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Richard E. Orville

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Richard E. Orville

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An optical multichannel analyzer slit spectrometer coupled to a minicomputer was used to record lightning spectra. This is the first successful application of a slit spectrometer to the study of individual lightning flashes and it was accomplished in the daytime. Over 300 spectra were obtained in 1978 and 1979 and are correlated with other experiments in the Thunderstorm Research International Program (TRIP). The spectra duplicate previously published nighttime data but reveal for the first time the relative intensity of H-alpha (656.3 nm) and H-beta (486.1 nm) emissions above their daytime absorption features. These are the characteristic Fraunhofer C and F lines in the solar spectrum. This result suggests that the observation of lightning from space may be accomplished by monitoring the hydrogen emissions from lightning which occur on Earth, or on other planets with hydrogen in their atmospheres, such as Jupiter and Venus where lightning recently has been reported.

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Richard E. Orville

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Richard E. Orville

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A model of the lightning return stroke is developed using the experimental data presented in Parts I and II and Gilmore's tables for the composition of dry air in thermodynamic equilibrium. The temperature, electron density, pressure, relative mass density, per cent ionization, and specie concentration in a 10-m section of a model-return stroke are given with 5-μsec resolution from 0–30 μsec. During this period, the temperature decreases from 30,000 to 16,000 K and the electron density decreases from 1018 to 1.5 × 1017 cm−3. The channel is characterized by an average pressure of 8 atm in the first 5 μsec and attains atmospheric pressure at approximately 20 μsec. A minimum relative mass density of 3×10−2 is attained at the same time. Per cent ionization is on the order of 100 in the first 15 μsec and then decreases.

The largest nitrogen specie concentration in the 0-15 μsec period is NII, followed by N-III, which in turn is followed by NI, All three concentrations decrease in the 0-15 μsec time period. In the 15-30 μsec period the NI concentration increases, the NII concentration attains a quasi-equilibrium, and the NIII concentration continues to decrease rapidly. The salient characteristics of the model return stroke are discussed and related to spectral observations.

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Richard E. Orville

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The first time-resolved spectra of return strokes between the cloud and ground have been obtained. During the summers of 1965 and 1966 twenty-two spectra were obtained at the Institute of Atmospheric Physics, Tuscon, Ariz. The spectra were recorded with two high-speed streaking cameras converted to slitless spectrographs. The conversion was accomplished by mounting Bausch and Lomb replica transmission gratings in front of the cameras' objective lenses. The gratings are blazed for 5500 Å and have 600 lines mm−1. Inverse dispersions from 70'140 Å mm−1 were used. Most of the data were obtained with a Beckman and Whitley high-speed camera. A 200-mm objective lens was used to focus the return stroke on a 0.5-mm horizontal slit. Thus, a 10-m section of the lightning channel was isolated for a discharge occurring at a distance of 4 km. Data have been obtained with a time resolution of 2-5 μsec. All spectra have been recorded on film calibrated for intensity and wavelength with a xenon source of known relative spectral emittance.

The following data have been obtained. Spectral emissions from 4000–6600 Å have been recorded with 10 Å wavelength resolution. All emissions have been attributed to neutral hydrogen or to neutral or singly ionized atoms of nitrogen and oxygen. No molecular or doubly ionized emissions have been identified in these spectra. The time for luminosity to rise from zero to its peak in a section of the channel is 10 μsec or less. Several faint lines due to neutral nitrogen and oxygen atoms persist for 150 μsec. The H-alpha line is present in these spectra. The recorded time sequence of spectral emissions from a section of the lightning channel is 1) line radiation from singly ionized atoms, 2) continuum, and 3) line radiation from neutral atoms. A flash has been recorded composed of at least 5 strokes. Two types of strokes are observed in this flash. The first type is characterized by intense short-lived emissions from singly ionized nitrogen atoms (NII) and a long lasting H-alpha emission. Continuum emission is relatively weak. In the second type, the singly ionized nitrogen emissions (NII) persist for a relatively long time and the H-alpha emission is very intense but short-lived. Continuum emission is relatively strong.

