Showing papers on "Zenith published in 1977"

Standard distributions of clear sky radiance

Abstract: Diffuse radiation from cloudless skies varies with solar height and atmospheric turbidity but its angular distribution remains generally characteristic of single scattering. It was found that when sky radiance measurements, N, were normalized with respect to horizontal diffuse irradiance, Sd, the angular distributions of N/Sd were remarkably independent of atmospheric turbidity. This enabled standard distributions of normalized sky radiance, based on a long series of measurements on clear skies, to be drawn up for different solar zenith angles. Due to the difference between radiometric and photometric units these distributions are not comparable with previously published luminance distributions. Departures due to variation in atmospheric turbidity, of the radiance distributions from the proposed standards, were analysed and found to be small for the range of turbidities (τα from 0·1 to 0·5) normally encountered in Britain.

Atmospheric transmittance of an absorbing gas. 2: A computationally fast and accurate transmittance model for slant paths at different zenith angles.

Abstract: Models exist which allow the calculation of atmospheric transmittance at a given zenith angle for an absorbing gas with a constant mixing ratio. However, many applications require transmittances at several zenith angles. A simple, fast, and accurate model for calculating the angular dependence is given. This model is computationally fast because only the four arithmetic operations are used. Details for calculating the expansion coefficients are provided. When this technique is combined with a procedure for calculating transmittances at a fixed angle, it is possible to calculate transmittances for slant paths at arbitrary zenith angles and temperature profiles, provided the mixing ratio is constant. This technique was evaluated with a method capable of calculating transmittances at zero zenith angle with an accuracy of 0.0031. For zenith angles ranging from 0° to 40°, transmittances agree with line-by-line calculations to within 0.0038.

Conjugate photoelectron fluxes observed on Atmosphere Explorer C

Abstract: The results are presented of a number of observations made with the aid of the Atmosphere Explorer C spacecraft under conditions which provided an unambiguous opportunity to study photoelectrons entering the atmosphere after flowing along the geomagnetic field from the magnetic conjugate point. A graph is presented with energy spectra of electrons observed precipitating into the atmosphere when the magnetic conjugate point was illuminated. Another graph shows the average energy spectra of electrons observed at 263 km for a range of solar zenith angles when the zenith angles at the magnetic conjugate point varied from 100 to 81 deg.

Atmospheric CO2 and N2 vibrational temperatures at 40‐ to 140‐km altitude

Abstract: Non-LTE (local thermal equilibrium) calculated vibrational temperatures of CO2 and N2 in the altitude z region 40 < z < 140 km are presented in this paper. Improvements in these calculations over those we have reported previously include (1) improved rate constants, (2) a detailed radiative transport treatment for the ν1ν2 modes of CO2, (3) the use of the most recently determined efficiency (33%) for the excitation of N2≠ via O(¹D) + N2 → ³P + N2≠, (4) the radiation transport in the weak CO2 ν3 bands, and finally, (5) a better selection of altitude mesh points in order to achieve numerically more accurate solutions and to extend the lower boundary of the calculation ZL from 50 down to 40 km. The improved calculation for TN2 and T001 are in good general agreement with our previous publications. The change in the efficiency for excitation of N2≠ by quenching O(¹D) made the most noticeable change in our most recent calculations: absorption of 4.3-µm sunlight in the weak bands also impacted these calculations significantly. The ν1ν2 calculations provide a decent fit to 15-µm zenith radiance data for altitudes z ≲ 95 km. At higher altitudes it will be necessary to account for thermal line and band broadening in order to achieve a better fit to the data. The thermal broadening increases rapidly as a function of increasing altitude in the lower thermosphere. With the exception of the fluorescent bands the qualitative behavior of the weak CO2ν3 band upper state vibrational temperatures Tν3j relative to TN2 and T001 may be understood on the basis of the relative efficiency of these bands for radiation to space in comparison with the efficiency of the optically thicker, strong ν3 band, the 12C16O2 (or 626) 001–000 fundamental. These weak bands contribute much more than the fundamental to 4.3-µm zenith radiance in the altitude range z ≲ 70 km. With regard to the 4.3-µm earth limb the nonfluorescent weak bands can be expected to contribute more than the fundamental for z ≲ 85 km, but the CO2 2.7- to 4.3-µm fluorescence of sunlight will still dominate the non-LTE 4.3-µm earth limb below 95 km. Resolved measurements of zenith or earth limb radiance in the fluorescent bands may be used to obtain information on the extent to which the upper levels of the hot 15-µm bands 1000, 02²0, and 0200 are in LTE among themselves in the mesosphere and lower thermosphere. This information would be useful for understanding non-LTE atmospheric radiative cooling near 15 µm.

