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A test technique was developed for applying a full scale mean value (30 kiloampere peak) simulated lightning return stroke current on a complete flight ready aircraft to assess the threat of lightning to aircraft electrical circuits. A computer-aided generator design was used to establish the parameters of the test system. Data from previous work done on development of low inductance current paths determined the basic system configuration.
The occurrence of cloud-to-ground flashes that effectively lower positive charge to earth (+CG flash) over flat terrain has been documented in the mature stage of severe thunderstorms. Of the 31 documented +CG flashes, most had only one return stroke. Zero-to-peak rise times for the strokes averaged 7 microsec. The +CG flashes averaged 520 ms in duration, with 25 percent lasting more than 800 ms. Many of these had field changes suggestive of continuing current. Positive flashes have been observed to emanate from several regions of severe storms: high on the back of the main storm tower, through the wall cloud, and from the downshear anvil. Visually most of these positive flashes have emanated from high in the storm, and acoustic mapping of two shows thunder sources to a height of about 15 km.
Techniques, equipment, and results of studies (1978-1980) to determine the relationships between electrical phenomena and the dynamics and precipitation of storms are reported. Doppler and conventional radar, video tapes and movies, and VHF recording devices were used to monitor an area 200 x 100 km, aligned SW to NE. The 23 cm radar and a Doppler radar were employed to acquire radar echoes from lightning. Observations of a squall line, a severe storm, and radar echoes from electrical discharges are described. Positively charged cloud-to-ground lightning was observed during the severe and final stages of severe storms; average lightning rates and total flashes for normal and severe storms are provided. Comparisons of lightning echoes and electric field changes indicated that abrupt increases in radar reflectivity were correlated with return strokes and K-type field changes.
A theory is presented for the spectrum of radiation from lightning applicable in the range LF to VHF. This is done by adopting the hypothesis that all events in the flash (e.g., return strokes, K-changes, etc.) can be modelled in terms of a current pulse propagating along a tortuous channel. Using a Fraunhofer approximation for radiation in the far field, a formula for the electric field radiated from an arbitrary event is obtained, and then this result is summed over all events to obtain an expression for the spectrum of the composite flash. The result predicts a spectral decrease as the inverse of frequency (1/v) in agreement with data.
A detailed analysis is presented of the electric-field variations preceding the first return strokes of 80 cloud-to-ground lightning flashes in nine different storms observed at the NASA Kennedy Space Center during the summers of 1976 and 1977. It is suggested that the electric-field variations can best be characterized as having two sections: preliminary variations and stepped leader. The stepped-leader change begins during a transition period of a few milliseconds marked by characteristic bipolar pulses; the duration of stepped leaders lies most frequently in the 6-20 millisecond range. It is also suggested that there is only one type of stepped leader, not two types (alpha and beta) often referred to in the literature.
Optical signals radiated by Florida lightning were recorded in correlation with wideband electric field signatures. The amplitudes and time behavior of the early portion of the signals produced by return strokes indicate that this lightning process produces a space and time averaged peak radiance.
Calculated time-domain waveforms and frequency spectra are presented for three of the most important processes in a lightning discharge to ground: the return stroke, the stepped leader, and the preliminary breakdown. For each of these processes, the model calculations are given for 200 m and 50 km. The calculations are compared with available time and frequency domain measurements.
The results of some measurements of earth lightning made with the functional unit of the Galileo probe RF lightning experiment are presented. The frequency range used is about 100 Hz to 100 kHz. The measurements were carried out in conjunction with simultaneous studies of lightning sources made by the University of Florida lightning research group. It is found that the typical RF pulse duration times are less than 30 microsec, independent of lightning source distance (less than 250 km). It is also found that the time between successive pulses is either quite short (less than 64 microsec), corresponding to the time between intracloud pulses and to the time between bipolar pulses, or occur around 100 ms, corresponding to the time between return strokes in ground discharges. The RF spectra are found to become less steep (relatively more power at the higher frequencies) during an interval of intense, nearby (less than 150 km) storms.
The question of X-ray generation in thunderclouds is examined. Energetic electrons may be produced by the large electric fields of return strokes and, in turn, give rise to bremsstrahlung X-rays. Estimates of the X-ray fluxes are presented.
The conditions leading to the breakup of the Atlas-Centaur 67 (AC-67) vehicle launched on March 26, 1987 during a typical winter-time storm are analyzed, and a most probable lightning strike scenario was developed based on inspection of the AC-67 debris, the AC-68 vehicle, and appropriate drawings, electrical diagrams, and photographs. It is shown that, during ascent, the vehicle encountered increasingly larger electric fields and flew through clouds that produced precipitation static on the vehicle, eventually triggering a cloud-to-ground lightning, comprised of at least four return strokes. The resulting lightning current coupled a signal into the wiring which goes to the AC-67 digital computer unit (DCU), effecting a single-word memory alteration and causing the DCU to issue a hardover engine gimbal command. This led to an excessive angle of attack, large dynamic loads, and the breakup of the AC-67.
