Function Of Solar Zenith Angle Research Articles

All‐Weather Comparison between Spectral and Broadband (Robertson‐Berger) UV Measurements

Abstract— Spectral measurements of UV irradiance at the Fraunhofer Institute at Garmisch‐Partenkirchen, Germany, are intercompared with data from two different Robert‐son‐Berger‐type meters. More than 21000 spectra gathered during 20 months of continuous operation were used for this analysis, sampling a large variety of atmospheric conditions. At low solar zenith angles the agreement (± 10%) was quite satisfactory, whereas at high solar zenith angles the broadband instruments gave systematically lower readings than the erythemally weighted spectral data. The deviation of the spectral response of the broadband instruments from the Commission Internationale de 1'Eclairage (CIE) erythema function and the cosine error provide an explanation of the observed differences that are similar for both broadband meters. Model calculations agree quite well with the observations, if an accurate measurement of the broadband spectral response is available. An analytical fit curve was established for the ratio between broadband and weighted spectral data, as a function of solar zenith angle, enabling quick and easy checks of the stability of the spectrora‐diometric system. The relative standard deviation of the data from the fit function was only2–3% in the case of cloudless sky conditions and 5% for all sky conditions, including all types of systematic and instrumental errors.

An Algorithm for Inferring Surface UV Irradiance Including Cloud Effects

Abstract Recent extratropical ozone depletion and the concomitant increase in surface ultraviolet (UV) radiation may be expected to adversely influence the biosphere. Since few long-term, high quality datasets of surface UV are available for assessing these effects, there is a need to develop techniques for estimating past levels of biologically harmful UV at a particular location and thus derive long-term trends. This paper presents a semiempirical algorithm, making use of readily available meteorological variables and total column ozone, for inferring historical UV levels at a particular location, including cloud cover effects. Where input data are available for a network of locations, the technique can be used to generate geographical distributions of surface UV. Measurements made at Lauder (45.04°S, 169.68°E), from November 1993 to October 1994, were used to establish the relationship between cloud-induced reductions of erythemal UV and broadband irradiance, as a function of solar zenith angle—termed ...

Observed influence of clouds on ultraviolet‐B radiation

A Brewer Spectrophotometer was utilized to make ground‐based measurements of solar ultraviolet (UVB) radiation received at Black Mountain, NC (35.66°N) in order to quantify the reduction of UV‐B due to cloud cover. Observations were made over the interval February–July 1995 with continuous coverage except for a three‐week period in May. A wide‐angle lens video camera at the site was used for the assessment of cloud conditions at the time of each scan. Integrated UV‐B (290–320 nm) values were evaluated and a regression through the values for clear conditions was produced for the range of zenith angles from 25°–70°. This regression effectively represents an average effect of the ambient aerosols and attenuating gases and was used to normalize the UV‐B values measured under different cloud cover amounts, providing an estimate of the reduction in irradiance due to a given cloud amount as a function of solar zenith angle. For a solar zenith angle of 50°, average UV‐B transmission was observed to be 30% for overcast skies, 61% for 8–9 tenths cloud cover, 74% for 6–7 tenths cover, and 79% for the scans during 4‐5 tenths cover. A number of fractional cloud cover cases were observed where cloud reflections resulted in measured surface irradiances exceeding the expected clear sky values by as much as 11 percent.

Cirrus Infrared Parameters and Shortwave Reflectance Relations from Observations

Abstract A summary of experimental observations and analysis of cirrus from high-altitude aircraft remote sensing is presented. The vertical distribution of cirrus optical and infrared cross-section parameters and the relative effective emittance and visible reflectance are derived from nadir-viewing lidar and multispectral radiometer data for observations during the 1986 and 1991 FIRE cirrus experiments. Statistics on scattering and absorption cross sections in relation to altitude and temperature are given. The emittance and reflectance results are considered as a function of solar zenith angle. Comparative radiative transfer calculations based on the discrete-ordinate method were carried out for three representative cloud phase function models: a spherical water droplet, an ice column crystal cloud, and a Henyey-Greenstein function. The agreements between observations of the effective emittance and shortwave reflectance and the model calculations were a function of the solar zenith angle. At angles bet...