Determination of the decameter wavelength spectrum of the quiet Sun.

Abstract: The Teepee Tee array of the Clark Lake Radio Observatory has been used to compare the flux of the Sun with that of the sidereal sources Tau A and Vir A at several frequencies in the range 109.0–19.0 MHz. Only the two central banks of the E-W arm of the array were used as elements of a phase switched interferometer so that the Sun could be observed as a point source and compared directly to the sidereal sources. The Sun was still partially resolved however, and appropriate corrections for this effect were made. The observations were taken at times when the Sun and either Tau A or Vir A were at the same declination. We have therefore been able to derive the values for the solar flux, without having to resort to a gain vs zenith distance correction. The observations, combined with those available in the literature, allow us to derive an accurate meter and decameter wavelength spectrum of the quiet Sun.

Apparatus for erecting and aligning articles

Abstract: Elongated articles are randomly received through the open top of a housing in which an inclined disc is rotatably mounted. The disc has a circumferential edge adjacent the housing and the edge defines equidistantly spaced openings capable of permitting article to pass therethrough into shafts associated therewith and descending therefrom. A stationary annular support plate is mounted in a plane immediately below the plane of the disc and in registry with the openings, the support plate defining an opening at the zenith of the disc for permitting communication between a respective one of the openings in the circumferential disc edge and the shaft associated therewith. This shaft has a perpendicular axis whereby the randomly received articles gravitate to the nadir of the disc, are revolved by the rotating disc to the zenith while supported in the openings on the annular support plate and pass sequentially through the opening at the zenith into the shaft while being erected therein. A horizontal elongated conveyor receives the sequentially erected articles from the shaft at the zenith and transports them in alignment away from the shaft.

Rocketborne measurement of mesospheric H2O in the auroral zone

Abstract: Infrared emission spectra of the long-wavelength wing of the 6.3 µm water band between 6.7 and 7.6 µm have been measured in the mesosphere using a cryogenic rocketborne spectrometer. The resulting zenith radiance profile has been compared with a high altitude radiance model resulting in a volume mixing ratio of 3.5 ± 2.2 ppm between 49 and 70 km.

Theoretical zenith angle distribution of atmospheric MeV gamma radiation at high balloon altitudes

Abstract: A simple model is developed in order to interpret the recently measured zenith angle distribution of MeV γ rays in the upper atmosphere, which shows a maximum of intensity at angles around 120° (upward moving direction). Because γ ray production by electron bremsstrahlung is by far the most important process in this energy range, an attempt has been made to derive the γ ray source functions within the atmosphere from measured electron fluxes. The transport of γ rays from their source position to the detector altitude is simulated in a Monte Carlo calculation. The calculation shows that the observed maximum around zenith angles of 120° is caused by those γ rays which were originally ejected preferentially in the horizontal direction but which were Compton-scattered on their way to the detector. Because the mean Compton scattering angle and the scattering probability depend on energy, the maximum in the zenith angle distribution varies with energy: with increasing energy it is more and more shifted toward 90°.

Very-high-resolution far-infrared measurements of atmospheric emission from aircraft

Abstract: Instrument problems and technical results are discussed for an experiment in which an absolute spectrometric radiometer was flown aboard a NASA aircraft at altitudes of 33,000 to 41,000 ft to measure atmospheric emission in the spectral range from 5 to 40 kaysers with a resolution of about 0.03 kayser apodized. The instrument used was actually a polarizing interferometer, and the atmosphere was observed at fixed zenith angles constant to within plus or minus 0.1 deg. The only problem noted was the spoilage of some interferograms by spikes due to electrical interference from the aircraft radio transmission system. A spectrum of the atmospheric-emission brightness temperature obtained with real-time calibration is examined, and the spectral resolution, S/N ratio, and total instrument efficiency are evaluated. The experimental S/N ratio is estimated to be between 40 and 100.

