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Showing papers on "Scintillation published in 1988"


Journal ArticleDOI
TL;DR: In this article, an extensive VHF/UHF scintillation data base covering the frequency range of VHF to a few gigahertz has been utilized to determine the magnitudes of phase and intensity scintillations and their temporal/spatial structures during the sunspot maximum and minimum periods.
Abstract: An extensive VHF/UHF scintillation data base covering the frequency range of VHF to a few gigahertz has been utilized to determine the magnitudes of phase and intensity scintillations and their temporal/spatial structures during the sunspot maximum and minimum periods. The equatorial portion of the study has been based on geostationary satellite observations at Huancayo, a station on the magnetic equator, and at Ascension Island, which is an equatorial anomaly station having an extremely disturbed irregularity environment. The high-latitude part of the study is based on quasistationary satellite measurements at a polar cap location (Thule) and two auroral locations (Goose Bay and Tromso). The Tromso observations are augmented with the Defense Nuclear Agency HiLat satellite beacon measurements during the solar minimum period. The data indicate a strong solar cycle control of scintillation activity at all locations, resulting in a drastic reduction of the magnitudes and occurrence of scintillations during the current solar minimum period. This pattern is consistent with both a reduction of F region ionization density and a reduction of irregularity generation in the solar minimum period. At the magnetic equator the magnitude of scintillations at 1.5 GHz seldom exceeds 3 dB with the percentage occurrence > 2 dB varying from 70% during high sunspot conditions to 30% during low sunspot conditions. At the crest of the equatorial anomaly, on the other hand, during the solar maximum in 1979, fades of 20 dB at 1.5 GHz are observed 30% of the time. At a decreased level of solar activity in 1982, a similar occurrence level is obtained at 1.5 GHz for fade levels of only 5 dB. During the solar minimum period, 1.5-GHz scintillations are virtually absent. Phase scintillation measurements made at Ascension Island indicate that the median value of rms phase deviation is about 5 rad for detrend intervals of 100 s. In the auroral region, during the solar maximum period under magnetically disturbed conditions, the median values of scintillation fades and rms phase deviation (82-s detrend) at 250 MHz are observed to be 15 dB and 3 rad, respectively. At Thule, located deep within the polar cap, the median values of scintillation fades and rms phase deviation at 250 MHz attain values as large as 20 dB and 4 rad during the sunspot maximum period. Unlike Ascension Island the scintillation activity at high-latitude stations exhibits a threshold effect and does not decrease until 1983. However, in 1986 with sunspot numbers in the vicinity of 10, fade levels as low as 5 dB at 250 MHz are recorded in the polar cap and auroral stations only 5% of the time. It is noted that at auroral locations the most prominent feature, namely the existence of magnetic L shell-aligned irregularity sheets, is equally evident at both sunspot maximum and minimum.

306 citations


Journal ArticleDOI
TL;DR: A signal of 5 events within 9.1 s was found in the Baksan scintillation telescope with a fiducial mass of 200 t at the KAMIOKANDE-II and IMB times on February 23, 1987 as discussed by the authors.

296 citations


Journal ArticleDOI
TL;DR: In this article, a method of predicting scintillation fading occurring on Earth-space paths is proposed that is based on the data obtained from a low-elevation 14/11 GHz wave propagation experiment at Yamaguchi, Japan.
Abstract: A method of predicting scintillation fading occurring on Earth-space paths is proposed that is based on the data obtained from a low-elevation 14/11 GHz wave propagation experiment at Yamaguchi, Japan. This model includes parameters representing meteorological conditions that have not been covered in existing models and can be applied to broad regions with varying climates. The prediction accuracy of the method is evaluated using experimental data collected at four sites in the western part of Japan, as well as data reported thus far on a worldwide basis. Although data used for evaluation are limited, the model shows good prediction accuracy for frequencies from 7 to 14 GHz and elevation angles from 4 degrees to 30 degrees . >