Radiation transfer in finite cylindrical clouds

An analytical method is developed for determining radiation transport in finite cylindrical clouds. The cylindrical form is taken as the idealized shape of cumulus clouds. In the shortwave part of the spectrum illumination by the direct solar beam is considered. In the infrared the cloud's emission as well as the radiation emitted by an underlying ground is taken into account. Numerical computations are carried out to illustrate the applicability of the technique developed in the present study. For a cloud in an absorbing midlatitude summer atmosphere results are shown for the shortwave region including spectral integration. Cloud transmission and albedo are presented as a function of solar zenith angle and as a function of the cloud's dimensions. The results are in agreement with published results from Monte Carlo calculations as far as a comparison is possible. Vertical profiles of the solar heating rate in the cylindrical cloud indicate the effect of the cloud's finite dimensions. Similarity as well as differences are found from comparison with results obtained for a cuboidal cloud from a finite analytical model that was tested successfully against Monte Carlo calculations.

The Downward Flux of O + over the Nightside of Venus

We have constructed a map of the downward flux of O + over the nightside of Venus at high and low solar activities through a combination of modeling and analysis of Pioneer Venus ion mass spectrometer data. O + density profiles were obtained for almost 400 inbound or outbound segments of orbits from the first 2 years of the mission and about 40 more in the recent reentry phase of the mission. We have determined the nearly linear relationship between the O + maximum density and the downward O + flux for several solar zenith angles and local times by constructing models of the nightside ionosphere of Venus for a range of downward ion fluxes at the upper boundaries. We find that the largest downward fluxes occur near the terminators, and the fluxes fall off sharply toward the antisolar point. Although the standard deviations in the data are large, there is a suggestion of a local maximum near 155°, and the location of this maximum correlates fairly well with structure in the peak electron density as a function of solar zenith angle reported for the Pioneer Venus radio occultation experiment. The average downward ion flux is inferred to be about 1.7 × 10 8 cm -2 sec -1 over the nightside hemisphere at solar maximum and a factor of 7 less at moderately low solar activity.

Use of FUV auroral emissions as diagnostic indicators

In an earlier study we modeled selected FUV auroral emissions (O I (1356 Å), N2 Lyman‐Birge‐Hopfield (LBH) (1464 Å), and LBH (1838 Å)) to examine the sensitivity of these emissions and their ratios to likely changes in the neutral atmosphere. In this paper we extend that study to examine the dependence of these same emissions and their ratios on the shape of the energy distribution of the auroral electrons. In particular, we wish to determine whether changes in energy spectra might interfere with our determination of the characteristic energy. Modeled column‐integrated emissions show relatively small (<30%) dependences on the shape and width of the incident energy spectrum, provided the average energy and total energy flux of the energy distribution are held constant. Long‐wavelength FUV emissions, which are relatively unaffected by O2 absorption losses, exhibit virtually no dependence on the shape of the incident energy distribution. Changes in ratios of FUV short‐ to long‐wavelength emissions as a function of characteristic energy are much larger than those due to changes in energy distribution. As a result, the determination of characteristic energy using these emission ratios is relatively unambiguous. We also examine the relative intensities of the aurora and the dayglow for various conditions. The intensities of modeled FUV auroral emissions relative to the dayglow emissions are presented as a function of solar zenith angle and incident energy flux. Under certain conditions (energy flux ≤ 1 erg cm−2 s−1 and solar zenith angle ≤50°) the dayglow will be the limiting factor in the detection of weak auroras.

Evidence for day‐to‐night ion transport at low solar activity in the Venus pre‐dawn ionosphere

Periapsis of the Pioneer Venus spacecraft dropped below 180 km on August 28, 1992 near midnight, and 42 orbits of low altitude data at moderately low solar activity in the pre‐dawn sector were obtained before contact was lost to the spacecraft in October, 1992. Through a combination of analysis of data from the PV orbiter ion mass spectrometer (OIMS) and modeling, we consider here what can be learned about the relative importance of plasma transport from the dayside and electron precipitation in maintaining the nightside ionosphere during the re‐entry period. In particular, we examine here the atomic ion density profiles. We compute the average peak density of O+ as a function of solar zenith angle and determine what fluxes of atomic ions or precipitating electrons would be necessary to produce those values. We then compare model calculations of the ion densities to those observed during the re‐entry period. We find that the low solar activity nightside ionosphere shows evidence of significant day‐to‐night plasma transport.