Remote Sounding of Cloudy Atmospheres. III. Experimental Verifications

Abstract: The cloud-filtering technique developed in Parts I and II of this study is experimentally verified in this paper. The verification is based on radiance data measured in the 4.3 and 15 micron CO2 bands using a multidetector sounder mounted on an aircraft. The results presented here show that, from the aircraft height of 7.6 km and in the presence of multiple cloud formations, it is possible to recover simultaneously: (1) the clear-column atmospheric temperature profile with an rms error of 1 K with respect to radiosondes, (2) the land and sea surface temperature at all sun zenith angles. The accuracy of the recovered sea-surface temperature is 0.5-1 K with respect to measured bucket temperatures, (3) the humidity profile (water vapor mixing ratio) with a precision of 10%, (4) the fractional covers and heights of up to three cloud formations, and (5) the types of clouds, i.e., whether convective or nonconvective.

Temperature sounding from the absorption spectrum of CO2 at 4.3 microns

Abstract: A new method is described for obtaining the temperature profile in the stratosphere and lower mesosphere from observations of the absorption spectrum of the high J lines of carbon dioxide at 4.3 microns. This concept is based upon the measurement of the integrated absorption of individual CO2 lines whose strengths depend strongly on temperature and that the absorption of these lines are obtained from measurements of the solar or stellar spectrum through an atmospheric path. The technique involves a rapidly converging iterative process in which the equivalent widths of the individual vibration-rotation lines of CO2 are used. Theoretical calculations are presented for balloon and satellite observations using a model atmosphere. Experimental results are given from spectra obtained with a balloon-borne Fourier interferometer spectrometer in which the sun was observed at low zenith angles. The experimental results are compared to rocketsonde data.

Minimizing atmospheric dispersion effects in compensated imaging

Abstract: A compensated imaging system that corrects optical path length distortions due to atmospheric turbulence by means of an achromatic corrector will have residual errors caused by the dispersion of the atmosphere. These errors become significant for astronomical objects at large zenith angles, but they may be minimized by special dispersion correctors.

Application of statistical inversion to ground-based microwave remote sensing of temperature and water vapor profiles

Abstract: Surface-based observations of downwelling microwave thermal emission are related to temperature and humidity profiles via a standard integral equation of radiative transfer. Both in clear and in cloudy atmospheres, statistical inversion techniques are used to retrieve profiles from a data vector of brightness observations and surface meteorological constraints. For the clear case, accuracy predictions and profile retrievals are illustrated for: (1) single frequency angular scanned data; (2) multi-frequency angular scanned data; and (3) multi-frequency zenith data. For the last case predicted and achieved accuracies were compared in a recently conducted radiometric experiment. Retrievals of cloud contaminated radiometric data are elaborated.

Daytime r(0) evaluated from vertical microthermal measurements.

Abstract: The usefulness of parameter r(0) for describing the effect of atmospheric turbulence on wavefront distortion sensitive optical systems has been widely discussed. In this paper we present in situ daytime r(0) measurements (5500 A, Zenith propagation) obtained from balloon-borne microthermal radiosondes. We find that the parameter r(0) is log normal distributed at various levels aloft and that r(0) follows a power law in the 100-10,000-m range. Comparing our results with nighttime ground based observations we conclude that local turbulence prevails in determining image quality.

Calculations of Antenna Temperature, Horizontal Path Attenuation, and Zenith Attenuation Due to Water Vapor in the Frequency Band 150-700 GHz

Abstract: The results of calculations of antenna temperature at zenith, both with and without the sun viewed as a source, are given. Horizontal path and total zenith attenuation are also calculated. Each of these calculations was made over the frequency band 150-700 GHz, using data from the 24 water-absorption lines between 150 and 1000 GHz.

Diurnal rhythm corticosterone in fasted rats.

Abstract: The diurnal rhythm of plasma and adrenal corticosterone level was examined in fed male Wistar rats and in these fasted for 24 and 48 h. In fed animals a remarkable diurnal rhythm of both parameters was found showing a nadir at the beginning and a zenith at the end of light period. After 24 h of fasting the zenith was shifted to the beginning of dark period without any other changes. After 48 h of fasting remarkable shifts were found resulting in a nadir at the beginning of dark period and a zenith at the middle of light period.

