On the inversion of O(+)(2D-2P) 7320 A twilight airglow observations - A method for recovering both the ionization frequency and the thermospheric oxygen atom densities
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Mcdade, I. C.↗
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A new spherical model for computing the radiation field available for photolysis and heating at twilight
Accurate computation of atmospheric photodissociation and heating rates is needed in photochemical models. These quantities are proportional to the mean intensity of the solar radiation penetrating to various levels in the atmosphere. For large solar zenith angles a solution of the radiative transfer equation valid for a spherical atmosphere is required in order to obtain accurate values of the mean intensity. Such a solution based on a perturbation technique combined with the discrete ordinate method is presented. Mean intensity calculations are carried out for various solar zenith angles. These results are compared with calculations from a plane parallel radiative transfer model in order to assess the importance of using correct geometry around sunrise and sunset. This comparison shows, in agreement with previous investigations, that for solar zenith angles less than 90 deg adequate solutions are obtained for plane parallel geometry as long as spherical geometry is used to compute the direct beam attenuation; but for solar zenith angles greater than 90 deg this pseudospherical plane parallel approximation overstimates the mean intensity.
Dahlback, Arne↗
Recent observations of the OI 8446 A emission over Millstone Hill
Evening twilight spectra of the OI 8446 A emission were obtained during May and June of 1993 using a single-etalon, pressure scanning, Fabry-Perot interferometer located in the Millstone Hill Optical Facility. The goals of this work are to positively identify the 8446 A emission in the twilight airglow and to determine the intensity decay as a function of solar depression angle. Also, a study of the relative triplet line strengths is performed in hopes of establishing the importance of the primary excitation mechanisms (photoelectron impact or Bowen fluorescence) during the twilight period. Although absent in most of the data, a distinct auroral influence is also found to contribute considerably, on occasion, to the emission over Millstone Hill. The ratio of the combined 8446.26 A and 8446.38 A intensities to the 8446.76 A intensity varies as 0.13 +/- 0.03 per degree of solar depression angle, indicating that secondary excitation mechanisms are becoming increasingly important as evening twilight progresses. Bowen fluorescence is not found to be the primary excitation mechanism at any time during twilight, contributing just a few Rayleighs at most. These observations are an important first step toward a better characterization of highly variable thermospheric oxygen concentrations through ground-based measurements of the OI 8446 A emission.
Lancaster, R. S.↗
Observations of diurnal variations of BrO and OClO at McMurdo Station, Antarctica (78S)
Observations of the diurnal variations of OClO and BrO during austral spring, 1987 using long-path visible and near-ultraviolet absorption spectroscopy are presented and compared to simplified model calculations. It is shown that care must be taken to compare model calculations and measurements along the line of sight of the instrument. Evening twilight observations of OClO are shown to be broadly consistent with current photochemical schemes, assuming ClO and BrO levels near 50 mb of about 0.5 ppbv and 7 pptv, respectively, throughout the observing period from late Aug. to mid-Oct. Nighttime observations of OClO obtained using the moon as a light source display evidence for growth through the night in late-Aug., but not in late-Sept. Further, the observed morning twilight OClO abundances are in agreement with model calculations in late August, but generally fall below in late September and October. Observations of BrO in mid-Sept. systematically show far greater evening twilight than morning twilight abundances.
Solomon, S.↗
Dust of Orionid meteor shower in the Earth atmosphere before and after Halley's Comet
Among the interesting questions concerning meteor streams associated with Comet Halley is the question of whether or not the activity of a meteor stream was connected with the approach of the comet to the terrestrial orbit in 1985 to 1986. Meteoric aerosols getting to the upper atmosphere can be detected by twilight sounding, as has been done previously. It turns out that not only parameters describing some properties of aerosol can be obtained by twilight sounding, but also characteristics concerning the structure of the stream can be derived. Among the yearly active streams, the Orionides have always attracted the attention of scientists. The period of activity of the Orionides is October 18 to 26, and the maximum stream activity is October 21. In detecting aerosol layers in the terrestrial atmosphere, a notion of the logarithmic intensity gradient of scattered twilight light is used, d log I/dH, where I is intensity and H is the real twilight beam height, which is a function of the wavelength observed. A photoelectric photometer with an interference filter at the wavelength of 610 nm is used. The observation were carried out in two points of the solar vertical; the zenith angle of the observation points was + or - 60 degrees. The recording was carried on continuously in each direction during a minute, then the system was switched to the other direction. A calibration standard was recorded before each observation. The observation dates in the Orionid periods of 1984, 1986, and 1987 are given.
