required measurements. Attempts are being made to study this question with existing NASA and DOD satellites. However, these investigators would, we believe, all be delighted to obtain some ``air-truth" measurements.
Summary of Questions
The discovery of sprites, jets and AOE's raises a number of interesting scientific questions. The first and most obvious question concerns the nature of the emission process: Are sprites the result of a DC heating process similar to tropospheric lighting? If they are, we would expect that there should be a net current delivered to the ionosphere and the exciting electric field should be predominantly vertical. Are they instead the result of an ac breakdown discharge drive by unusually large transient electric fields? In this case, there would be no net current to the ionosphere and the exciting electric field would be predominantly horizontal. The various possibilities that arise in considering answers to the first question point to the second question: Are sprites carrying any current? The third question will probably be answered if we can answer the first question: What are the differences between the three types of event? In particular, what are the electrical differences between them? The fourth question is: are the Fishman et al.  gamma ray bursts actually produced by sprites, jets, or AOE's? Which? More importantly, how are they produced, by direct electron acceleration or by precipitation of trapped radiation belt particles? The fifth question is a generalization of the fourth question: What are the effects of sprites, etc., on the ionosphere and magnetosphere? Do they precipitate trapped radiation belt particles? Do they produce ``Trimpi" effects? Do they launch whistlers? Are they more or less effective in launching whistlers than ordinary flashes? The sixth question asks : What is the effect of sprites, etc., on the global circuit. The seventh and most technologically important question asks: What hazard do sprites pose for civil aviation and spacecraft operations. This question is of considerable economic importance. This list is incomplete. However, it pretty much covers the range of questions that will be addressed by the balloon experiment that we propose.
b:1. At the University of Houston:
The personnel required to accomplish the research at the University of Houston are the Principle Investigator, the Co-I and one graduate student. The principle investigator is a faculty member in the Physics Department of the University of Houston. The principle investigator and Co-I have a total of more than fifty years experience in the study of upper atmospheric physics and space and atmospheric electricity using ground instruments, balloons and sounding rockets. The design of the instrumentation will be done by the graduate student under direction of the principle investigator. The instrumentation and electronics will be fabricated, tested, and calibrated by all personnel working together in the electronics shop of the Space Physics Group at the University of Houston. Analysis of the data will be done by the graduate student under the direction and guidance of the investigators.
b:2. At the University of Alaska:
The personnel required to accomplish the research at the University of Alaska are the Co-Investigation Principle Investigator, the Co-I's and graduate students. The three investigators are faculty members in the Geophysical Institute of the University of Alaska. They have a total of more than eighty years experience in the study of upper atmospheric physics and space and atmospheric electricity using ground-based video instruments, balloons and sounding rockets. No new instrument construction is required. The entire team will deploy to the two field sites to take data. Analysis of the data will be done by the graduate student under the direction and guidance of the investigators.
b:3. At FMA Research Inc.:
The personnel required to accomplish the research at FMA Research are the Co-Investigation Principle Investigator and the staff of FMA Research, Inc. The team will be lead by Dr. Walter Lyons, Senior Scientist, who has been involved in sprite research since 1989 when he assisted Prof. J.R. Winckler in the analysis of the original sprite video record obtained by the University of Minnesota. He has over 15 years of experience in lighting research, having veen involved in the design and implimentation of the prototype National Lighting Detection Network. He received his Ph.D. in 1970 from the University of Chicago in the area of severe convective storms. Also on the FMA Research staff is Mr. Thomas Nelson, who has been technical program manager for the last three SPRITE campaigns conducted at by Dr. Lyons under NASA and USAF auspices.
