Solar Illumination Conditions Research Articles

Some recent developments in understanding auroral electron acceleration processes

Discrete auroral arcs and the associated inverted-V-type electron precipitation are accompanied by potential structures, upgoing ion beams and plasma density depletions in the acceleration region altitude and above. However, exactly how these phenomena depend on altitude in various Kp and solar illumination conditions as well as at various magnetic local time (MLT) and invariant latitude (ILAT) is not so well known. We review the main results of our recent large statistical studies of the altitude dependence of these parameters using Polar data. We also include similar statistical results on some additional parameters that appear to have major importance in inverted-V auroral acceleration. These parameters include middle-energy (/spl sim/50-500 eV) electron anisotropies, certain types of electrostatic wave bursts as well as ion shell distributions. One of the most interesting results of the statistical studies is that many of the parameters mentioned change their behavior rather abruptly at around 4 R/sub E/ radial distance. We draw the conclusion that the region for auroral energization processes seems to take place within a closed potential structure below /spl sim/3-4 R/sub E/. These conclusions lead to a new scenario for auroral plasma physics and the energy flow in the auroral acceleration process. The observational results are in agreement with simulations that we also summarize in this paper.

Molecular Beams in Space: Sources of OH(A→X) Emission in the Space Shuttle Environment

OH(A→X) emission bands have been observed in the molecular beam jets produced by Space Shuttle engine exhaust using the GLO imager spectrograph located in the payload bay. Spectra were collected at a resolution of 4 Å for both daytime and nighttime solar illumination conditions, all at an altitude of ∼390 km. A spectral analysis is presented that identifies and quantifies four separate OH(A) excitation processes. These include (i) solar-induced fluorescence of the OH(X) in the exhaust flow, (ii) solar-induced photodissociation of H2O in the exhaust at the strong Lyman-α solar emission line (1216 Å), (iii) solar-induced photodissociation of H2O in the far UV, at shorter wavelengths than Lyman-α, and (iv) luminescent collisions between atmospheric species and exhaust constituents, most probably the reaction O + H2O → OH(A) + OH(X). Process (i) produces a very rotationally cold and spectrally narrow component due to the rapid cooling of the OH(X) in the supersonic expansion of the exhaust flow. Processes (ii) and (iii) produce extremely excited OH(A), not well characterized by thermal vibrational or rotational distributions. The O + H2O chemiluminescent reaction has a substantial activation energy, 4.79 eV, and is only slightly above threshold for the ram geometry, where the engine exhaust is directed into the atmospheric wind. Evidence for process (iv) is observed in the night ram but not the night perpendicular exhaust atmospheric interaction, consistent with the threshold energy. Through the use of a nonequilibrium spectral emission model for OH, the integrated intensity, spectral distribution, and OH(A) internal state characterization for each of the above processes was deduced. Additional confirmation of the analysis is provided through the use of a model simulation of the space experiment to predict the total integrated intensities for processes (i) and (ii), for which the underlying spectroscopy, absorption cross sections, and solar excitation intensities are well established. Analysis of process (iii) has established, for the first time, a value for the far-UV conversion efficiency of absorbed photons to OH(A) photons of 0.26, which is twice the established value for Lyman-α. Under the assumption that O + H2O collisions are the source of process (iv), the analysis has established a chemiluminescence cross section at ram conditions of 1.7 × 10-2 Å2. Evidence of OH(A) emission bands from predissociated vibrational levels suggests that the total reaction cross section for process (iv) may be significantly higher. While this cross section assumes a single-step reaction of O with H2O, the possibility of a two-step process of O with other plume species has yet to be explored.

The physical parameterization of the top-of-atmosphere reflection function for a cloudy atmosphere—underlying surface system: the oxygen A-band case study

The paper is devoted to the physical parameterization of the top-of-atmosphere reflection function. The accuracy of the parameterization is checked against exact radiative transfer calculations in a cloudy atmosphere for various cloud-top-heights, cloud optical and geometrical thicknesses, solar illumination and surface reflection conditions. It was found that the error of approximation is smaller than 5% for most cases studied at the wavelength interval 758 –768 nm , which corresponds to the oxygen A-band. This band is routinely used in cloud-top-height retrievals. The model proposed can be used for cloud-top-height and cloud geometrical thickness retrievals. This allows to avoid a standard look-up-table retrieval scheme, involving complex numerical procedures.