Mateshvili, G.↗
The solar occultation technique for remote sensing of particulates in the earth's atmosphere. I - The inversion of horizon radiances from space
The aerosol scattering coefficient as a function of height can be recovered from a direct inversion of the single-scattering horizon radiance provided the sun is above the horizon and an independent measurement of extinction as a function of height is made. Aerosol detection is effected by means of spacecraft measurements of the horizon radiance made during periods of spacecraft twilight. A solar occultation technique which allows the twilight measurements to be made when the sun is still above the horizon greatly reduces the complexity of the inversion problem. The second part of the paper reports on the use of a coronograph aboard Skylab to photograph the horizon just before spacecraft twilight in order to monitor the aerosol component above the tropopause. The coronograph picture, centered on 26.5 degrees E longitude and 63.0 degrees S latitude, shows that the aerosol layer peaks at a height of 48 plus or minus 1 km.
Schuerman, D. W.↗
Self Validation of Radiance Measurements from the CERES (TRMM)Instrument
Eight continuous months of earth-nadir-viewing radiance measurements from the 3-channel Tropical Rainfall Measuring Mission (TRMM,) Clouds and the Earth's Radiant Energy System (CERES) scanning radiometric measurement instrument, have been analyzed. While previous remote sensing satellites, such as the Earth Radiation Budget Experiment (ERBE) covered all subsets of the broadband radiance spectrum (total, longwave and shortwave.) CERES has two subset channels (window and shortwave) which do not give continuous frequency coverage over the total band. Previous experience with ERBE indicated the need for us to model the equivalent daytime longwave radiance using a window channel regression, which will allow us to validate the performance of the instrument using a three-channel inter-comparison. Limiting our consideration to the fixed azimuth plane, cross-track, scanning mode (FAPS), each nadir-viewing measurement was averaged into three subjective categories called daytime, nighttime, and twilight. Daytime was defined as any measurement taken when the solar zenith angle (SZA) was less than 90 ; nighttime was taken to be any measurement where the SZA was greater than 117 ; and twilight was everything else. Our analysis indicates that there are only two distinct categories of nadir-view data; daytime, and non-daytime (i.e., the union of the nighttime and twilight sets); and that the CERES longwave radiance is predictable to an accuracy of 1%, based on the SZA, and window channel measurements.
Paden, Jack↗
Mariner 9 ultraviolet spectrometer experiment: Afternoon terminator observations of Mars
It is possible to model the ultraviolet terminator observations of Mariner 9 either with a homogeneous atmosphere or with a scattering layer in addition to a homogeneous atmosphere. The characteristic shape of the twilight intensity variation is dependent on the latitude of observation. Observations north of 55N require a homogeneous model. Twilight measurements south of 55N indicate that a scattering layer may be a general Martian occurrence, at least in the evening. The day-to-day surveillance of the Martian terminator during the first 216 Mariner 9 revolutions indicates haze formation in the evening, and its continued presence until early morning is a daily occurrence in mid-latitudes up to 50N.
Ajello, J. M.↗
OH emission intensity measurements during the 1969 NASA Airborne Auroral Expedition
Absolute intensity measurements of the (8, 6) OH band obtained during 10 flights of the December 1969 NASA Auroral Airborne Expedition are presented. Nightglow intensities higher by a factor of 2 than the usual values are recorded during flights 8, 14, and 15. The OH variations are compared with the evolution of the green line and O2(1 Delta g) emissions measured by other experimenters on board the aircraft. Before sunrise the twilight variations of OH down to a solar depression angle of 5 deg show a rapid decrease. A theoretical prediction of the OH, O I 5577 A, and O2(1 Delta g) emissions is evaluated by means of an extensive time-dependent oxygen-hydrogen model of the 25- to 150-km region. Twilight decrease of the OH emission is interpreted in terms of mesospheric ozone photodissociation. Nighttime variations of the emissions may be reproduced if modifications of the dynamic regime are introduced into the model.
Moreels, G.↗
Ca(+) emission in the sunlit ionosphere
In the course of a program of twilight airglow observations, the resonance fluorescence emissions from calcium ions were measured. In particular, twilight sequences were obtained during the period of December 19-22, 1987, which coincided with the Ursids meteorite shower. During this meteorite event the intensities of the Ca(+) emission lines at 3934A increased to the point that the surface brightness profiles could be inverted to volume emission rate profiles. These profiles show evidence for strong spatial redistribution of the Ca(+) over the course of three days. Observations of these emissions during meteor shower periods could provide a valuable tracer for the processes responsible for the transport of ions in the D, E, and F region, allowing the full altitude and latitude extent of the distribution to be determined.