It is quite clear we need high time resolution measurements of the vector electromagnetic field in the volume above sprite, jet and elve producing storm systems [ Marshall et al., 1996]. These data are required in order to answer the questions posed above and to select between the various models of these phenomena. There are a number of platforms that might be used to make such measurements, including rockets [e.g. Kelley et al., 1985, 1990; Baginsky et al., 1988], aircraft [Gish and Wait, 1950; Markson, 1976; Blakeslee et al., 1989], balloon-borne altitude profiles made with short duration small balloons [Byrne et al., 1989; Marshall et al., 1996], and constant-level high altitude stratospheric balloons [Benbrook et al., 1974; Bering et al., 1980b; Holzworth et al., 1986; Pinto et al., 1988]. This proposal will describe a set of experiments based on the latter technique. We have chosen this technique because we believe it affords several unique advantages over the other techniques. The region of the central plains where ground based instruments in the foothills of the Rocky Mountains can observe sprites, jets, and elves is too densely inhabited to make rockets practical. Aircraft can be flown directly above a sprite producing storm. However, the maximum ceiling of available aircraft is less than half either the minimum altitude of sprites or the float altitude of stratospheric balloons. Furthermore, the perturbations introduced by the airframe ( a large, irregularly shaped conductor) pose formidable calibration problems to investigators wishing to make precise quantitative measurements. Altitude profiles obtained with radiosonde balloons are a traditional and very successful method of studying the electrodynamics of thunderstorms. The use of small balloons and small (< 6 kg) payloads has the advantage of allowing investigators to drive under promising weather systems and thus control the location of their observations. Disadvantages include the fact that 6 kg does not permit enough instrumentation to make the high sampling rate vector measurements that are required, the very short interval of time that the balloons are above the storm and what appears in the literature to be a rather limited maximum altitude. Stratospheric balloons also have a number of drawbacks. First, they are unsteerable. It takes considerable preplanning and the order of a day of down time to move launch sites. Second, it takes about three hours from the moment a decision to launch is made for a balloon to be launched and reach float altitude. On the advantage side, high altitude balloon payloads operate at much higher altitude than any other platform except a sounding rocket, they stay aloft for many hours and they can carry a substantial, diverse payload of sophisticated, well calibrated instruments.
c:1. Overview of A Balloon-Borne Sprite Observatory:
The reason that we have chosen high altitude balloons as the vehicles for these proposed experiments is that the drawbacks mentioned above have been shown to be relatively minor problems. First, it has been shown that sprites are predictable enough to make long duration of balloon launches tolerable. The discovery that sprites are generated with some regularity by large (2.0 x 105 km2) mesoscale convection complexes means that the occurrence of sprites can be predicted several hours in advance. The forecasting record of the staff of FMA Research is impressive. With one to 3 hours notice, they hit on 100% of their forecasts for sprites from mesoscale convective systrems during 1995 and 1996. That is for more than 50 cases. Many sprite storms last in excess of three hours. This predictability opens up the possibility of studying sprites with balloon-borne payloads flying at a constant altitude (> 35 km) in the stratosphere. The use of such payloads to study the electrical environment above thunderstorms has been very fruitful historically [Benbrook et al., 1974; Bering et al., 1980b; Holzworth et al., 1986; Pinto et al., 1988]. The vector electric field detectors that will be flown on these payloads will be built with enough dynamic range to detect perturbations similar to observed sprite related perturbations [Marshall et al., 1996] at distances from 10's to 1000's of km [Bering and Benbrook, 1995]. In midsummer, when a Sprite Campaign might be conducted, the winds at 3-5 mbar above the great Plains will be easterlies. Thus, it should be possible to select a centrally located site near, for example, Iowa City, from which to launch suitably equipped payloads carried by stratospheric balloons. A dusk or early evening launch on a promising night will then position the balloon over the heart of the region viewed from the Colorado video observatories during the best observing hours. Such a launch strategy will guarantee that the payload will be well within 300-500 km of the sprite activity on perhaps half of the flights in a twelve flight campaign.
c:2:2. Electric Field:
The most critical parameter that must be measured to address the questions posed above is the electric field. What is required is a vector measurement with substantial dynamic range (up to at least 1000 V/m) and bandwidth (100 kHz). It must be remembered that the fields at 35-40 km will be considerably weaker than the 104-105 V/m field found near and within the storm. The necessary measurement must be made above the originating thunderstorm, preferably as close as possible to the light emitting volume. This requirement points very specifically to a stratospheric balloon measurement as the technique of choice. For a prior project, the Space Physics Group at the University of Houston developed preamplifiers with sufficiently high dynamic range, slew rate, and input impedance to make the necessary measurements [Byrne et al., 1988]. Unfortunately, the combination of wide dynamic range and high slew rate means that the input amplifiers will be relatively power hungry, which precludes the use of a sub 6 kg radiosonde type of payload. Therefore, we envision a three axis measurement on a medium sized (~100-200 kg) payload with a complement of other instruments. A combination of three telemetry approaches will be used. First, analog waveforms from two of the axes will be transmitted in real time using low gain and high band-width (100 kHz). Second, a relatively slow 1 kHz sample rate gain switched PCM will be transmitted in real time via a radio telemetry link. Third, an event trigger will be used to capture bursts of 200 kHz sampling rate data of all six components of the E-M field that will be stored in an on-board recorder for post-flight recovery.