216 citations


Journal ArticleDOI
TL;DR: In this paper, a combination of thin-wall glass microspheres filled with high-pressure /sup 3/He gas dispersed in plastic scintillation was used to detect low-energy protons.
Abstract: The authors have been investigating the heterogeneous combination of neutron-conversion materials in a plastic or liquid scintillation matrix with the goal of developing neutron detectors of high detection efficiency and fast response They report on one such combination, consisting of thin-wall glass microspheres filled with high-pressure /sup 3/He gas dispersed in plastic scintillation They have developed glass formulations that are capable of containing the helium in shells with wall thickness as low as 1 mu m with leak rates at room temperature that correspond to several years half life The energy loss of the reaction products in the shell wall and the low light yield of plastic scintillations for low-energy protons result in low light yield per event The authors discuss the use of an optical modeling code to investigate the incorporation of wavelength-shifting elements to aid in the collection of this light from highly scattering composite scintillation media >

189 citations


Journal ArticleDOI
TL;DR: In this paper, the properties of pure CsI have been studied with respect to its application as a scintillation material and its properties have been investigated in terms of its properties and properties.
Abstract: The properties of pure CsI have been studied with respect to its application as a scintillation material.

114 citations


Journal ArticleDOI
TL;DR: In this paper, the phase structure function showed a rolloff at large spacings as was expected because the outer scale of the turbulence and log-intensity and phase difference are Gaussian random variables.
Abstract: Scintillations of intensity and phase difference were measured at millimeter wavelengths in a horizontally homogeneous atmospheric surface layer. Simultaneous micrometeorological and optical propagation measurements characterized the clear-air turbulence. Predicted and measured propagation statistics are in good agreement. It is shown that the phase structure function showed a rolloff at large spacings as was expected because the outer scale of the turbulence and log-intensity and phase difference are Gaussian random variables. The mutual coherence function is exp(-D/2) to great accuracy, where D is the sum of phase and log-amplitude structure functions. Estimating heat and humidity fluxes from intensity variances is shown to be valid. >

82 citations


Journal ArticleDOI
TL;DR: In this article, a quad scintillator dual-channel detector module using two avalanche photodiodes as photodetectors was proposed to achieve nearly isotropic spatial resolution and uniform sensitivity throughout the useful field of view.
Abstract: A novel type of positron camera achieving nearly isotropic spatial resolution and uniform sensitivity throughout the useful field of view is proposed. The design is based on a quad scintillator dual-channel detector module using two avalanche photodiodes as photodetectors. Original features include bilayered scintillators to measure the depth of interaction, tilted detectors relative to the radial direction to ensure proper sampling in a stationary mode, multiring capability, and an independent acquisition channel for every discrete detector. Significantly better resolution is obtained toward the edges of the camera field, and the sampling uniformity near the center is improved. Multiring systems with a slice thickness of 5 mm can be implemented to achieve true volume imaging with symmetrical 3-D spatial resolution. The performance of the proposed design is analyzed in terms of resolution, sensitivity, contrast, and field uniformity. >

75 citations


Journal ArticleDOI
TL;DR: In this paper, the efficiency of counting probabilities of different kinds of scintillation pulses upon energy of beta particles interacting with scintillator is derived and compared for double and triple detectors.

67 citations


Journal ArticleDOI
TL;DR: In this paper, simple models have been used to estimate characteristic height and intensity of tropospheric refractive turbulence from fluctuations of the field amplitude received by two antennas of diverse sizes on the OTS space-Earth path.
Abstract: Simple models have been used to estimate characteristic height and intensity of tropospheric refractive turbulence from fluctuations of the field amplitude received by two antennas of diverse sizes on the OTS space-Earth path. Data refer to particular events of high scintillation, probably associated with the presence of nonprecipitating clouds, and yield elevated values of cn2.

60 citations


Journal ArticleDOI
TL;DR: In this paper, a comparison between the propagation data and ground-level meteorological measurements indicated a high correlation between the scintillation characteristics and the water vapor contribution to the radio refractive index inferred from local humidity and temperature data.
Abstract: Radio waves with frequencies above 10 GHz at low-elevation-angle paths are significantly affected not only by rain but also by atmospheric irregularities in the troposphere The tropospheric scintillation due to irregularities of the refractive index in the troposphere is investigated using 14/11-GHz low-elevation measurements made during 1983 at Yamaguchi, Japan The diurnal and seasonal variations, frequency dependence derived from a comparison of the 11- and 14-GHz signals, and elevation-angle dependence of the scintillation data are presented A comparison between the propagation data and ground-level meteorological measurements indicated a high correlation between the scintillation characteristics and the water vapor contribution to the radio refractive index inferred from local humidity and temperature data This suggests a method for predicting the severity of scintillation fading using local measurements of meteorological parameters >

