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At least 613 records · Page 34

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NASA NTRS1980

Submicrosecond risetimes in lightning return-stroke fields

Measurements of lightning electric field, E, and dE/dt signatures have been made near Tampa Bay, Florida, under conditions where the lightning locations were known and where the results were not significantly affected by the response time of the measuring system or groundwave propagation. The fast transitions found on the initial portion of return-stroke fields have 10-90% risetimes ranging from 40 to 200 nsec, with a mean of 90 nsec. The maximum field derivatives during return strokes range from 5 to 75 V/m per microsec with a mean of 29 V/m per microsec when normalized to a distance of 100 km. These field risetime and derivative values suggest that return-stroke currents contain large, submicrosecond components, and this in turn suggests that it may be necessary to reevaluate the possible effects of lightning and the performance of lightning-protection devices in many situations.

Weidman, C. D.↗

NASA NTRS1981

Preliminary results of the study of lightning location relative to storm structure and dynamics

Lightning is being studied relative to storm structure using a VHF space-time discharge mapping system, radar, a cloud-to-ground flash locator, acoustic reconstruction of thunder, and other instrumentation. The horizontal discharge processes within the cloud generally propagate at speeds of 10,000-100,000 m/s. Horizontal extents of lightning were found up to 90 km. In an analysis of a limited number of flashes, lightning occurred in or near regions of high cyclonic shear. Positive cloud-to-ground flashes have been observed emanating from several identifiable regions of severe storms. Lightning echoes observed with 10-cm radar generally are 10-25 dB greater than the largest precipitation echo in the storm.

Rust, W. D.↗

NASA NTRS1981

Investigation of severe lightning strike incidents to two USAF F-106A aircraft

The results of the inspection and analysis of two F-106A aircraft that were struck by separate lightning strikes within a few minutes of each other are presented. Each aircraft sustained severe lightning strikes to the pitot booms, resulting in extensive damage to the pitot heater power harness, number 8 ground wire, and lightning suppressors, but there was no damage to either aircraft's electrical or avionic systems. A simulated lightning current of 226 kA and 3.8 million A(2)*S was required to reproduce the damage to the ground wires in the radomes. Photographs and detailed assessments of the damage are included.

Plumer, J. A.↗

NASA NTRS1982

Lightning protection of wind turbines

Possible damages to wind turbine components due to lightning strikes are discussed and means to prevent the damage are presented. A low resistance path to the ground is noted to be essential for any turbine system, including metal paths on nonmetal blades to conduct the strike. Surge arrestors are necessary to protect against overvoltages both from utility lines in normal operation and against lightning damage to control equipment and contactors in the generator. MOS structures are susceptible to static discharge injury, as are other semiconductor devices, and must be protected by the presence of static protection circuitry. It is recommended that the electronics be analyzed for the circuit transient response to a lightning waveform, to induced and dc current injection, that input/output leads be shielded, everything be grounded, and lightning-resistant components be chosen early in the design phase.

Dodd, C. W.↗

NASA NTRS1982

Lightning to the ionosphere

It is pointed out that most lightning discharges take place within clouds or between clouds and the ground. There are a few reports, however, of a much rarer form of lightning that has been observed to come out of the top of the thunderstorm cloud and extend vertically for some distance into the clear air above it. A description is presented of the observations of such an incident made by a NASA pilot. The incident occurred when the pilot was flying a T-38 jet aircraft at an altitude of about 40,000 feet over Amarillo in Texas. The pilot found himself behind an extremely fast-moving very large cold front. A large number of lightning flashes could be seen. While most of the lightning flashes were typical, approximately every 50th or 100th discharge would go from the top of a cloud vertically upwards through the clear air towards the ionosphere. Explanations for these observations are discussed.

Vaughan, O. H., Jr.↗

NASA NTRS1982

Production of nitric oxide by lightning on Venus

The first measurements of the production of nitric oxide (NO) by a laboratory discharge in a simulated Venus atmosphere are presented. The average NO yield over a range of energies was found to be 3.7 + or - 0.7 x 10 to the 15th molecules/joule. Simultaneous measurements of carbon monoxide (CO) resulting from the lightning-induced dissociation of carbon dioxide (CO2) indicated a CO yield of about 4 x 10 to the 17th molecules/joule. These measurements suggest that at and below cloud level, a region where solar ultraviolet radiation cannot penetrate, the dissociation of CO2 by lightning may be a significant source of oxygen atoms. Depending on the assumed value for the total energy dissipated by lightning on Venus, the production of NO by lightning may be a significant sink of atmospheric nitrogen over the history of Venus.

