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At least 1,261 records · Page 70
All sourcesNASA NTRSDOE OSTI
NASA NTRS1977

The effect of pulse interval statistics on the spectrum of radiation from lightning

An analysis of atmospheric radio noise originally appearing in the literature to describe the VLF structure of atmospherics is repeated here, keeping a term discarded in the previous work and extending the results to include all frequencies and some simple effects of amplitude variations. It is shown that at high frequencies, atmospheric radiation appears to be the result of incoherent sources, whereas at low frequencies the sferics appear to originate from coherent sources. These conclusions are valid with only weak restrictions, regardless of the actual statistical model assumed for the process. An implication of these results is that at high frequencies the magnitude of the spectrum of the received signal can be related to the spectrum of the source current, a means for the study of lightning current wave forms thus possibly being provided.

D M Le Vine↗
NASA NTRS1977

Lightning Protection of Aircraft

The current knowledge concerning potential lightning effects on aircraft and the means that are available to designers and operators to protect against these effects are summarized. The increased use of nonmetallic materials in the structure of aircraft and the constant trend toward using electronic equipment to handle flight-critical control and navigation functions have served as impetus for this study.

Franklin A Fisher↗
NASA NTRS1978

The influence of tortuosity on the spectrum of radiation from lightning return strokes

An investigation was made of the influence of tortuosity on the spectrum of radiation from lightning return strokes. The shape of the spectrum obtained by including effects of tortuosity was in keeping with data: The spectrum had a peak in the correct frequency regime followed by an initial decrease as the inverse of frequency. This spectrum was in better agreement with data than the spectrum predicted by the same model without tortuosity (i.e. the long straight channel), which decays at a rate proportional to 1/v squared.

Levine, D. M.↗
NASA NTRS1978

An unusual lightning flash at Kennedy Space Center

A lightning flash that struck the 150-meter weather tower at Kennedy Space Center was studied by several research groups using various techniques. The flash had unusually large peak currents and a stepped leader of relatively short duration. The charged regions neutralized by the three return strokes were located within a horizontal layer between heights of about 6 and 8 kilometers, where environmental temperatures were about -10 to -20 C. The charge source for the first return stroke coincided with a vertical shaft of precipitation inferred to have been graupel or hail. Charge sources for subsequent strokes were near the edge of the detectable precipitation echo. The overall channel length was about 10 kilometers. A vertically oriented intracloud discharge occurred after the three return strokes.

Uman, M. A.↗
NASA NTRS1978

Simulation of radiation from lightning return strokes - The effects of tortuosity

A Monte Carlo simulation has been developed for the electromagnetic fields radiated from a tortuous lightning channel. This was done using a piecewise linear model for the channel and employing for each element the field radiated by a traveling wave on an arbitrarily oriented filament over a conducting plane. The simulation reproduces experimental data reasonably well and has been used to study the effects of tortuosity on the fields radiated by return strokes. Tortuosity can significantly modify the radiated waveform, tending to render it less representative of the current pulse and more nearly unipolar than one would expect based on the theory for a long straight channel. In the frequency domain the effect of tortuosity is an increase in high frequency energy as compared with an equivalent straight channel. The extent of this increase depends on the mean length of the elements comprising the channel and can be significant.

Levine, D. M.↗
NASA NTRS1980

Lightning protection for aircraft

Reference book summarizes current knowledge concerning potential lightning effects on aircraft and means available to designers and operators to protect against effects. Book is available because of increasing use of nonmetallic materials in aircraft structural components and use of electronic equipment for control of critical flight operations and navigation.

Fisher, F. A.↗
NASA NTRS1978

Lightning current waveform measuring system

An apparatus is described for monitoring current waveforms produced by lightning strikes which generate currents in an elongated cable. These currents are converted to voltages and to light waves for being transmitted over an optical cable to a remote location. At the remote location, the waves are reconstructed back into electrical waves for being stored into a memory. The information is stored within the memory with a timing signal so that only different signals need be stored in order to reconstruct the wave form.

Wojtasinski, R. J.↗
NASA NTRS1977

Measured electric field intensities near electric cloud discharges detected by the Kennedy Space Center's Lightning Detection and Ranging System, LDAR

For a summer thunderstorm, for which simultaneous, airborne electric field measurements and Lightning Detection and Ranging (LDAR) System data was available, measurements were coordinated to present a picture of the electric field intensity near cloud electrical discharges detected by the LDAR System. Radar precipitation echos from NOAA's 10 cm weather radar and measured airborne electric field intensities were superimposed on LDAR PPI plots to present a coordinated data picture of thunderstorm activity.

Poehler, H. A.↗
NASA NTRS1979

RF radiation produced by intracloud lightning discharges

The RF radiation produced during intracloud lightning flashes is presented, together with associated fast and slow electric field changes. These data were collected during the Thunderstorm Research International Project in Florida during the summer of 1977. The RF radiation is essentially simultaneous with the fast-field change but has a tendency to peak during the initial half-cycle of the bipolar field changes associated cloud processes. This is in marked contrast to previous observations of RF radiation during return strokes in cloud-to-ground discharges and provides a clue to the physics of the intracloud discharge.