60 citations


01 Jun 1988
TL;DR: In this paper, the effects of clear-air turbulence as well as atmospheric turbidity on optical communications are reviewed. But, the authors do not consider the effect of opaque clouds on the propagation of light.
Abstract: A number of atmospheric phenomena affect the propagation of light. This article reviews the effects of clear-air turbulence as well as atmospheric turbidity on optical communications. Among the phenomena considered are astronomical and random refraction, scintillation, beam broadening, spatial coherence, angle of arrival, aperture averaging, absorption and scattering, and the effect of opaque clouds. An extensive reference list is also provided for further study, Useful information on the atmospheric propagation of light in resolution to optical deep-space communications to an earth-based receiving station is available, however, further data must be generated before such a link can be designed with committed performance.

Journal ArticleDOI
TL;DR: In this article, translucent scintillator ceramics of Gd2O2S:Pr, Ce, F were fabricated using a hot isostatic pressing (HIP) technique.
Abstract: Translucent scintillator ceramics of Gd2O2S:Pr, Ce, F were fabricated using a hot isostatic pressing (HIP) technique. The optical transmission is about 60% of the incident light and the X-ray stopping power is also quite high. Owing to the enhancement of these properties by the HIP densification process, the highest light output from a 1 mm thick specimen combined with a silicon photodiode, when excited by 120 kV X-rays, has reached 1.8 times that of CdWO4 single crystals. The present ceramics are very promising candidates for scintillator materials in X-ray computed tomography (CT) detector applications.

Journal ArticleDOI
TL;DR: In this article, the drift characteristics of ionospheric irregularities were analyzed for the UHF and L band signals from the Pacific Marisat (50° elevation angle) from April 1982 through May 1, 1983.
Abstract: Ultrahigh frequency satellite signals from the Pacific FleetSat (53° elevation angle) and L band signals from the Pacific Marisat (50° elevation angle) were recorded in Guam, an equatorial station, from April 1982 through May 1, 1983. Three receivers aligned in the magnetic east-west direction were used, with the most widely separated receivers (457.2 m separation) recording the UHF signal amplitudes and the middle receiver recording the L Band signal amplitudes. These data are analyzed for the drift characteristics of ionospheric irregularities. The digitized spaced receiver data are processed to yield the scintillation index, the cross-correlation function and the self-power spectrum. By assuming a locally frozen flow, several methods have been devised to compute the drift velocity, V0, and the characteristic random velocity, VC. These methods seem to show internal agreement. They also show that the velocity fluctuations decrease markedly during the first few hours after local sunset. The spectral index of the self-power spectrum roll-off was also determined during times of both weak and strong scintillation. For the weak scintillation cases, the slopes were observed to gradually steepen as the night progressed into the postmidnight period. This steepening suggests the decaying of smaller-sized irregularities. There were a few exceptions to this observed decay. For the strong scintillation cases, the shape and slope of the self-power spectrum were observed to be influenced by the drift velocity and scattering strength of the irregularities.

Journal ArticleDOI
TL;DR: In this article, a theory of refractive scintillation due to a thin phase-changing screen with an anisotropic power-law spectrum of phase fluctuations is presented, and the application of the theory to the analysis of two occultations by Neptune is addressed.
Abstract: A theory of refractive scintillation due to a thin phase-changing screen with an anisotropic power-law spectrum of phase fluctuations is presented. Scintillation theory for an isotropic medium is discussed, and anisotropy of the mean density and anisotropy in the scattering are discussed. The theory of refractive scintillation in an anisotropic medium is developed, deriving a general expression for the cross-correlation of flux variations at two points on the 'observer screen'. From this, estimates of the coherence lengths and amplitudes of flux variations are obtained for important parameter regimes. The application of the theory to the analysis of two occultations by Neptune is addressed. The projected dimensions of the occulting stars, a normalization constant to describe the phase fluctuations, and an anisotropy parameter are determined, and the theory is shown to agree well with observation. The significance of the theory for understanding of the physics of Neptune's atmosphere is discussed.