Levine, J. S.↗

NASA NTRS1982

Interpretation methodology and analysis of in-flight lightning data

A methodology is presented whereby electromagnetic measurements of inflight lightning stroke data can be understood and extended to other aircraft. Recent measurements made on the NASA F106B aircraft indicate that sophisticated numerical techniques and new developments in corona modeling are required to fully understand the data. Thus the problem is nontrivial and successful interpretation can lead to a significant understanding of the lightning/aircraft interaction event. This is of particular importance because of the problem of lightning induced transient upset of new technology low level microcircuitry which is being used in increasing quantities in modern and future avionics. Inflight lightning data is analyzed and lightning environments incident upon the F106B are determined.

Rudolph, T.↗

NASA NTRS1983

RF radiation from lightning correlated with aircraft measurements during storm hazards-82

During the Storm Hazards Experiment 1982, the Goddard Space Flight Center monitored radiation from lightning from a site at the Wallops Flight Facility, Wallops Island, VA. Measurements were made while the NASA F106 penetrated thunderstorms to obtain data on lightning strikes to the aircraft. The objective of the ground-based measurements was to help determine if the events recorded by the F106 were part of lightning discharges. During the experiment, 53 cases were obtained in which events were recorded aboard the aircraft while reliable quality RF radiation was recorded on the ground. These cases came from 12 different storms occurring from June through August 1982. The data confirms that the aircraft was measuring events which were part of lightning and indicates that the events recorded on the aircraft tend to occur early in the flash.

Levine, D. M.↗

NASA NTRS1982

The effect of polarization on radar detection of lightning

The polarization dependence of lightning echoes received with a 23-cm wavelength radar is estimated theoretically and experimentally. The reduction in the lightning radar cross-section due to the transmission of circular polarization instead of linear is measured to be about 4 dB and is much smaller than the 15-30 dB reduction in reflectivity of weak precipitation regions. The 8.7 dB average reduction in reflectivity maxima has been observed. This leads to an increase in the detected lightning flash rate by as much as 40%. The radar cross-section ratio of circular to linear polarization obtained experimentally suggests that lightning elements tend to be horizontally oriented.

Mazur, V.↗

NASA NTRS1982

Comparison of Venusian lightning observations

Spacecraft observations of Venusian lightning are reviewed to determine the energy dissipation rate. Optical sensors provide data on the total energy dissipated per flash, so long as the contributions of atomic oxygen and ionized carbon are accounted for. Radio probes yield information on the total lightning occurrences. Venera 9-12 radio surveys are reviewed, and event rates of 20-30/sec for Venera 11 are mentioned. Venera 9 observed a sole, 70 sec event period on the Venusian nightside. Comparisons are made between the recorded electromagnetic energy levels and terrestrial lightning data. The nightside occurrence rate is calculated to be 0.08 strokes/sq km per year, while the dayside exhibits 85 flashes/sq km per sec. Energy dissipation rates ranging from 0.0012-0.25 W/sq m are calculated for various flash durations. It is suggested that the lightning occurs in the clouds as part of an organized meteorological system.

Borucki, W. J.↗

NASA NTRS1983

Saturn's electrostatic discharges - Could lightning be the cause?

It is proposed that Saturn's electrostatic discharges (SED) might be generated in the planet's equatorial atmosphere, perhaps as lightning from a storm system. The 10-h-10-min periodicity of the signal envelope duplicates that of Saturn's equatorial jet. The rings shield the atmosphere from solar EUV photons, and thereby substantially reduce the local ionospheric cutoff frequency to allow low-frequency SED to leak out. Many of the unusual properties of SED could be explained in terms of changes in the storm system, the relative spacecraft position in the beaming pattern of the source, local refraction of the signal by the highly disturbed ionosphere, and the influence of the ring particles on the highest frequency component of SED. A comparison of SED with planetary lightning on other planets shows that the two are similar in general character and some time behavior; the power output of SED may be higher than most planetary lightning, but that is unclear because of uncertainties in the measurements and variations in the signal's spectrum. It is suggested that lightning could be a viable source for SED and that exotic ring mechanisms are not necessarily required.

Burns, J. A.↗

NASA NTRS1983

Lightning mapper sensor design study

World-wide continuous measurement of lightning location, intensity, and time during both day and night is to be provided by the Lightning Mapper (LITMAP) instrument. A technology assessment to determine if the LITMAP requirements can be met using existing sensor and electronic technologies is presented. The baseline concept discussed in this report is a compromise among a number of opposing requirements (e.g., ground resolution versus array size; large field of view versus narrow bandpass filter). The concept provides coverage for more than 80 percent of the lightning events as based on recent above-cloud NASA/U2 lightning measurements.