Levine, D. M.↗
NASA NTRS1979

Sources of the strongest RF radiation from lightning

Experiments performed at the Kennedy Space Center, Florida during TRIP-78 identified sources of the strongest RF radiation from lightning in the HF-VHF frequency range. Measurements were made of electric field changes associated with RF radiation using a field change system triggered on the output of an RF detector. The field changes associated with the strongest RF radiation are very fast (10 - 20 microseconds), bipolar pulses having an initial negative going half-cycle followed by a positive overshoot. These fast pulses consistently produced more RF radiation than was associated with return strokes, and their shape was remarkably consistent, independent of frequency.

Levine, D. M.↗
NASA NTRS1978

Lightning protection for traffic control systems

The probable cause of the several types of problems found related to lightning are determined. Protective measures were selected for each type of problem and the results are presented.

Plumer, J. A.↗
NASA NTRS1979

Lightning Detection and Ranging system LDAR system description and performance objectives

The instruments used at the six remote stations to measure both the time-of-arrival of the envelope of the pulsed 60 MHz to 80 MHz portion of the RF signal emitted by lightning, and the electric field waveforms are described as well as the two methods of transmitting the signal to the central station. Other topics discussed include data processing, recording, and reduction techniques and the software used for the 2100S, 2114, and 2116 computers.

Poehler, H. A.↗
NASA NTRS1978

The fine structure of lightning return stroke wave forms

The paper describes and analyzes the structure of the radiation fields produced just prior to and during the development of the first and subsequent return strokes in lightning discharges to ground. Stroke data are summarized, and the waveforms of the fields are found to contain several features which are quite alike for different strokes in the same discharge and also for different discharges. The possibility of large currents in the upward connecting discharges, the existence of a large submicrosecond component in all return strokes, and the cause of the small second peak and of the large subsidiary peaks in first strokes are discussed. The similarity of subsequent stroke fields within a flash suggests that the currents and velocities of different subsequent strokes in the same discharge are often very nearly the same.

Weidman, C. D.↗
NASA NTRS1979

The temporal structure of the HF and VHF radiation produced by intracloud lightning discharges

Measurements of the radio frequency emissions produced by large-amplitude intracloud lightning impulses are presented in correlation with the associated electric radiation field. The RF radiation starts at almost the same time as the field, and the peak RF tends to occur during the initial half cycle of the field pulse. This temporal behavior is in marked contrast to that exhibited by the RF during the return stroke portions of cloud-to-ground flashes.

Krider, E. P.↗
NASA NTRS1980

Nighttime/daytime optical survey of lightning and convective phenomena experiment (NOSL)

The Nighttime/Daytime Optical Survey of Lightning experiment was selected for flight on Orbital Flight Test 2 of the Space Shuttle. The background and rationale for the experiment are presented and the experiment hardware, the operating procedures, and the data analysis technique are discussed. Some preliminary results from the prototype hardware development are also presented.

Vonnegut, B.↗
NASA NTRS1980

A wave guide model of lightning currents and their electromagnetic field

Lightning channels are considered as resonant wave guides in which only standing resonant wave modes can be excited. Two types of discharging currents develop. Type 1 is an aperiodic wave; type 2 is a damped oscillation. The electromagnetic radiation field of both types of currents is calculated and compared with the observation.

Volland, H.↗
NASA NTRS1980

Calculations of lightning return stroke electric and magnetic fields above ground

A lightning return stroke model with which the two station electric and magnetic fields measured at ground level can be reproduced is used to compute fields at altitudes up to 10 km and at ranges from 20 m to 10 km. These calculations provide the first detailed estimates of the return strokes fields that are encountered by aircraft in flight. With the advent of modern aircraft utilizing low voltage digital electronics and reduced electromagnetic shielding by way of structures containing advanced composite materials, these calculations are of considerable practical interest. Further, since airborne electric and magnetic field measurements are presently being attempted, a comparison of the calculations presented with appropriate experimental data, when they are available, will constitute a test of the return stroke model.

Uman, M. A.↗
NASA NTRS1980

Submicrosecond risetimes in lightning radiation fields

Measurements of lightning electric fields, E, and dE/dt signatures were made near Tampa Bay, Florida, under conditions where the field propagation from the source to the detector was entirely over sea water. The fast transitions found on the initial portion of return stroke E waveforms have 10 to 90 percent risetimes ranging from 40 to 200 nsec, with a mean of 90 nsec. The maximum dE/dt values during these transitions range from 5 to 80 (V/m)microsec, with a mean of about 30 (V/m)microsec when normalized to a distance of 100 km. The initial risetimes of stepped-leader impulses that occur just prior to the first return stroke in cloud-to-ground flashes are very similar to the fast transitions in return strokes. The dE/dt values during leader steps range from 10 to 40 (V/m)/microsec with a mean of 20 (V/m)/microsec when normalized to 100 km. The fast impulses superimposed on large-amplitude intracloud waveforms have E risetimes and dE/dt values similar to those of the leader steps.

Weidman, C. D.↗