c:2:2. Magnetic Field:
The ELF magnetic field signature of the sprites can be used to determine the amount of current being carried, at least in principle. In previous studies of the EM field above thunderstorms, we have developed and flown a set of three axis induction or search coil magnetometers with 100 kHz bandwidth.
The standard University of Houston electric field experiment measures the conductivity owing to both positive and negative ions at 4 minute intervals [Bering et al., 1980b, 1991; Byrne et al., 1988, 1990, 1991]. Two methods are used, the relaxation method [Benbrook et al., 1974; Hu et al., 1989; Holzworth, 1991; Hu, 1994] and a version of the blunt probe method. Since it is somewhat unlikely that we will fly directly through a sprite, this time resolution should be sufficient to monitor any large scale conductivity changes in the stratosphere.
A balloon-borne version of the air-Earth current sensor used in our measurements at South Pole station [Byrne et al., 1993] has been developed and successfully test flown. We will including this measurement in the balloon payload, which means that we will measure all three terms in Ohm's Law simultaneously.
c:2:4. X Ray Counting Rate:
The University of Houston Space Physics Group has extensive experience in measurements of auroral X rays [Bering et al., 1980a, 1988; Matthews et al., 1988]. Bremsstrahlung X rays and gamma rays of the energies reported by Fishman et al.  can penetrate to a depth of 5 gm/cm2 with relatively little attenuation [Berger and Seltzer, 1972]. Thus, a balloon borne X ray counter will be capable of monitoring X ray and gamma ray production by sprites. We plan on flying a NaI scintillation counter with a 125 mm diameter crystal and a 16 channel pulse height analyzer that will be read out to telemetry every 100 ms. Following event triggers, higher sampling rate data will be stored in the on-board recorder. A set of particle detectors including a Geiger-Muller counter and two solid state MeV electron counters will be included to look for in situ acceleration processes. It must be noted that the ambient air pressure at balloon altitude will probably make it impossible to devise a particle detector that can directly observe the ~10 eV electrons that appear to be responsible for exciting the visible emissions from sprites [Mende et al., 1995; Hampton et al., 1996].
c:2:5. Optical Instruments:
The payload that we presently envision will have enough available volume, power and telemetry to accommodate several photometers. We will install two broadband, rapid response all sky photometers to provide one source of event triggers for the high sampling rate on-board data recording system. They will also monitor the temporal relationship between tropospheric lightning and sprite emissions.
Balloon location information will be provided by an on-board GPS receiver, which will also provide accurate time tagging for the on-board recorder.
The task of calculating the electromagnetic field at the balloon that is predicted by the various models requires fairly precise knowledge of the timing, location and size of the emission being modeled. We propose to meet this requirement by making low-light-level video observations of the sprites, jets, and elves from three sites located on the eastern edge of the Rocky Mountains. Simultaneous multiple site video recordings will permit the use of triangulation methods to establish the location and size of the observed events. The observations will be carried out by the Co-Investigation teams from the Geophysical Institute at the University of Alaska and FMA Research. Detailed descriptions of the sites and instrumentation may be found in the two appendicies.
The University of Alaska group will operate low light level cameras from two ground sites separated by sufficient distance to allow accurate triangulation of sprties and other optical features. Since we will be using the star backgrounds for reference, the sites need to be as free of light contamination as possible. We propose to use the Wyoming Infrared Observatory (WIRO) atop Jelm Mountain, Wyoming (2943 m) and the Denver University Astronomical Observatory on Mt. Evans, Colorado (4313 m) where excellent data was obtained in previous operations in 1995 and 1996. Both ground sites will be equipped with a Trimble model Six-Vee 6 Global Positoning System (GPS) receiver to provide position and time. GPSD timing information will be recorded directly onto the video images using a Horita model GPS-2 smpte time code generator. A master sync generator at each using the GPS time will synchronize all camera systems to 32 s absolute. This method will allow accurate triangulation and comparison with balloon data.