Fractal Analysis of Satellite-Detected Urban Heat Island Effect

This study reports on a study of the urban heat island (UHI) phenomenon in Guangzhou, China. In the study, surface radiant temperatures derived from Landsat TM thermal infrared images of December, 13, 1989, March 3, 1996, and August 29, 1997 were used. To examine the spatial distribution of surface radiant temperatures, transects were drawn and analyzed from each temperature image. The fractal dimensions of these transects were then computed using the divider method, to better understand the spatial variability of surface radiant temperatures caused by the thermal behavior of different land-cover types and landscape pattern characteristics. The effect of urban development on the geographical distribution of surface radiant temperatures and thus on the UHI was also investigated. The results revealed two major heat islands, one in the southwest and the other in the east of the city. The distribution of the UHIs was closely associated with industrial land uses but not with residential land uses. Changes in fractal dimension in different seasons were not only attributed to solar illumination and climatologic conditions relating to soil moisture and air temperature, but also to the topographic variation and the spatial arrangement and extent of different land cover types. Urban development increased the spatial variability of radiant temperatures, resulting in higher fractal dimension values.

Evaluation of Landsat-7 ETM+ Panchromatic Band for Image Fusion with Multispectral Bands

The Landsat-7 ETM+ panchromatic band is taken simultaneously with multispectral bands using the same sensor system. The two data sets, therefore, are coregistered accurately and the solar illumination and other environmental conditions are identical. This makes ETM+ Pan advantageous to SPOT Pan for resolution fusion. A spectral preserve image fusion technique, Smoothing Filter-Based Intensity Modulation (SFIM), can produce optimal fusion data without altering the spectral properties of the original image if the coregistration error is minimal. With TM/SPOT Pan fusion, the technique is superior to HSI and Brovey transform fusion techniques in spectral fidelity, but has slightly degraded edge sharpness as a result of TM/SPOT Pan coregistration error because SFIM is sensitive to coregistration accuracy and temporal changes of edges. The problem is self-resolved for ETM+ because there is virtually no coregistration error between the panchromatic band and the multispectral bands. Quality fusion imagery data thus can be produced.

Modelling of the non-LTE populations of thenitricacid and methane vibrational states in themiddleatmosphere

A modelling of the non-LTE populations of the HNO 3 and CH 4vibrational levels in the middle atmosphere has been carried out and the results are presented.Thework is oriented to assess the potential impact of non-LTE effects on the remote sensing ofthesegases. The models developed for this purpose include a complete set of radiative andcollisionalprocesses. In order to cover typical and extreme remote sensing scenarios, the modelshave beenapplied to different atmospheric and solar illumination conditions. The vibrationallevelsresponsible for the major emissions of HNO 3 are found to be in LTE up to thelowermesosphere, driven by the dominant V–T processes with the air molecules. In thenon-LTEregion, the absorption from the warmer tropospheric layers and solar direct excitationproducesmall enhancements over the equilibrium populations. The mesospheric CH 4vibrational temperatures are mainly determined by two mechanisms: the radiative absorption oftheupcoming radiation emitted by the lower layers of the atmosphere, and thenear-resonantvibrational coupling between the CH 4 levels and the first vibrationallyexcited level ofO 2. By day, non-LTE is significantly enhanced as a consequence of thecollisionalrelaxation of overtone and combinational states excited by the solar radiation at 3.3 μm.The effects derived from the uncertainties in the parameters of the models have beenstudied.

A procedure for the detection and removal of cloud shadow from AVHRR data over land

Although the accurate detection of cloud shadow in AVHRR scenes is important for many atmospheric and terrestrial applications, relatively little work in this area has appeared in the literature. This paper presents a new multispectral algorithm for cloud shadow detection and removal in daytime AVHRR scenes over land. It uses a combination of geometric and optical constraints, derived from the pixel-by-pixel cross-track geometry of the scene and image analysis methods to detect cloud shadow. The procedure works well in tropical and midlatitude regions under varying atmospheric conditions (wet-dry) and with different types of terrain. Results also show that underdetected cloud shadow ran produce errors of 30-40% in observed reflectances for affected pixels. Moreover, radiative transfer calculations show that the effects of cloud shadow are comparable to or exceed those of aerosol contamination for affected pixels. The procedure is computationally efficient and hence could be used to produce improved weather forecast, land cover, and land analysis products. The method is not intended for use under conditions of poor solar illumination and/or poor viewing geometry.