Cloud-to-ground (CG) lightning strike data on the NASA F-106B research aircraft obtained during the 1984-86 storm seasons in the vicinity of Wallops Island, Virginia, are analyzed. The results suggest that CG strikes may represent a significant portion of the total number of lightning strikes encountered by the aircraft at altitudes below 6 km. It is unlikely that an aircraft encounters the first return stroke of the CG flash. The current values of the CG strikes are not different from currents in other types of strikes.
Lightning-related transient electric fields and conduction current densities due to the redistribution of charge by the return stroke, but not directly associated with the high-intensity current channel, are investigated via a simulation of Maxwell's equations. The investigation presents evidence to suggest that the lightning-related charge redistribution and subsequent relaxation, rather than the high intensity current, is the source of the major component of the transient field and energy coupled to the ionosphere. A time domain finite element analysis is employed in the simulation, using a conductivity profile obtained simultaneously with electric field measurements which agree, in general terms, with the simulations. It is concluded that prior depictions of these phenomena underestimate the fraction of the total energy that couples to and dissipates within the ionosphere and magnetosphere.
The goal of the F106B Thunderstorm Research Program is to characterize the lightning environment for aircraft in flight. This report describes the application of numerical electromagnetic models to this problem. Topics include: (1) Extensive application of linear triggered lightning to F106B data; (2) Electrostatic analysis of F106B field mill data; (3) Application of subgrid modeling to F106B nose region, including both static and nonlinear models; (4) Extension of F106B results to other aircraft of varying sizes and shapes; and (5) Application of nonlinear model to interaction of F106B with lightning leader-return stroke event.
A device to measure the propagation speeds of cloud-to-ground lightning has been developed. The lightning propagation speed (LPS) device consists of eight solid state silicon photodetectors mounted behind precision horizontal slits in the focal plane of a 50-mm lens on a 35-mm camera. Although the LPS device produces results similar to those obtained from a streaking camera, the LPS device has the advantages of smaller size, lower cost, mobile use, and easier data collection and analysis. The maximum accuracy for the LPS is 0.2 microsec, compared with about 0.8 microsecs for the streaking camera. It is found that the return stroke propagation speed for triggered lightning is different than that for natural lightning if measurements are taken over channel segments less than 500 m. It is suggested that there are no significant differences between the propagation speeds of positive and negative flashes. Also, differences between natural and triggered dart leaders are discussed.
In 1988, ONERA carried out a complete airborne lightning characterization program. Among other features, the program has provided a significant amount of data from analog records of electric and magnetic field sensors to give a description of all stages of the lightning process. The initiation phase has a description of all stages of the lightning process. The initiation phase has been described in the past, so the present objective is to give a description of the other stages of the discharge, based on the observation of analog electromagnetic waveforms and of fast video (200 f/s) image processed pictures. During lightning attachment, the aircraft remains electrically connected to the lightning channel where a large variety of physical processes occur. These physical processes include recoil streamers, return strokes, and even other initiation processes of secondary discharges. Typical records of analog electromagnetic field along with relative luminosity variation of the channel, which shows that there is no extinction of the continuous current throughout the discharge process. A computation of current and potential waveforms using the transmission line formalism is presented and gives satisfactory results for the representation of two of the typical waveforms observed in the recoil streamer processes.
Gated, wideband, magnetic direction finders (DFs) were originally designed to measure the bearing of cloud-to-ground lightning relative to the sensor. A recent addition to this device uses proprietary waveform discrimination logic to select return stroke signatures and certain range dependent features in the waveform to provide an estimate of range of flashes within 50 kms. The enhanced ranging techniques are discussed which were designed and developed for use in single station thunderstorm warning sensor. Included are the results of on-going evaluations being conducted under a variety of meteorological and geographic conditions.
Measurements were characterized of simultaneous vertical electric fields and voltages induced at both ends of a 448 m overhead power line by artificially initiated lightning return strokes. The lightning discharges struck ground about 20 m from one end of the line. The measured line voltages could be grouped into two categories: those in which multiple, similarly shaped, evenly spaced pulses were observed, which are called oscillatory; and those dominated by a principal pulse with subsidiary oscillations of much smaller amplitude, which are called impulsive. Voltage amplitudes range from tens of kilovolts for oscillatory voltages to hundreds of kilovolts for impulsive voltages.
Development of the Sandia Transportable Triggered Lightning Instrumentation Facility (SATTLIF) was motivated by a requirement for the in situ testing of a munitions storage bunker. Transfer functions relating the incident flash currents to voltages, currents, and electromagnetic field values throughout the structure will be obtained for use in refining and validating a lightning response computer model of this type of structure. A preliminary shakedown trial of the facility under actual operational conditions was performed during summer of 1990 at the Kennedy Space Center's (KSC) rocket-triggered lightning test site. A description is given of the SATTLIF, which is readily transportable on a single flatbed truck of by aircraft, and its instrumentation for measuring incident lightning channel currents and the responses of the systems under test. Measurements of return-stroke current peaks obtained with the SATTLIF are presented. Agreement with data acquired on the same flashes with existing KSC instrumentation is, on average, to within approximately 7 percent. Continuing currents were measured with a resolution of approximately 2.5 A. This field trial demonstrated the practicality of using a transportable triggered lightning facility for specialized test applications.