We will use one Dage-MTI VE-1000 SIT monochromatic camera at each site. They have a sensitivity of 1.6 x 10-7 W-m-2 at 550 nm and 300 lines resolution. The second camera at each site will be an intensified CCD camera using Fairchild CCD's. The sensitivity is comparable to the Dage-MTI Systems. We have a variety of focal length lenses for use as appropriate. Both sites will have access to real time satellite cloud data, and NLDN lightning data for assisting in the decision processes of launching the balloon and selecting storms to view with the camera.
One site will also have a high speed photometer that is bore sited with the cameras. The low-light level cameras, which will be used for event identification, can time events to the closest TV field, or to 16.66 ms. In order to relate the optical emissions with the high bandwidth balloon data, closer timing is needed. This timing will be provided by use of a photometer. A high speed photometer system has been developed for auroral research by one of the Co-Investigators, Hans Stenbaek-Nielsen, in conjunction with Major Geoff McHarg of the air FOrce Academy. The system, as presently configured, consists of a 200 mm, 5" circular aperture reflector lens with a 25 mm Hamamatsu photomultiplier. The maximum field of view is 7.2 degrees. Aperture plates can be inserted in the image plane to provide different field of views. The photometer system is controlled by a computer and the output of the photomultiplier is recorded on disk. The time resolution of the output can be varied up to 110,000 samples per second.
FMA Research, Inc. will operate a third ground observatory at Yucca Ridge, Colorado. The facilites and instrumentation of this observatory have been extensively described in the literature [Lyons, 1994a, Winckler et al., 1996]. The Yucca Ridge Field Station (YRFS) operated by FMA Research has hosted SPRITES observation campaigns each summer since 1993. It has recently greatly expanded its physical facilitiies by adding almost 2000 sq ft of climate controlled laboratory space including a large viewing platform with a commanding view of more than 400,000 sq km. Ideal viewing conditions allowed the 1996 campaign to record over 1100 sprites and elves during a five week cmapaign in July and August 1996. Scientific reserach teams from over a dozen organizations worked at YRFS to make coordinated optical and RF measurements of sprites and elves. The sensors successfully deployed included numerous low-light televisions, spectrometers, photomeetrs, fast imagers, and various ELF and VLF receiving systems. A 28 MHz radar was also deployed nearby.
YRFS is set up for complete command and control functions associated with a mutli-site field program. Real time weather data support includes satellite delivered radar, weather satellite and lighting network data. Also, data are accessed and archived from the Internet onto dedicated workstation.
Dedicated YRFS facilities include dual Xybion LLTV imagers, a pointing pohotometer, and several VLF receivers. It is hosting several additional VLF systems operated by Stanford University and the University of Otago. A cooperative ELF Schumann Resonance station is run in conjunction with MIT/Lincoln Labs. ELF, VLF and photometer data can be digitally sampled using high speed A/D system. All LLTV images are GPS time stamped and recorded on SVHS tapes, with four channel audio including voice annotation, VLF, ELF and WWV time. During SPRITES;96 sprites and eleves were successfully imaged for distances out to 1000 km, and often coordinated with other LLTV sites in Wyoming (University of Alaska) and in New Mexico (NM Tech).
We propose to build three of the payloads just described during year 1 of the project, with sufficient preamplifiers and electric field probe booms to refurbish the payloads in real time and conduct twelve flights. We will also conduct detailed site surveys of potential launch and ground observing sites. These balloons will be flown during the summer of 1998. The anticipated campaign strategy will depend on real time predictions from campaign headquarters in Colorado, followed by a late evening launch. The campaign will take two moon down periods, in June and July. We expect to launch from base airports in Missouri (June window) and Iowa (July window), with down range telemetry stations located at the most central ground observatory corresponding to each launch site. Cut-down and recovery will take place east of the Front Range. Real-time field refurbishment for reflight should then enable us to make twelve flights. Given the vagaries of weather prediction, perhaps half of these will get close enough to a sprite producing mesoscale convection complex to provide useful data.