Torr, Marsha R.↗
A procedure for the extraction of airglow features in the presence of strong background radiation
A technique is developed that can be used to derive the total intensity of band emissions from twilight airglow measurements when the basic spectral signature of the band to be considered is known. The method is designed to automatically extract total band or line intensities of a signal imbedded in background radiation several orders of magnitude greater in brightness. It is shown that the technique developed can reliably measure the intensity of both weak and strong band and line emissions in the presence of strong twilight background radiation. The method of extraction is shown as part of a general purpose spectral analysis program written in VAX FORTRAN. This extraction procedure has been used successfully on emissions of Fel, Ca(+), N2(+) (1N) (0-0) and (0-1), OH in the near UV. OI red (630nm) and green (558nm) lines in the visible, and the OH Meinel bands and O(+) (2P) 732 nm in the near IR.
Swift, W. R.↗
Southern Hemisphere Lidar Measurements of the Aerosol Clouds from Mt. Pinatubo and Mt. Hudson
On 19 Jul., 1991, during tests to determine the ability of the newly-modified CSIRO Ns:YAG lidar to measure signals from the stratosphere before the arrival of dust from the eruption of Mt. Pinatubo, a strongly scattering layer was detected at an altitude of 2 km. That evening, the spectacular sunset and twilight were typical of volcanically disturbed conditions. Lidar measurements at 532 nm were made between 1400 and 1500 EST (0400-0500 UT) on 19 Jul. through broken cloud. Approximately 3800 laser firings were averaged in 256 shot blocks. These and subsequent data have been analyzed to produce profiles of aerosol volume backscatter function and scattering ratio. Clouds again prevented a clear view of the twilights on the next two nights, although there was some evidence for an enhanced glow. The evidence suggested that the aerosol layer had disappeared. An explanation for this disappearance and the earlier than expected arrival of the layer over Melbourne was required. Nimbus 7 TOMS data for 23 Jun. showed that the SO2 from the eruption had extended at least 11000 km to the west and that the southern boundary of the cloud had reached 15 degrees S just 8 days after the climactic eruption. It can be assumed that this cloud also contained dust and sulphuric acid aerosol. It was proposed that a section had then been broken away from the main cloud and carried south by a large scale eddy between the low latitude easterlies and the strong mid-latitude westerlies which finally carried the aerosol cloud over southern Australia. Accompanying 30 mb wind data showed a counter clockwise circulation, responsible for the transport, located in the South Atlantic Ocean.
Young, Stuart A.↗
Long-lived artificial ion clouds in the earth's ionosphere
In two barium releases made over the Caribbean during the summer of 1991, the ion cloud was observed to last more than 10 hours. Observations were made with a low light level TV imager and a TV spectrograph operated onboard a Russian research vessel conducting ionospheric research and participating in the NASA-CRRES mission. The two clouds were observed near the release region for 15 to 25 minutes during the twilight following the initial release. One of the releases was made during evening twilight and the ion cloud reappeared the following morning, while the other was a morning release and the late-time observations made the next evening. The observations are highly surprising since, generally, it has been assumed that the ion cloud would dissipate within a few hours. However, the long life time appears consistent with current knowledge about dissipative processes. The long lifetime of barium ion clouds may provide for the most sensitive experimental testing of ionospheric convection models, a possibility with clear implications for future barium release experiments.
Milinevskij, G. P.↗
PSC and volcanic aerosol routine observations in Antarctica by UV-visible ground-based spectrometry
Polar statospheric clouds (PSC) and stratospheric aerosol can be observed by ground-based UV-visible spectrometry by looking at the variation of the color of the sky during twilight. A radiative transfer model shows that reddenings are caused by high altitude (22-28 km) thin layers of scatterers, while low altitude (12-20 km) thick ones result in blueings. The color index method applied on 4 years of observations at Dumont d'Urville (67 deg S), from 1988 to 1991, shows that probably because the station is located at the edge of the vortex, dense PSC are uncommon. More unexpected is the existence of a systematic seasonal variation of the color of the twilight sky - bluer at spring - which reveals the formation of a dense scattering layer at or just above the tropopause at the end of the winter. Large scattering layers are reported above the station in 1991, first in August around 12-14 km, later in September at 22-24 km. They are attributed to volcanic aerosol from Mt Hudson and Mt Pinatubo respectively, which erupted in 1991. Inspection of the data shows that the lowest entered rapidly into the polar vortex but not the highest which remained outside, demonstrating that the vortex was isolated at 22-26 km.