Threshold functions for automated cloud analyses of global meteorological satellite imagery

Abstract Polar-orbiting meteorological satellites collect imagery across a wide range of solar illumination and atmospheric conditions. Algorithms used to create automated cloud analyses from these data must compensate for variations in cloud signatures caused by changes in atmospheric attenuation and solar scattering geometry that occur as the satellite orbits the Earth. In this paper, a methodology is presented that describes the variations in cloud spectral signatures that result from changes in satellite observational conditions. Relationships are developed that describe the impact of solar illumination and scattering geometry on the spectral signature of optically-thick water clouds in the daytime Advanced Very High Resolution Radiometer (AVHRR) visible and near-infrared imageryxs. Additional relationships are presented that describe the impact of total integrated water vapour on the spectral signatures of optically-thin cirrus and stratus clouds in night-time AVHRR infrared. Threshold functions are ...

Visible and near‐ultraviolet spectroscopy at McMurdo Station, Antarctica: 10. Reductions of stratospheric NO2 due to Pinatubo aerosols

Visible absorption spectroscopy was employed for stratospheric measurements at McMurdo Station, Antarctica, during the summer and fall seasons in 1990, 1991, 1992, and 1993. Observed column amounts of NO2 were as much as 50% smaller in 1992 and 1993 than in 1990 and 1991. The measured decreases in NO2 are believed to be due to the hydrolysis of N2O5 on the enhanced volcanic aerosols injected by the massive eruption of Mount Pinatubo in mid‐1991. Such decreases in NO2 are broadly consistent with observations at other latitudes and with model calculations. However, substantially reduced NO2 was also observed during the polar day period in summer under conditions of continuous local solar illumination. The formation of the key N2O5 intermediate and reduction of NO2 at this time of year probably occurs via two mechanisms: (1) excursions of air parcels to lower latitudes where nighttime chemistry can produce N2O5 and (2) conversion of a small fraction of the NO3 formed during the day to N2O5. Model calculations show that the latter mechanism can be important in spite of the fast daytime photolysis of NO3.

Northern forest classification using temporal multifrequency and multipolarimetric SAR images

Characterizing forest ecosystem dynamics for global change studies requires updated knowledge in terms of species composition, carbon storage, and biophysical functioning. Often, significant areas of forest are obscured by clouds or are under reduced solar illumination conditions, which limit acquisition of optical satellite data. Synthetic aperture radar (SAR) images, however, can be acquired under these conditions. Several SAR image data sets were acquired over the Northern Experimental Forest near Howland, Maine as part of the Forest Ecosystem Dynamics-Multisensor Aircraft Campaign. A SAR data processing and analysis sequence, from calibration through classification, is described. The usefulness of multifrequency temporal polarimetric SAR image data for identifying ecosystem classes is discussed. Our results show that with principal component analysis of temporal data sets (winter and late summer) SAR images can be classified into general forest categories such as softwood, hardwood, regeneration, and clearing with better than 80% accuracy. Other nonforest classes such as bogs, wetlands, grass, and water were also accurately classified. Classifications from single date images suffered in accuracy. The winter image had significant confusion of softwoods and hardwoods with a strong tendency to overestimate hardwoods. Modeling results suggest that increased double-bounce scattering of the radar beam from conifer stands because of lowered dielectric constant of frozen needles and branches was the contributing factor for the misclassifications.

Diffuse reflectance of oceanic waters II Bidirectional aspects

For visible wavelengths and for most of the oceanic waters, the albedo for single scattering (?) is not high enough to generate within the upper layers of the ocean a completely diffuse regime, so that the upwelling radiances below the surface, as well as the water-leaving radiances, generally do not form an isotropic radiant field. The nonisotropic character and the resulting bidirectional reflectance are conveniently expressed by the Q factor, which relates a given upwelling radiance L(u) (θ',φ) to the upwelling irradiance E(u) (θ' is the nadir angle, φ is the azimuth angle, and Q = E(u)/L(u)); in addition the Q function is also dependent on the Sun's position. Another factor, denoted f, controls the magnitude of the global reflectance, R (= E(u) /E(d), where E(d) is the downwelling irradiance below the surface); f relates R to the backscattering and absorption coefficients of the water body (b(b) and a, respectively), according to R = f(b(b)/a). This f factor is also Sun angle dependent. By operating an azimuth-dependent Monte Carlo code, both these quantities, as well as their ratio (f/Q) have been studied as a function of the water optical characteristics, namely ? and η; η is the ratio of the molecular scattering to the total (molecular + particles) scattering. Realistic cases (including oceanic waters, with varying chlorophyll concentrations; several wavelengths involved in the remote sensing of ocean color and variable atmospheric turbidity) have been considered. Emphasis has been put on the geometrical conditions that would be typical of a satellite-based ocean color sensor, to derive and interpret the possible variations of the signal emerging from various oceanic waters, when seen from space under various angles and solar illumination conditions.