Journal ArticleDOI
TL;DR: In this article, high-resolution in situ Dynamics Explorer 2 data on thermal plasma densities are used to study the small-scale irregularity structure of the F layer patches, and spatially discrete density structures associated with polar cap patches can be detected fairly high in the topside by an in situ irregularity sensor and correspond to temporally discrete scintillation patches.
Abstract: High-resolution in situ Dynamics Explorer 2 data on thermal plasma densities are used here to study the small-scale irregularity structure of the F layer patches. It is shown that spatially discrete density structures associated with polar cap patches can be detected fairly high in the topside by an in situ irregularity sensor and that they correspond to temporally discrete scintillation patches. It is also shown that it is possible to model phase and amplitude scintillation occurrence from a knowledge of irregularity amplitude at a satellite altitude of about 800 km provided that independent measurements of the peak density and scale height of the F region are available.

Journal ArticleDOI
TL;DR: In this paper, a /sup 137/Cs (662-keV) photopeak resolution of better than 5% FWHM was obtained due to the low noise characteristics and high quantum efficiency of the mercuric iodide photodetector.
Abstract: Scintillation spectrometers with mercuric iodide as the photodetector were evaluated for their spectral response to gamma rays in the energy range of 60 keV to 1332 keV. A /sup 137/Cs (662-keV) photopeak resolution of better than 5% FWHM was obtained due to the low noise characteristics and high quantum efficiency of the mercuric iodide photodetector. Photopeak broadening due to the scintillator, electronic noise, and statistical noise vs. gamma energy are detailed. >

Journal ArticleDOI
TL;DR: In this paper, the authors detected scintillation photons emitted by LaF3 single crystals doped with Nd3+ in a multiwire chamber filled with TMAE vapour.
Abstract: Scintillation photons emitted by LaF3 single crystals doped with Nd3+ have been detected in a multiwire chamber filled with TMAE vapour. This scintillation (λ = 173 nm) is ascribed to a 5d–4f transition of Nd3+ and is characterised by a decay time of 6.3±0.5 ns.

Journal ArticleDOI
TL;DR: In this article, the amplitude and phase scintillation measurements have been made at a high-latitude site over northern Europe during a 2-year period, with more than 12,000 satellite passes being monitored to yield in excess of 400,000 values of each of the S4 and στ indices.
Abstract: Amplitude and phase scintillation measurements have been made at a high-latitude site over northern Europe during a 2-year period. Transmissions from the polar-orbiting multisatellite Navy Navigation Satellite System have been used to achieve both latitudinal and temporal coverage of the observations, with more than 12,000 satellite passes being monitored to yield in excess of 400,000 values of each of the S4 and σф indices for amplitude and phase scintillation respectively. Results are presented of scintillation occurrence for season, time of day and geomagnetic activity. A premidnight maximum and more scintillation activity in summer and autumn than in winter and early spring are apparent. The results demonstrate the importance of propagation geometry to high-latitude scintillation measurements, and estimates of the geometrical factors in both S4 and σϕ, computed for a model using different irregularity anisotropies, are discussed.

Journal ArticleDOI
TL;DR: In this paper, the first synoptic study of Doppler scintillation transients caused by interplanetary disturbances was conducted, where a total of 148 separate transients covering a heliocentric distance range of 5-179 solar radii were detected, including 26 transients detected by more than one spacecraft.
Abstract: This paper conducts the first synoptic study of Doppler scintillation transients caused by interplanetary disturbances. The Doppler scintillation data used are part of the 2.3-GHz navigation data collected by the NASA Deep Space Network when tracking planetary spacecraft during 1979-1983, a period that includes solar maximum. A total of 148 separate transients covering a heliocentric distance range of 5-179 solar radii were detected, including 26 transients detected by more than one spacecraft. The frequency of occurrence was highest near the sun and decreased with radial distance, a reflection of the radial evolution of the transients and the sensitivity of the Doppler scintillation measurements to transients. Since transients can be disruptive, as was demonstrated during the encounter of Saturn by Pioneer 11 in 1979, information on Doppler scintillation transients is essential.