e. Institutional commitment and sources of additional support
e:1. Additional support:
The ground-based instrumentation that will be used has been developed and acquired with the support from several agencies for a variety of previous projects, including several NASA sponsored investigations of the aurora and of sprites, jets and elves. Dr. Lyons and FMA Research will be proposing a sprite UAV mission, most likely in 1999, in which the P.I. of this proposal will participate. This proposal will be submitted as a NASA SBIR. The UAV will only get to 25 km. However, the balloon experience in 1998 will be invaluable for us in designing and selecting instrumentation.
e:2. Institutional commitment:
The University of Houston Physics Department has three faculty members working in the area of Upper Atmospheric and Space Physics. The Space Physics group occupies 3800 square feet of laboratory and office space. Of particular importance is the complete balloon and rocket instrument fabrication facility operated by the Space Physics Group. The University has also provided the Space Physics Group with two computers for data analysis. The main Physic Department computer is a DEC AlphaServer 2000/233 with 64 MB RAM, 18 GB of disk, of which 8.6 GB is reserved for Space Physics, a TZ87 20GB cartridge tape drive, a CD-ROM reader, and network connections to a variety of other peripherals. The Space Science Data Center facilities include the University of Houston Space Physics Computer, which is a dedicated VAX 11/750 with 16 MB of main memory and 1.6GB of disk space, two tape drives and a variety of other peripherals. In addition, the group has a total of 8 PC workstations all connected to the Campus LAN.
The Geophysical Institute, University of Alaska Fairbanks, is a research organization dedicated to basic geophysical research. It has a relatively large group, about 15 academic and research faculty, engaged in auroral and space physics. Drs. Sentman and Wescott have pioneered the work on upward lightning which may have significant implications for the Earth's DC and AC electric environment. Of particular interest to this proposal are the fully staffed and equipped machine and electronics shops specializing in research support. The Geophysical Institute also has a large computer shop which can provide programming and computer maintenance support.
The UAF proposing team have all imaging equipment required for the observations, and they have a range of computers and special software available for the analysis of the data. Furthermore, they have access to the Arctic Region Supercomputer Center (ARSC) for large computing tasks.
FMA Research operates the Yucca Ridge Field Station near Ft. Collins described above. The group has published as prime author or contributor several dozen papers and technical articles on sprites, lightning detection and severe local storms.
f. Impact on infrastructure of science and engineering
The primary infrastructure impact of this research will be on the education of scientists and engineers at the University of Houston, University of Alaska and FMA Research. Balloon-borne observing programs and the development of new balloon-borne instruments are, in general, excellent preparation for students interested in both the scientific and engineering aspects of research into atmospheric science and space exploration. Students will acquire practical, hands-on experience in instrumentation development, telemetry system development and operation, data recovery, data analysis, etc. Operation of remote balloon-borne sensors to study atmospheric electricity constitutes one of the broadest possible practical interdisciplinary technical education opportunities.
g. Analysis Plan
Initial data analysis will be conducted as a parallel effort. One major task is the reduction of the balloon borne data from telemetry levels into physical units represented in an earth fixed geophysical coordinate system. Since the vertical axis of the payload is a known, fixed direction, measurement of a single vector suffices to determine payload attitude. We will use two axis fluxgate magnetometers as aspect sensors. Aspect will be determined during analysis by least-squares fitting a sine wave to the magnetometer output. Once the aspect is known, the low sampling rate real time PCM data will be analyzed to find spin averaged values of the slowly varying vector electric and the dc levels of the electric field sensors using well established procedures [Mozer and Serlin, 1969; Liao et al., 1994]. Once the payload aspect and instrument dc levels have been determined, the individual vector electric and magnetic field data from the balloon payload will be ``despun" point-by-point; that is, each observed vector from the real time data will be rotated into an earth-fixed coordinate system individually. This 1 kHz data set will have insufficient time resolution for detailed study of sprites, but it will have enough resolution to survey for major field discontinuities. This survey will provide one of the initial event lists used to select intervals for more detailed analysis.
The video data from the three ground stations will be searched for images of sprites, jets, and elves. This search will be guided in part by the real-time viewers log. The images from the three stations will be used to triangulate the location and extent of the sprite activity using published video analysis techniques [Sentman and Wescott, 1993; Sentman et al., 1994, 1995; Wescott et al., 1995] that have been developed at the University of Alaska. The analysis of the video data is expected to yield three data products: an event list, with times and event types; positions of many of these events , with error bars; and geographic outlines of the major emitting volumes.