Reflectance calibration of aerial video imagery with automatic gain compensation on and off

Abstract Video imaging systems are noi radiometrically calibrated, thus it is difficult to obtain quantitative remotely-sensed imagery for natural resource applications. Video camera automatic gain controls (AGC) present potential problems in calibrating video systems for quantitative analysis because they compensate for changing solar illumination conditions. In this experiment aerial video calibrations to ground reflectance standards were compared for AGC turned on and off, The calibrated aerial video was evaluated for guinea grass (Panicum maximum L.) biomass treatments on two dates. Results showed that there was more atmospheric light scattering in the red than in the NIR video band. Light scattering affects could be detected only when the AGC was ofT because when AGC was on light scattering affects were masked. These results also showed that video imagery produced significant correlations with guinea grass biomass that were comparable to ground reflectance measurements.

Universal time effect in the occurrences of large-scale ionospheric disturbances

The auroral storm-related enhancements in the virtual height ( Δh′ F) of the F-region were analysed for a period of 3 years (1980–1982), using hourly ionosonde data from six sub-auroral stations positioned in a geographic longitude range 4–284°E. The night-time height enhancements, due to the magnetic storm-generated large-scale ionospheric disturbances (LSTIDs), followed (with an average time delay of 0.6 h) peaks of auroral surges deduced from the hourly auroral electrojet ( AE) index. Since the average auroral activity had a distinct diurnal distribution, peaking between 13:00 and 19:00 U.T., the most distinct height rises tended to occur for the stations which were then at night-time, i.e. in the Australian-Asian sector. Other stations, West of this sector and towards the sunlit region of the Earth, showed progressively smaller LSTID activity. Thus, the average height enhancements were smaller in the European sector and were very much reduced in the North American sector which was in full daylight during the period of maximum auroral activity. The largest hemispheric differences in ΔhF in the Australian-Asian sector were in the solstitial periods; the height rises tended to be larger and of a longer duration in the winter hemisphere. In the equinoctial months the height rises in the Australian-Asian sector were approximately of the same magnitude and duration in both hemispheres. The morphology in the occurrence of LSTIDs confirms previous results on the strong effect of solar illumination conditions on the propagation of LSTIDs.

Venera-15 and -16 middle atmosphere profiles from radio occultations: Polar and near-polar atmosphere of Venus

Results of 42 radio occultation measurements of the thermal structure of the Venus atmosphere at altitudes of 42 to 90 km, carried out with the help of Venera-15 and Venera-16 satellites during three occultation cycles from October 1983 to September 1984, are presented. The measurements were made in polar and near-polar regions of the southern and northern hemispheres, at latitudes from −89° to −60° and from 64° to 89° with solar zenith angles of 65° to 118°. The results point out the similarity of the thermal structures of both hemispheres. In particular, the southern hemisphere has the same cold collar region as the northern. The thermal structure of the atmosphere in both hemispheres depends mainly on the latitude and only weakly on solar illumination conditions. In the polar regions, at latitudes beyond 80°, the temperature profiles above the tropopause are close to isothermal, while in the mesosphere region of the near-polar atmosphere, there exists a large temperature inversion with a mean value of 19 K and a maximum of about 35 K. The transition from the near polar to polar regions in the northern atmosphere is characterized by rapid lowering of the tropopause altitude by 5 km, rising of temperature by 12 K, and rising of pressure by 170 mbar. In the southern hemisphere, the corresponding magnitudes change by 3.1 km, 11 K, and 86 mbar, respectively. There is a substantial difference between the polar regions at the tropopause level. Temperature and pressure at the tropopause in the southern polar atmosphere are 15 K and 68 mbar lower than in the Northern region. The thermal structure difference between the near-polar regions of the hemispheres is less significant. Comparison of these results with data of radio occultations obtained by the Pioneer Venus orbiter leads to the conclusion that the thermal structure of the atmosphere persists over a time interval on the order of 5 years.