Sarkissian, A.↗
Global surface-based cloud observation for ISCCP
Visual observations of cloud cover are hindered at night due to inadequate illumination of the clouds. This usually leads to an underestimation of the average cloud cover at night, especially for the amounts of middle and high clouds, in climatologies on surface observations. The diurnal cycles of cloud amounts, if based on all the surface observations, are therefore in error, but they can be obtained more accurately if the nighttime observations are screened to select those made under sufficient moonlight. Ten years of nighttime weather observations from the northern hemisphere in December were classified according to the illuminance of moonlight or twilight on the cloud tops, and a threshold level of illuminance was determined, above which the clouds are apparently detected adequately. This threshold corresponds to light from a full moon at an elevation angle of 6 degrees or from a partial moon at higher elevation, or twilight from the sun less than 9 degrees below the horizon. It permits the use of about 38% of the observations made with the sun below the horizon. The computed diurnal cycles of total cloud cover are altered considerably when this moonlight criterion is imposed. Maximum cloud cover over much of the ocean is now found to be at night or in the morning, whereas computations obtained without benefit of the moonlight criterion, as in our published atlases, showed the time of maximum to be noon or early afternoon in many regions. Cloud cover is greater at night than during the day over the open oceans far from the continents, particularly in summer. However, near noon maxima are still evident in the coastal regions, so that the global annual average oceanic cloud cover is still slightly greater during the day than at night, by 0.3%. Over land, where daytime maxima are still obtained but with reduced amplitude, average cloud cover is 3.3% greater during the daytime. The diurnal cycles of total cloud cover we obtain are compared with those of ISCCP for a few regions; they are generally in better agreement if the moonlight criterion is imposed on the surface observations. Using the moonlight criterion, we have analyzed ten years (1982-1991) of surface weather observations over land and ocean, worldwide, for total cloud cover and for the frequency of occurrence of clear sky, fog and precipitation The global average cloud cover (average of day and night) is about 2% higher if we impose the moonlight criterion than if we use all observations. The difference is greater in winter than in summer, because of the fewer hours of darkness in the summer. The amplitude of the annual cycle of total cloud cover over the Arctic Ocean and at the South Pole is diminished by a few percent when the moonlight criterion is imposed. The average cloud cover for 1982-1991 is found to be 55% for northern hemisphere land, 53% for southern hemisphere land, 66% for northern hemisphere ocean, and 70% for southern hemisphere ocean, giving a global average of 64%. The global average for daytime is 64.6% for nighttime 63.3%.
Source record↗
The effect of moonlight on observation of cloud cover at night, and application to cloud climatology
Ten years of nighttime weather observations from the Northern Hemisphere in December were classified according to the illuminance of moonlight or twilight on the cloud tops, and a threshold level of illuminance was determined, above which the clouds are apparently detected adequately. This threshold corresponds to light from a full moon at an elevation angle of 6 deg, light from a partial moon at higher elevation, or twilight from the sun less than 9 deg bvelow the horizon. It permits the use of about 38% of the observations made with the sun below the horizon. The computed diurnal cycles of total cloud cover are altered considerably when this moonlight criterion is imposed. Maximum cloud cover over much of the ocean is now found to be at night or in the morning, whereas computations obtained without benefit of the moonlight criterion, as in our published atlases, showed the time of maximum to be noon or early afternoon in many regions. The diurnal cycles of total cloud cover we obtain are compared with those of the International Satellite Cloud Climatology Project (ISCCP) for a few regions; they are generally in better agreement if the moonlight criterion is imposed on the surface observations. Using the moonlight criterion, we have analyzed 10 years (1982-91) of surface weather observations over land and ocean, worldwide, for total cloud cover and for the frequency of occurrence of clear sky, fog, and precipitation. The global average cloud cover (average of day and night) is about 2% higher if the moonlight criterion is imposed than if all observations are used. The difference is greater in winter than in summer, because of the fewer hours of darkness in summer. The amplitude of the annual cycle of total cloud cover over the Arctic Ocean and at the South Pole is diminished by a few percent when the moonlight criterion is imposed. The average cloud cover for 1982-91 is found to be 55% for Northern Hemisphere land, 53% for Southern Hemisphere land, 66% for Northern Hemisphere ocean, and 70% for Southern Hemisphere ocean, giving a global average of 64%. The global average for daytime is 64.6%; for nighttime 63.3%.
Hahn, Carole J.↗
BAND-HEAD WAVELENGTHS OF C SUB 2, CH SUB 6, CN, CO, NH, NO, O SUB 2, OH, AND THEIR IONS RESEARCH PAPER NO. 6
Band-head wavelengths of some radicals & their ions for intepreting ultraviolet spectra of dayglow, twilight & aurorae of earth, mars & venus
ION↗
Observed characteristics of ionospheric winds
Ionospheric winds during twilight determined from observations of growth and motion of sunlit alkali vapor trails
VAPOR↗