The two event lists that have been produced in the two parallel portions of the data analysis will be combined to select events for detailed analysis of the on-board recorder data. Events will be selected for complete reduction of the high sampling rate data based on several criteria. First, there must have been a trigger that caused high rate data to be recorded. Second, we will focus on events that occurred close to the balloon. Third, priority will be given to events for which good triangulation solutions were found with relatively small error bars. Once such a selection has been made, the high rate data for an interval of ~1 s after each event will be despun and the calibrations applied as described above. In addition to waveform representations of the event, we will also prepare value added data products such as power spectra and cross-spectra.
Standard, previously published analysis techniques will be used to reduce the conductivity [Benbrook et al., 1974; Byrne et al., 1988], vertical current [Byrne et al., 1991, 1993] and X-ray data [Berger and Selzer, 1972; Bering et al., 1980a] and to produce time series data files in geophysical units. Energetic electron spectra will be prepared directly from the solid state telescope data and by inference from the X ray energy spectra.
We have proposed a series of experiments to study the electromagnetic fields responsible for the production of visible emissions in the mesosphere and ionosphere above thunderstorms. We propose to conduct a series of stratospheric balloon flights to make these observations.
h:1. Work Plan:
We propose to build three of the payloads just described during year 1 of the project. We will also procure sufficient preamplifiers and electric field probe booms to refurbish the payloads in real time and conduct twelve flights. We will survey possible launch sites and choose two primaries, one for the June and one for the July campaign. Corresponding ground observatory sites will also be selected. During the summer of year 2 of the project, these balloons will be flown from a location to be selected in the central Great Plains. The anticipated campaign strategy will depend on real time predictions of sprite producing mesoscale convective complex activity from campaign headquarters with input from Yucca Ridge Observatory in Colorado, followed by a late evening launch. We expect to launch from base airports in the western Iowa (July) or Missouri (June), with a down range telemetry station located at the ground observatory closest to due west of the launch site. Cut-down and recovery will take place east of the Front Range. Real-time field refurbishment for reflight should then enable us to make twelve flights. Given the vagaries of weather prediction, perhaps half of these will get close enough to a sprite producing mesoscale convection complex to provide useful data. The second half of year 2 and year 3 will be devoted to reduction, analysis and interpretation of the data.
h:2. Scientific Closure:
The analysis plan presented above will produce a body of reduced data products that consists of a list of sprite, jet and elve events that occurred during the intervals of the balloon flights, balloon and event positions at those times, event volumes, 200 kHz sampling rate six component vector electromagnetic field measurements during the events, along with value-added data products such as spectrograms and cross-spectra of the electromagnetic field data. Available data will also include x-ray and charged particle counting rates, vertical electric current and ambient conductivity. Ground based observations will include triangulation solutions from the multi-station image data, high sampling rate high gain photometer data, and stroke locations from the National Lightning Detection Network.
The process of obtaining scientific closure will be initiated by comparing the observed vector signatures with the predictions of the various models, evaluated for the appropriate storm size, ambient quasi-static electric field and relative distance. If, as we expect, several of the competing models produce qualitatively similar agreement with the data, quantitative hypothesis testing methods will be used to refine the choices. In the case that none of the extant models fit all of the data satisfactorily, we will construct new models that may do better. As a check of the selected model, the light intensity time profiles observed by a high speed photometer at one of the ground observatories will be compared with the expected emission rate time profile inferred from the observed field amplitude profile using the leading candidate models. Since all of the presently extant excitation models for sprites, jets and elves make very specific predictions of the EM field that one expects to observe, this process of observation, modeling and detailed quantitative comparison will produce a substantial increase in our understanding of the first and third questions posed above. Detailed model calculations using the model that is selected as the best, augmented with the current and conductivity data, will answer the second, sixth and seventh questions posed above. The fourth and fifth questions will be most directly addressed by examining the counting rate and spectra of X-rays and energetic electrons observed during the events, if any. If enhanced X ray counting rates are observed, then the basic answer to the fourth question is yes. Details of the acceleration mechanism can be inferred by comparing the shape of the energy spectrum with model predictions.
In summary, the data that will be returned by the proposed set of balloon experiments are likely to prove rich enough to make major contributions to finding the answers of most of the questions posed above.
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