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At least 1,315 records · Page 73
All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

University of Florida lightning research at the Kennedy Space Center

A variety of basic and applied research programs are being conducted at the Kennedy Space Center. As an example of this research, the paper describes the University of Florida program to characterize the electric and magnetic fields of lightning and the coupling of those fields to utility power lines. Specifically, detailed consideration is given to the measurements of horizontal and vertical electric fields made during the previous three summers at KSC and the simultaneous measurements of the voltages on a 500 m test line made during the past two summers at KSC. Theory to support these measurements is also presented.

Uman, Martin A.↗
NASA NTRS1987

Review of measurements of the RF spectrum of radiation from lightning

A review is presented of the measurements reported in the literature of the spectrum of electromagnetic radiation from lightning in the frequency range from 1 kHz to 1 GHz. Measurements have been made either by monitoring the power received at individual frequencies using a narrow bandwidth recording device or by recording the transient (time dependent) radiation with a wide bandwidth device and then Fourier transforming the waveform to obtain a spectrum. Measurements of the first type were made extensively in the 1950's and 1960's and several composite spectra have been deduced by normalizing the data of different investigators to common units of bandwidth and distance. The composite spectra tend to peak near 5 kHz and then decrease roughly as 1/(frequency) up to nearly 100 MHz where scatter in the data make the behaviour uncertain. The spectrum obtained with measurements of the second type is similar. Recent experiments have extended the frequency range covered to several 10's of MHz and include spectra for return strokes, the stepped leader and for some intracloud processes. The spectrum of first return strokes obtained in this manner is very similar to the spectrum of the composite flash obtained from the narrowband measurements.

Radiation↗
NASA NTRS1987

Optical observations of lightning from a high-altitude airplane

Near-infrared optical measurements of about 1300 optical pulses produced by 79 lightning flashes were obtained by a NASA U-2 high-altitude airplane. Ninety percent of the flashes produced peak radiant energy densities of 4.7 microJ/sq m per sr or greater relative to the full field of view of the instrument. A median pulse rise time of 240 microsec and FWHM of 370 microsec are found, and these slow optical rise times and broad pulse widths are suggested to be due to multiple scattering within the cloud. The spectral characteristics in the near-infrared of the neutral emission lines noted above the clouds are shown to be similar to those found with ground-based measurements.

Christian, H. J.↗
NASA NTRS1988

Horizontal electric fields from lightning return strokes

An experiment to measure simultaneously the wideband horizontal and vertical electric fields from lightning return strokes is described. Typical wave shapes of the measured horizontal and vertical fields are presented, and the horizontal fields are characterized. The measured horizontal fields are compared with calculated horizontal fields obtained by applying the wavetilt formula to the vertical fields. The limitations and sources of error in the measurement technique are discussed.

Thomson, E. M.↗
NASA NTRS1988

Evidence of lightning and volcanic activity on Venus - Pro and con

It is argued that the impulsive 100-Hz noise bursts detected with the use of the electric field antenna on the Pioneer Venus Orbiter (PVO) have plasma wave charcteristics that can only be explained if they are whistler mode signals of a type that can be produced by atmospheric discharges. It is further argued that these data are evidence for lightning and volcanic activity on Venus. A reply contends that the PVO electric field measurements are unrelated to either the lower atmosphere or the surface of Venus.

Scarf, Frederick L.↗
NASA NTRS1988

An experimental test of the 'transmission-line model' of electromagnetic radiation from triggered lightning return strokes

Peak currents, two-dimensional average propagation speeds, and electric field waveforms for a number of subsequent return strikes in rocket-triggered lightning flashes were measured in order to test the 'transmission-line model' of return-stroke radiation of Uman and McLain (1970). Reasonable agreement is found between the propagation speeds measured with the streak camera and those deduced from the transmission-line model. A modification of the model is proposed in which two wave fronts travel upward and downward away from a junction point a short distance above the ground.