Proceedings ArticleDOI
27 Jun 1988
TL;DR: Modular scintillation cameras are gamma cameras with relatively small crystal faces, a small number of photomultiplier tubes (PMTs), and independent processing electronics and logarithmic matched filtering and likelihood windowing are introduced, two processing techniques that result from exploitations of the Poisson model of the distribution of photopeak events.
Abstract: Modular scintillation cameras are gamma cameras with relatively small crystal faces, a small number of photomultiplier tubes (PMTs), and independent processing electronics. Our prototypical module has a 10 cm square crystal face, four PMTs, and digital processing electronics. Scintillation event information is transferred to images by mapping digitized PMT response combinations to optimal position estimates of event locations. In our prototype, a look-up table is used to perform this mapping. To encode scintillation event information more effectively, we use nonlinear compression of each of the PMT signals. Also introduced are logarithmic matched filtering and likelihood windowing, two processing techniques that result from exploitations of the Poisson model of the distribution of photopeak events. Logarithmic matched filtering is a method of obtaining estimates of mean detector response functions having greater accuracy than that indicated by the digitization of the PMT responses. Likelihood windowing is the utilization of a likelihood threshold, rather than the familiar energy window, as a discriminant of photopeak and scatter events. We have implemented each of the above on our prototypical module. Performance characteristics of this module include energy resolution of 10% full width at half maximum (FWHM) at 140 keV and spatial resolution of better than 4mm FWHM over 90% of the crystal.

Patent
20 May 1988
TL;DR: In this article, a single photon emission computer tomography (SPECT) embodiment is initialized by storing weighting factors which are derived from placing a source of radiation in a plurality of known locations.
Abstract: A system for locating the position of a source of a detachable emittance is particularly adapted for locating the position of a scintillation event in a scintillation crystal which has arranged adjacent thereto a plurality of photomultiplier tubes arranged in a predetermined configuration for detecting the scintillation event. In a single photon emission computer tomography (SPECT) embodiment, a system which utilizes the present invention is initialized by storing weighting factors which are derived from placing a source of radiation in a plurality of known locations. The output values of the photomultiplier tubes are stored in a calibration file and subjected to a mathematical process to produce the weighting factors which are subsequently stored in a memory. During operation of the system, the signal values from the photomultiplier tubes are combined with the weighting factors to produce weighted signals which are then summed together to produce a composite weighted signal. The addresses of the weighting factors are correlated to position information; the correct position information being determined in response to the composite weighted signal assuming a predetermined value.

Journal ArticleDOI
TL;DR: Estimates of the level of turbulence C(2)(n) and the inner scale of turbulence lambda(0) are obtained from measurements of the spatial covariance of laser scintillation in weak scattering conditions.
Abstract: Estimates of the level of turbulence Cn2 and the inner scale of turbulence λ0 are obtained from measurements of the spatial covariance of laser scintillation in weak scattering conditions. Boundary layer turbulence is wide-sense nonstationary over time scales of <1 min but becomes locally wide-sense stationary on time scales of 2 min. The interpretation of the measurements is very sensitive to the form of the turbulence spectrum in the dissipation region.

Journal ArticleDOI
07 Jan 1988-Nature
TL;DR: In this article, a series of timing measurements at the Arecibo Observatory were conducted to obtain more accurate values for the period and dispersion measure and to determine the period derivative and scintillation parameters.
Abstract: Shortly after the discovery1 of a 39.5-ms pulsar in the supernova remnant CTB 80, we began a series of timing measurements at the Arecibo Observatory to obtain more accurate values for the period and dispersion measure and to determine the period derivative and scintillation parameters. Data acquired over a span of 11 days yielded a surprisingly small period derivative, Ṗ = (5.92 ± 0.06) x 10− 15 s s− 1. If the pulsar slows by magnetic dipole radiation, this spin-down rate implies a surface field strength of only 5 x 1011 gauss yet the pulsar is believed to be much too young for its magnetic field to have decayed. Scintillation observations reveal unusually rapid fluctuations, indicating a large pulsar transverse velocity. Here we describe the timing and scintillation measurements and their results, and briefly explore some astrophysical consequences.

Patent
02 Mar 1988
TL;DR: In this article, a high-temperature well logging instrument of the scintillation detector type includes a metal retaining cup with the SCI inside, capped at its open end by a silicon rubber pad that is transparent to photons generated by the scI in response to detected radiation.
Abstract: A high-temperature well logging instrument of the scintillation detector type includes a plastic scintillation element susceptible to softening and deformation at high operating temperatures. To maintain the shape of the scintillation element at such high temperatures, a metal retaining cup holds the scintillation element in its original shape. The retaining cup with the scintillation element inside is capped at its open end by a silicon rubber pad that is transparent to photons generated by the scintillation element in response to detected radiation. A biasing spring holds the end face of a photomultiplier tube against the silicon rubber pad so as to sandwich the pad between the photomultiplier tube and the open end of the metal retaining cup. Such a configuration maintains excellent optical coupling between the scintillation element and photomultiplier tube during high-temperature well logging.