Studies of the angular distribution and of the altitude dependence of the cosmic-ray muon and general components using scintillation telescopes

Abstract: The data of measurements taken near Elbrus in the (600÷1000) mb atmospheric-depth range are used to study the angular distribution and the altitude dependence of the cosmic-ray general and muon components. The measurements were taken with a system of scintillation telescopes at 0°, 30°, 45° and 60° zenith angles and at 100°, 80°, 65° and 40° apertures, respectively. The muon component was isolated with a 10 cm thick lead screen. The angular distribution was set in the formI(θ,h)=I0(h)[cos θ]n(h), where θ is the zenith angle,h is the atmospheric depth. Relations have been obtained which permit the parametern(h) to be plotted on the basis of the measured values of cosmic-ray intensity at various depths by using the above-mentioned four telescopes. It has been found that, ash varies from 600 to 1000 mb,n varies from 0.6 to 1.6 for the muon component and from 1.2 to 1.9 for the general component. The found altitude dependences permit the barometric coefficients to be determined for the various detection directions as functions of the pressure at the observation level.

Zenith Angle Variation of Satellite Thermal Sounder Measurement.

Abstract: : A simple procedure is presented for normalizing satellite spectral radiance measurements taken at zenith angle theta to corresponding values which would have been observed at zero zenith angle. From simulated data for the DMSP SSH sounder, it is shown that the variation of observed radiance with zenith angle is approximately linear when taken as a function of the square root of the secant of the angle. This relationship is employed to develop the correction formula. (Author)

Undistorted three-dimensional image projection - uses photograph taken through fish eye lens and projected onto hemispherical screen surface

Abstract: The scene is to be photographed through a fish-eye lens which provides the transformation y = c sin THETA, where c is a constant, THETA the zenith distance of the point considered, and y the distance of the image of the point from the image of the zenith. The resulting diapositive is to be projected on to a spherical (hemispherical) surface. Projection may be either in to the convex or the concave side of the surface, and is preferably on to a transparent 1 m hemisphere, on the side remote from the observer.

Zenith photoelectric measurements during the annular solar eclipse of April 29, 1976

Abstract: Measurement of the light intensity in the zenith, obtained during the annular solar eclipse of April 29, 1976 are given in three wavelength bands (centered on λ's 4800, 5400 and 6100 A). The observed differences in the three wavelength regions have been examined and compared with calculations taking into account limb darkening.

Measurements of Net Atmospheric Irradiance in the 0.7- to 2.8-Micrometer Infrared Region.

Abstract: : The objective of this experiment was to measure atmospheric irradiance within the 0.7- to 2.8-micrometer infrared wavelength region from a balloon platform at altitudes ranging from 5 to 39 kilometers. An Epply precision spectral pyranometer pointed in a general downward direction and equipped with a 7-degree field of view field stop was successfully employed to collect irradiance data for a period of 34 hours on 23, 24, and 25 September 1976. The results showed that at low solar zenith angles and in the absence of clouds the atmospheric irradiance values usually remained within 2.2 x 1/100 cal/sqcm min and 2.6 x 1/100 cal/sqcm min. Clouds attenuated the irradiance down to magnitudes below 4 x 1/10000 cal/sqcm min which was the combined noise level of the pyranometer and associated electronics. At high solar zenith angles, specular reflections of sunlight which entered the pyranometer's field view caused enhancements of the measured irradiance to reach magnitudes greater than the instrument could measure (3.5 x 1/100 cal/sqcm min). No significant variations in irradiance were detected with changes in altitude above 5 kilometers or terrain viewed; therefore, the background intensity is attributed to the radiant emittance of the lower atmosphere.

A study of high-energy cosmic-ray interactions at sea level. I. Experimental and interpretation

Abstract: The burst-size spectrum due to cosmic rays of energy approximately 1 TeV interacting in a lead target at sea level has been measured. The zenith angle distribution of the particles initiating the bursts has also been determined. The results are compared with predicted spectra based on theoretical calculations. No unusual features are observed and it appears that the observed data could be explained in terms of hadronic and muonic fluxes at sea level. However, the measured hadronic contribution is higher than that predicted by the calculations.