Willett, J. C.↗
NASA NTRS1989

Interpretation of F106B and CV580 in-flight lightning data and form factor determination

Two topics of in-flight aircraft/lightning interaction are addressed. The first is the analysis of measured data from the NASA F106B Thunderstorm Research Aircraft and the CV580 research program run by the FAA and Wright-Patterson Air Force Base. The CV580 data was investigated in a mostly qualitative sense, while the F106B data was subjected to both statistical and quantitative analysis using linear triggered lightning finite difference models. The second main topic is the analysis of field mill data and the calibration of the field mill systems. The calibration of the F106B field mill system was investigated using an improved finite difference model of the aircraft having a spatial resolution of one-quarter meter. The calibration was applied to measured field mill data acquired during the 1985 thunderstorm season. The experimental determination of form factors useful for field mill calibration was also investigated both experimentally and analytically. The experimental effort involved the use of conducting scale models and an electrolytic tank. An analytic technique was developed to aid in the understanding of the experimental results.

Rudolph, T.↗
NASA NTRS1988

Storm diagnostic/predictive images derived from a combination of lightning and satellite imagery

A technique is presented for generating trend or convective tendency images using a combination of GOES satellite imagery and cloud-to-ground lightning observations. The convective tendency images can be used for short term forecasting of storm development. A conceptual model of cloud electrical development and an example of the methodology used to generate lightning/satellite convective tendency imagery are given. Successive convective tendency images can be looped or animated to show the previous growth or decay of thunderstorms and their associated lighting activity. It is suggested that the convective tendency image may also be used to indicate potential microburst producing storms.

Goodman, Steven J.↗
NASA NTRS1988

Lightning and precipitation history of a microburst-producing storm

Quantitative measurements of the lightning and precipitation life cycle of a microburst-producing storm are discussed. The storm, which occurred on July 20, 1986 at Huntsville, Alabama, was studied using Doppler radar data. The storm produced 116 flashes, 6 of which were discharges to the ground. It is suggested that an abrupt decrease in the total flash rates is associated with storm collapse, and serves as a precursor to the arrival of the maximum microburst outflows at the surface. Ice-phase precipitation is shown to be an important factor in both the formation of the strong downdraft and the electrification of the storm.

Goodman, Steven J.↗
NASA NTRS1988

Electric field pulses in close lightning cloud flashes

Data obtained in 1984 for lightning cloud flashes at Florida were used to investigate electric field pulses, their times of occurrence in the flash, and their relationship to slower electric field variations. It is found that large pulses with a width of a few microseconds tended to occur more frequently early in the flash. The results suggest that the process that produces K change pulses is different from that which produces the large pulses.

Bils, John R.↗
NASA NTRS1988

Comparison of fast electric field changes from subsequent return strokes of natural and triggered lightning

Fast electric field changes from subsequent return strokes of natural and triggered lightning with propagation paths almost entirely over water are compared. Data were collected at the Kennedy Space Center, Florida. Comparisons have been made of the average shape, the rise time and the spectrum of the electric field changes. The electric field changes from the triggered flashes tend to rise to peak faster and decay faster than do their counterparts in natural cloud-to-ground flashes.

Le Vine, D. M.↗
NASA NTRS1989

Analysis of lightning field changes during active Florida thunderstorms

A computer algorithm for deriving accurate values of lightning-caused changes in cloud electric fields under active storm conditions was developed and applied to data obtained during two thunderstorms from a network of ground-based electric field mills at the NASA Kennedy Space Center and the Cape Canaveral Air Force Station. The resulting field changes were analyzed using a least-squares optimization procedure and point-charge (Q) and point-dipole (P) models. The values and the time variations of the Q-model parameters under active storm conditions were found to be similar to those reported by Maier and Krider (1986) for small storms, when the computations were carried out with the same analysis criteria and comparable biases. The parameters of the P solutions were found to vary with time within the storm interval and from storm to storm.