Journal ArticleDOI
01 Nov 1988
TL;DR: The significance of tilt anisoplanatism is established, and a measurement theory, based on aperture-averaging intensity scintillation, is developed in this article, which is a direct extension of the technique currently used to determine the isoplanatic angle as defined by Fried et al.
Abstract: The significance of tilt anisoplanatism is established, and a measurement theory, based on aperture-averaging intensity scintillation, is developed. The theory is a direct extention of the technique currently used to determine the isoplanatic angle as defined by Fried [ J. Opt. Soc. am.72, 52 ( 1982)]. By using this theory, a physically realizable binary aperture-weighting function is derived for a particular case of interest. It is noted that direct quantitative measurements of tilt anisoplanatism can also be made, under specific circumstances, by tracking the relative centroid motion of a binary star pair. Thus independent verification of the remote-sensing theory for tilt anisoplanatism, based on aperture-averaging scintillation measurements, should be possible.

Journal ArticleDOI
Zs. Kajcsos, Ch. Sauer, A. Holzwarth, R. Kurz, W. Zinn, M. A. C. Ligtenberg1, G. Van Aller1 
TL;DR: In this paper, a low-noise scintillation counter has been developed for the detection of low-intensity, low-energy (several keV) electrons in high vacuum.
Abstract: A low-noise scintillation counter has been developed for the detection of low-intensity, low-energy (several keV) electrons in high vacuum. Performance comparisons with a channeltron for 57 Fe depth-selective conversion electron Mossbauer spectroscopy (DCEMS) show a substantial improvement in counting time in favour of the scintillation detector.

Journal ArticleDOI
TL;DR: It is shown that at practically available sample sizes the region of the most probable values of the estimated higher-order moment is almost independent of the scintillation probability density function (PDF).
Abstract: The statistical distribution of the experimentally obtained higher-order moments of optical scintillation probability density is studied. It is shown that this distribution is strongly dependent on the size of the data sample. At reasonable sample sizes the correct estimation of the theoretical value is improbable. At practically available sample sizes the region of the most probable values of the estimated higher-order moment is almost independent of the scintillation probability density function (PDF). The distinction between the candidate PDFs is almost impossible at reasonable sample sizes.

Journal ArticleDOI
TL;DR: Spatial resolution has been achieved with a simple pulse shape discriminator in the new scintillation material CsI investigated for a position sensitive detector which is used together withCsI(Tl) coupled to a common PMT.
Abstract: The new scintillation material CsI has been investigated for a position sensitive detector which is used together with CsI(Tl) coupled to a common PMT. Spatial resolution has been achieved with a simple pulse shape discriminator. Combinations of CsIBGOCsI(T1) and CsIBaF 2 CsI(T1) are also shown to be useful as position sensitive detectors.

Journal ArticleDOI
TL;DR: In this article, a deuterated anthracene scintillation crystal and a pulse shape discriminator are used to select forward recoiling deuterons by means of their pulse shape signatures, making use of the direction dependence of the pulse height response and scindillation decay characteristics of the crystal.
Abstract: A neutron spectrometer consisting of a deuterated anthracene scintillation crystal and a pulse shape discriminator is described. Forward recoiling deuterons are selected by means of their pulse shape signatures, making use of the direction dependence of the pulse height response and scintillation decay characteristics of the crystal. The line shape of the spectrometer for monoenergetic neutrons is a single peak and the neutron energy resolution (FWHM) varies from 7% at 9 MeV to 3.5% at 22 MeV neutron energy.

Journal ArticleDOI
TL;DR: Calculations of the scintillation index are presented that result from a plane wave of light impinging on the atmosphere using the two-scale theory and assuming a Kolmogorov structure function with an altitude-dependent structure constant.
Abstract: We present calculations of the scintillation index that result from a plane wave of light impinging on the atmosphere. Calculations are made using the two-scale theory and assuming a Kolmogorov structure function with an altitude-dependent structure constant. A plot of the scintillation index for typical nighttime viewing is given as a function of angle from the zenith. A further approximation is then introduced which allows the calculation of the scintillation index in terms of a single nondimensional parameter. This parameter is a direct generalization of the parameter used when the structure constant is an absolute constant.