Eaton, L. R.↗

NASA NTRS1984

Lightning Observations above and below clouds

The quantitative optical characteristics of cloud to ground (CG) and intracloud (IC) lightning above clouds were studied. A data base of a number of pulse paramaters such as energy, rise times, pulse widths and pulse intervals was complied and categorized for first return strokes, subsequent strokes, the intracloud part of CG flashes and IC flashes. It is found that: (1) single stroke CG's are more readily distinguishable from IC flashes than multiple stroke CG's; (2) there is no significant difference between the energy of first and subsequent return stroke pulses; and (3) the pulse rise times and pulse widths are time broadened. Lightning activity in a mesoscale convective weather system (MCS) was examined. The CG flash rates average almost 50 per minute for 7 hours. It is shown that lightning above storms embedded within the MCS IC lightning activity can be much greater than CG activity at certain times in the MCS lifecycle.

Goodman, S. J.↗

NASA NTRS1984

Lightning observations from the Space Shuttle

Motion pictures were taken at night from the space shuttle that show lightning discharges spreading horizontally at speeds of .00001 m/sec for distances over 60 km. Tape recordings were made of the accompanying optical pulses detected with a photocell optical system. The observations show that lightning is often a mesoscale phenomenon that conveys large amounts of electric charge and energy derived from an extensive cloud system into a cloud-to-ground discharge. Several video tape recordings of lightning discharges were obtained on shuttle flights since the termination of the NOSL program. The size and location of the lightning illuminated cloud images is now being analyzed, and comparisons are made with meteorological data concerning the cloud system obtained from the McIDAS.

Vonnegut, B.↗

NASA NTRS1983

Lightning measurements from the Pioneer Venus Orbiter

The plasma wave instrument on the Pioneer Venus Orbiter frequently detects strong and impulsive low-frequency signals when the spacecraft traverses the nightside ionosphere near periapsis. These particular noise bursts appear only when the local magnetic field is strong and steady and when the field is oriented to point down to the ionosphere thus; the signals have all characteristics of lightning whistlers. We have tried to identify lightning sources between the cloud layers and the planet itself by tracing rays along the B-field from the Orbiter down toward the surface. An extensive data set, consisting of measurements through Orbit 1185, strongly indicates a clustering of lightning sources near the Beta and Phoebe Regios, with an additional significant cluster near the Atla Regio at the eastern edge of Aphrodite Terra. These results suggest that there are localized lightning sources at or near the planetary surface.

Scarf, F. L.↗

NASA NTRS1984

Correlated observations of three triggered lightning flashes

Three triggered lightning flashes, initiated during the Thunderstorm Research International Program (1981) at Langmuir Laboratory, New Mexico, are examined on the basis of three-dimensional return stroke propagation speeds and peak currents. Nonlinear relationships result between return stroke propagation speed and stroke peak current for 56 strokes, and between return stroke propagation speed and dart leader propagation speed for 32 strokes. Calculated linear correlation coefficients include dart leader propagation speed and ensuing return stroke peak current (32 strokes; r = 0.84); and stroke peak current and interstroke interval (69 strokes; r = 0.57). Earlier natural lightning data do not concur with the weak positive correlation between dart leader propagation speed and interstroke interval. Therefore, application of triggered lightning results to natural lightning phenomena must be made with certain caveats. Mean values are included for the three-dimensional return stroke propagation speed and for the three-dimensional dart leader propagation speed.

Idone, V. P.↗

NASA NTRS1984

Characteristics of lightning strikes experienced by the NASA F-1068 airplane

Thunderstorm penetrations were made during the 1980-1983 thunderstorm seasons by a NASA F-106B instrumented research aircraft in order to record lightning strike data and associated flight conditions. In 1980 and 1981, 10 direct lightning hits were received each year, with most penetrations occurring near the freezing level. In 1982 and 1983, a ground-based UHF-band radar was used to guide this aircraft through the upper, electrically active regions of thunderstorms (at altitudes above 6 km, corresponding to temperatures of less than -20 C), and 370 direct hits were recorded. The results of analyses of the radar, photographic, and surface inspection data are presented. The highest risk of lightning strike in the upper portions of a thundercloud occurred in regions where the ambient temperature was -40 to -50 C. Lightning strikes appear to be triggered by the aircraft.

Fisher, B. D.↗

NASA NTRS1986

Plotting Lightning-Stroke Data

Data on lightning-stroke locations become easier to correlate with cloudcover maps with aid of new graphical treatment. Geographic region divided by grid into array of cells. Number of lightning strokes in each cell tabulated, and value representing density of lightning strokes assigned to each cell. With contour-plotting routine, computer draws contours of lightning-stroke density for region. Shapes of contours compared directly with shapes of storm cells.

Tatom, F. B.↗