Koshak, William J.↗
NASA NTRS1989

Lightning rates relative to tornadic storm evolution on 22 May 1981

Lightning and Doppler radar data for two tornadic storms in Oklahoma on May 22, 1981 are used to analyze ground flash rates relative to the time of tornadoes. It is found that the ground flash rates had no obvious relationship with the tornado times, although the stroke rate in both storms was greatest after the tornadic stage ended. The variations in the cyclone shear and the intracloud flash rates within 10 km of the mesocyclone region are examined. The results suggest that most tornadic storms have an increase in total flash rates near the time of the tornado and that this increase is often dominated by intracloud flashes.

Macgorman, Donald R.↗
NASA NTRS1989

A photoelectric technique for measuring lightning-channel propagation velocities from a mobile laboratory

The present device for lightning channel propagation-velocity determination employs eight photodetectors mounted behind precision horizontal slits in the focal plane of a photographic camera lens. The eight photodetector pulses, IRIG-B time, and slow and fast electric field-change waveforms are recorded on a 14-track analog tape recorder. A comparison of the present results with those obtained by a streaking camera shows no significant differences between the velocities obtained from the same strokes with the two systems; neither is there any difference in pulse characteristics or in the velocities calculated from them.

Mach, Douglas M.↗
NASA NTRS1990

Determining Polarities Of Distant Lightning Strokes

Method for determining polarities of lightning strokes more than 400 km away. Two features of signal from each stroke correlated. New method based on fact each stroke observed thus far for which polarity determined unambiguously, initial polarity of tail same as polarity of initial deflection before initial-deflection signal altered by propagation effects. Receiving station equipped with electric-field-change antenna coupled to charge amplifier having time constant of order of 1 to 10 seconds. Output of amplifier fed to signal-processing circuitry, which determines initial polarity of tail.

Blakeslee, Richard J.↗
NASA NTRS1989

Photoelectric return-stroke velocity and peak current estimates in natural and triggered lightning

Two-dimensional photoelectric return stroke velocities from 130 strokes are presented, including 86 negative natural, 41 negative triggered, one positive triggered, and two positive natural return strokes. For strokes starting near the ground and exceeding 500 m in length, the average velocity is 1.3 + or - 0.3 X 10 to the 8th m/s for natural return strokes and 1.2 + or - 0.3 X 10 to the 8th m/s for triggered return strokes. For strokes with lengths less than 500 m, the average velocities are slightly higher. Using the transmission line model (TLM), the shortest segment one-dimensional return stroke velocity, and either the maximum or plateau electric field, it is shown that natural strokes have a peak current distribution that is lognormal with a median value of 16 kA (maximum E) or 12 kA (plateau E). Triggered lightning has a medium peak current value of 21 kA (maximum E) or 15 kA (plateau E). Correlations are found between TLM peak currents and velocities for triggered and natural subsequent return strokes, but not between TLM peak currents and natural first return stroke velocities.

Mach, Douglas M.↗
NASA NTRS1989

Comparison of fast electric field changes from subsequent return strokes of natural and triggered lightning

Fast electric field changes from subsequent return strokes of natural and triggered lightning with propagation paths almost entirely over water are compared. Data were collected at the Kennedy Space Center, Florida, during the summer of 1987. Comparisons have been made of the average shape, the risetime, and the spectrum of the electric field changes. To a first approximation, the waveforms are very similar; however, the electric field changes from the triggered flashes tend to rise to peak faster and decay faster than do their counterparts in natural cloud-to-ground flashes.

Le Vine, D. M.↗
NASA NTRS1988

Description of a new full scale lightning simulation facility

A lightning test facility has been constructed in Utah, and the facility has initially been used to test and certify solid rocket motor (SRM) segment shipping containers. The simulator equipment consists of a 17-stage 1.7 MV Marx generator and three current generators. The performance of the Marx is enhanced by use of a distributed peaking capacitor to produce maximum dl/dt's in excess of 4 x 10 to the 11th Amps/sec into large test objects. The current generators consist of the following: (1) a 200 KA underdamped 500 kilojoule high current bank, (2) a critically damped intermediate current bank and (3) an over damped continuing current bank. Total charge transfer for these banks is 270 Coulombs with an action integral in excess of 8 x 10 to the 6th sq Amps-sec. Representative results for the simulator and SRM segment shipping containers are presented.

Papazian, Peter B.↗