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Richard E. Orville
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Richard E. Orville

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The National Lightning Detection Network, composed of 114 wideband magnetic direction finders for locating cloud-to-ground lightning flashes, was operated with full coverage of the contiguous United States for the first time in 1989. More than 13.4 million flashes were recorded during that year. Ground flash density contours were drawn on a grid with 120 horizontal points and 100 vertical points. This produces a flash density resolution of 50 km in the east-west direction and 30 km in the north-south direction. The peak lightning flash density occurred northeast of Tampa, Florida, with yearly values of 10 km−2. An annual flash density of 8 km−2 was recorded over the Gulf Stream off the Carolina Coast. Local flash density maxima were observed in eastern Texas, Kansas, on the Illinois-Indiana border, and inland along the Carolina Coast extending into Virginia.

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Richard E. Orville

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The first significant results of a long detailed lightning study are reported in which the distribution of global midnight lightning is plotted from 60°S to 60°N for the months of September. October and November 1977. Lightning positions obtained from DMSP photographic data are plotted with an accuracy of ∼100 km and the cumulative plots show a remarkable agreement with features of the general circulation. Land-ocean lightning flash ratios are calculated to range from 1.5 to 2.0 for the three months, September (1.6), October (1.5) and November (2.0).

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Richard E. Orville

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A quantitative analysis has been completed of the first time-resolved spectra of return strokes. All values refer to approximately a 10-m section of the return-stroke channel. Ten return-stroke spectra, eight with 5-μsec resolution and two with 2-μsec resolution, have been analyzed to determine their temperature-time curves. The peak temperature in five of the ten spectra is in the 28,000–31,000 K range despite the use of different slitless spectrographs and different multiplet intensity ratios for the measurements. The highest peak temperature was calculated to be 36,000 K. Temperature errors are on the order of 10–25%. A temperature rise in two of the strokes has been calculated in the first 10 μsec from data having 5-μsec resolution. The two recorded strokes with 2-μsec resolution have monotonically decreasing temperature-time curves. It is shown that if the number density of a particular emitting species is known, the relative channel radius within which the particular radiators are contained can be calculated as a function of time. The NII radiation reaches peak intensity in 5–10 μsec, the continuum radiation attains maximum within 10–15 μsec, while the H-alpha emission is most intense in the 20–50 μsec period. The effective excitation potential of the continuum radiation lies between that of the ions and the neutrals and may therefore be due to radiative recombination or radiative attachment.

Two spectra with H-alpha emissions have been quantitatively analyzed. The first spectrum shows an increasing intensity to 50 μsec followed by a monotonic decrease. The second H-alpha spectrum attains maximum intensity in 20 μsec, decreases to a local minimum at 35 μsec, and then decreases monotonically after a small maximum at 45 μsec. The second maximum, or luminosity enhancement, is probably associated with a branch providing additional charge to the return-stroke channel. The Stark-broadened half-width of the H-alpha line has been measured as a function of time with 5-μsec resolution. From the half-width measurement an electron density on the order of 1018 cm−3 has been calculated in the first 5 μsec, decreasing to 1–1.5 × 1017 cm−3 in 25 μsec. Errors are on the order of 50%.

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Richard E. Orville

Abstract

A high-speed slitless spectrograph has been used to record photographically with 9 μs resolution the spectral emissions from five dart leaders. All occurred in the same lightning flash which produced at least 13 return strokes. Dart leader spectral emissions in the 398-510 nm region are characterized by singlyionized nitrogen (NII) emissions. These emissions from a 13 m isolated section of the channel decrease in intensity after the passage of the dart. They are, however, continuously recorded between the dart and return stroke spectra. Three dart temperatures are calculated to be approximately 20,000 K±10%. Five dart velocities are calculated; one value (3.4×107 m s−1±20%), exceeds the highest value previously reported in the literature (2.3×107 m s−1). The relative intensity ratio of the return stroke to the dart in the isolated channel section is measured to be 8–9±15% in these data.

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