Observations of atomic deuterium in the mesosphere from ATLAS 1 with ALAE Instrument

During the first ATLAS mission, the ALAE Lyman α spectrophotometer collected various measurements of hydrogen and deuterium atoms, from the mesosphere, the thermosphere, the exosphere and the interplanetary medium. In this paper is presented a preliminary analysis of some observations of atomic deuterium, which Lyman α emission is excited by resonance scattering of solar photons. Nadir measurements along the sunlit Earth part of the orbit show that the emission changes as a function of solar zenith angle. Comparison with a simple model shows that, from the shuttle altitude of 300 km and at low solar zenith angles, the line‐of‐sight probes atomic deuterium down to 80 km of altitude (where O2 absorption is complete), whereas at angles from 60° to 90°, the mesospheric part of the emission progressively vanishes. Then, the remaining emission mainly consist of the thermospheric part (z ≥ 100 km). This type of observations provides a sounding of atomic deuterium at its peak production and concentration, and D atoms can be used as a proxy to H atoms (which cannot be observed from a satellite) in this particularly active region of the mesosphere.

An empirical model of the non-auroral D-region

An empirical model of the D-region electron densities is presented, which is based on rocket borne wave propagation data from mid-latitude and equatorial regions obtained during quiet conditions only. The electron densities are defined for constant neutral densities and are given as a function of solar zenith angle.

An empirical model of the nonauroral D Region

An empirical model of the D region electron densities is presented, which is based on rocket‐borne wave propagation data from mid‐latitude and equatorial regions obtained during quiet conditions only. The electron densities are defined for constant neutral densities and are given as a function of solar zenith angle.

Positive and negative ion densities and mobilities in the middle atmosphere over India—rocket measurements

This paper discusses the results from four rocket experiments conducted from Thumba, India, during the Indian Middle Atmosphere programme (IMAP). These rockets carried instrumented Gerdien Condenser payloads to measure ion densities and their mobilities. In the first two flights only positive ion measurements were attempted while the other two measured both positive and negative ion values. The results show that the positive ion density profiles go through a minimum around 62 km, as expected from the ion production models for this region. The ion density distribution is a function of solar zenith angle. An asymmetry with respect to noon is seen in these measurements, which is not expected theoretically. The positive ion mobilities indicate the ions to be water clusters, of the type H + (H 2O) n with n = 2 or 3, similar to the earlier reported ones. The negative ion density profile exhibits a maximum around 85 km, which is not predicted by the currently available ion density models and theories of D-region ionisation processes. The negative ion mobility measurements show the ions to have a mass range of 30–60 amu, which is within the range of mass spectrometric measurements.

Diurnal variation of the linear polarization across the H? and H? Fraunhofer lines of the terrestrial atmosphere, and a detection of a ?daylight flash?, II

In Paper I, the ability of using the polarimetric technique to observe the filling-in of the Fraunhofer spectrum line profiles—the Ring effect—has been presented (using the Hβ Fraunhofer line only). It has also been found that the added light, which caused the filling-in, is polarized. Here, a description is given of some observations, which, hopefully, will lead to a better understanding of the Ring effect using the Hα and Hβ Fraunhofer spectral lines. The observations of the filling-in effect at the zenith have covered many conditions which might be considered as controlling its behaviour at these two spectral regions. The data reveal variations of polarization at the centres of those Fraunhofer lines as a function of solar zenith angle. The results show no uniformity for the added light intensity, its value was depending on the kind of day (clear and turbid) and/or on the solar zenith angle. Also the photometric observational studies reported here suggest that a new phenomenon behaving like airglow has been discovered. It is referred to here as a ‘daylight flash’ and was recorded (for short periods) on three days at both the Hα and Hβ Fraunhofer lines with different strengths. The cores of these lines were filled in while the near continua were unaffected.

Solar zenith angle effects on vegetation indices in tallgrass prairie

Nadir spectral reflectances for a range of grassland conditions on the Konza prairie in Kansas were acquired in 1987 from June to October during the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment, or FIFE. Measurements in the red (0.668 ± 0.005 μm) and near-infrared (0.821 ± 0.006 μm) were obtained at numerous solar zenith angles (SZAs) during the diurnal cycle. Reflectance factors were utilized for computation of two spectral vegetation indices (VIs) — the simple ratio (SR) and the normalized difference vegetation index (NDVI). The diurnal change in the narrow red and near-infrared bands and their VIs as a function of SZA, and the relationship of these spectral data to biophysical measurements, were examined throughout the growing season. The assumption of independence of VIs from illumination geometry was specifically addressed. Both the SR and the NDVI were significantly affected by SZA for many prairie conditions. SR was significantly related to more canopy variables over a wider SZA range (27°–61°) than was NDVI. For intermediate green LAI (0.5 < LAI < 2.0), both indices were at a minimum value at solar noon, increasing with SZA. Conversely, the higher biomass sites (LAI > 2.0) with especially tall grass exhibited either aa maximum value at solar noon which decreased as SZA increased or a moderate to high constant VI. Low density canopies (LAI < 0.5) exhibited no change in their low VI values over the diurnal course. These qualitatively different responses precluded the use of a general “correction” to remove the observed SZA effects on the measured VI values. The strength of the relationships between VI and LAI also varied, depending on SZA, with lower predictive capability at the traditionally used “high sun” condition than at oblique sun angles (SZA > 40°). The restriction of surface measurements to a standard acquisition SZA of 45° produced a good correspondence of both the SR and NDVI to LAI (r 2 = 0. 69). The best predictive linear model for the SR was obtained at SZA = 45°(r 2 = 0.82) and was based on two canopy variables, the green LAI and the canopy standing green to standing dead dry weight ratio. Therefore, a standard SZA for data acquisitions at 45° is recommended over the traditional “high sun” practice. The results suggest that the VIs measure the instantaneous, projected, and illuminated LAI canopy fraction relative to the illuminated substrate.

The high latitude lower ionosphere observed by EISCAT

Routine observations of lower ionosphere electron densities are now available from the EISCAT radar facility. High resolution (3km in altitude) power profile measurements are obtained from the altitude interval 55–170km for approximately 400 hours per year, during continuous operations of 24 hours or more. Variations of D and E region electron densities as a function of solar zenith angle, during undisturbed conditions, and as a function of simulated 30MHz riometer absorption, during disturbed conditions, are presented.

A D-region electron density model based on rocket borne wave propagation data

The temporal and spatial coverage by rocket borne radio wave propagation experiments is rather poor, these data, however, are considered the most reliable in the lower ionosphere. Published electron density profiles from Andoya, Kiruna, South Uist, Red Lake, Wallops Island, Thumba and Punta Chilca were fitted to simple functions of solar zenith angle at different latitudes. Where applicable comparisons are made to the corresponding results from IRI.

Bidirectional reflectance factors of agricultural targets: A comparison of ground-, aircraft-, and satellite-based observations

The bidirectional reflectance factor (BRF) of most natural surfaces exhibits substantial departure from lambertian behavior, yet the magnitude of its effect in off-nadir imagery from the SPOT satellite remains mostly undocumented. Field investigations were conducted at the Maricopa Agricultural Center in Arizona to explore this phenomenon using three wavelength intervals (green, 0.50-0.59 μm; red, 0.61-0.68 μm; and NIR, 0.79-0.89 μm). During 1988 and 1989 observations spanned two consecutive days during which the SPOT radiometer viewing angles differed by 34°. Ground-based BRFs were obtained for nadir and off-nadir viewing angles over replicated transects in cultivated soil, cotton, and wheat fields using radiometers mounted on a backpack. Aircraft data were measured from flight lines about 150 m above ground level with a nadir-oriented radiometer. SPOT digital imagery was converted to ground equivalent BRFs using specific calibration factors for the sensor, estimates of exoterrestrial radiation, solar zenith angles, and measurements of atmospheric optical depths. Reflectance factors varied with sensor, target, wavelength interval, and viewing and illumination geometry. Ground data were a strong function of solar zenith angle, revealing systematic differences in BRF behavior of each cover type. Good agreement was found between ground- and aircraft-based observations collected with nadir-pointed radiometers. For each wavelength interval, linear regression yielded R 2 > 0.99 and regression slopes averaging 0.96. Ground and aircraft data also revealed negligible changes in day-to-day reflectance from cultivated soil, cotton, and wheat targets. However, large differences were observed in SPOT imagery acquired on consecutive days. As expected, correlations between nadir aircraft observations and off-nadir satellite data were relatively poor ( 0.66 < R 2 < 0.80; slope ≈ 0.76 ) partially due to differences in sensor viewing angles. Off-nadir satellite and off-nadir ground data were better correlated ( 0.91 < R 2 < 0.96; slope ≈ 0.83 ). However, satellite-based BRFs were nearly double the ground-based values at low levels of reflectance (visible bands over dense vegetation). Possible reasons for discrepancies between satellite and ground off-nadir measurements include systematic bias in surface reflectance measurements, inadequate atmospheric correction of satellite data, adjacency effect from nearby surfaces, and inaccurate satellite sensor calibration.

Bidirectional measurements of surface reflectance for view angle corrections of oblique imagery

An apparatus for acquiring bidirectional reflectance-factor data was constructed and used over four surface types. Data sets were obtained over a headed wheat canopy, bare soil having several different roughness conditions, playa (dry lake bed), and gypsum sand. Results are presented in terms of relative bidirectional reflectance factors (BRFs) as a function of view angle at a number of solar zenith angles, nadir BRFs as a function of solar zenith angles, and, for wheat, vegetation indices as related to view and solar zenith angles. The wheat canopy exhibited the largest BRF changes with view angle. BRFs for the red and the near-infrared (NIR) bands measured over wheat did not have the same relationship with view angle. NIR/Red ratios calculated from nadir BRFs changed by nearly a factor of 2 when the solar zenith angle changed from 20° to 50°. BRF versus view angle relationships were similar for soils having smooth and intermediate rough surfaces but were considerably different for the roughest surface. Nadir BRF versus solar-zenith angle relationships were distinctly different for the three soil roughness levels. Of the various surfaces, BRFs for gypsum sand changed the least with view angle (10% at 30°).

Aeronomical calculations of valley size in the ionosphere

Real heights in the ionospheric F region, as calculated from ionograms, are subject to appreciable uncertainties because of our poor knowledge of the size and shape of the valley region between the E and F layers. Even with good extraordinary ray data we need model values for the valley depth, if we are to obtain the valley width (and hence the heights in the F region) with good accuracy. Theoretical profiles across the E/F valley region have been calculated using the MSIS86 atmospheric model, with full allowance for the effects of different ionising wavelengths and night-time ionisation sources. These profiles are used to determine the valley depth and width at all local times. Midlatitude results are presented as a function of solar zenith angle x ∼ for summer and winter conditions at solar minimum and solar maximum. Valley widths are consistently about 20 km at x > 40°, increasing to 40 km at x 90°. Daytime valley depths are greater than expected, giving a mean ratio NV/NE of about 0.7 for zenith angles less than 85°.

Removal of Atmospheric Effects prom AVHRR Albedos

Based on numerical simulations, coefficients are determined to be used in a linear relationship between clear-sky planetary albedo and surface albedo. Thew coefficients are given as functions of solar zenith angle and atmospheric parameters for both the AVHRR (Advanced Very High Resolution Radiometer) channels 1 and 2 separately. The atmospheric effects in these channels are systematically different, because of the different spectral behavior of atmospheric parameters. Accordingly the use of the narrowband coefficients will improve the determination of surface properties from AVHRR data, compared with analogous methods using broadband coefficients.