Auroral electrojet effect on the global occurrence pattern of large scale travelling ionospheric disturbances

Hourly values of the virtual height enhancements ( Δh' F) of the F-region, from 15 ionosonde stations in both hemispheres, were compared with the corresponding hourly values of the auroral electrojet (AE) index for the year of sunspot maximum (1980). It is evident that surges in the auroral electrojet generate trains of large scale travelling ionospheric disturbances (LSTIDs) in both hemispheres, which subsequently propagate equatorwards causing substantial height enhancements ( Δh' F) from sub-auroral to equatorial latitudes. A high degree of correlation exists between the current surges (represented by the variability of AE-index) and Δh' Fin the night-time sector, with an average correlation coefficient of 0.7 and on some occasions reaching 0.9. The optimum correlation between these two different parameters exists for an average time lag of 1.6 h between the onset of an auroral surge and the corresponding value of Δh' F in mid-latitudes. The ratio of the distance (from the auroral oval to a mid-latitude location) of travel of LSTIDs to the time lag corresponds to an average equatorward velocity (500 m s −1) of the disturbance. The correlation between AE and Δh' F depends strongly on solar illumination condition and is absent in the daytime sector. The amplitude of LSTIDs, as represented by the magnitude of Δh' F, tends to be largest in sub-auroral locations near the source region. It rapidly decreases in mid-latitudes and remains fairly constant from mid to low latitudes, with a slight tendency to increase at the equator.

Possibility of extraction of relative terrain heights from space images by a radiometric method

Abstract The paper describes a possibility to extract relative terrain heights by a radiometric method from Landsat MSS data. The proposed radiometric method to extract relative terrain heights is based on determination of a surface normal vector at a terrain point from multitemporal image data under an assumption of a certain spatial reflectance model. Spatial characteristics of reflectance of two types of forests covered with deciduous and coniferous trees in the mountainous test area were investigated in reference with digital elevation data and Landsat MSS images under five different conditions of solar illumination. The proposed radiometric method assuming Lambertian surface were applied to multitemporal Landsat MSS band 7 images in the test area to extract relative terrain profiles. The results show the potential of extraction of relative terrain heights from multitemporal space images by the proposed radiometric method in an area with homogeneous land cover.

A global study of large scale travelling ionospheric disturbances (TIDS) Following a step-like onset of auroral substorms in both hemispheres

A study has been undertaken on the conjugate effects in the generation and propagation of large scale ionospheric disturbances (LSTIDs), as deduced from the virtual height ( h′ F) rises obtained from ionosonde chains in the Japanese and Australian longitudinal sectors. Hitherto, the results of such studies have revealed the presence of simultaneous trains of LSTIDs propagating at night-time over a large range of southern and northern latitudes, following the onset of step-like auroral particle precipitation events in the conjugate locations. In the present case, the onset of an intense auroral substorm ( K p -index = 7−) occurred in the morning when the Japanese and Australian stations were at different solar illumination conditions unlike in the previous cases when both chains of ionosonde stations were in the night sector. Only the Japanese and other Asian stations which were in the same longitudinal sector and at night-time, showed the presence of pronounced LSTIDs. Simultaneously, all the ionosonde stations positioned in eastern Australia, which were then in daytime, showed that no detectable height rises were present in the sunlit portion of the Earth. The wavefronts of LSTIDs had a large longitudinal extent, spanning the night sectors of the Northern and Southern Hemispheres, as deduced from the h′ F hourly data obtained from 43 ionosonde stations. LSTIDs moved with a considerable velocity of 1.3 km s −1, from high to low latitudes, originating at auroral latitudes ( L-value > 6). It is suggested that a substantial increase in the ionization density in the F-region during daytime, inferred from an increase in the critical frequency (ƒ0 F2) of the layer for the stations in the sunlit sector of the Earth, was a dominant factor in the attenuation of large scale gravity waves responsible for the generation of LSTIDs.

Measurement of the condensation nuclei profile to 31 km in the Arctic in January 1989 and comparisions with Antarctic measurements

The first measurement of the condensation nuclei (CN) profile in the Arctic during winter was made to 31 km on 30 January 1989 from Kiruna Sweden (68° N). Enhanced levels of CN were observed in the colder regions above 18 km suggesting homogeneous or ion nucleation of CN as observed previously in Antarctica. A CN layer reaching a concentration of about 40 cm−3 was observed between 22.5 and 26 km. Comparisons with data obtained in Antarctica in 1987 and 1988 indicate that this layer is similar to those observed at the same altitude in Antarctica under similar solar illumination conditions. The latter are believed to be of photochemical origin as suggested by measurements before and after stratospheric sunrise. This CN layer may thus serve as a measure of the amount of time an air parcel has spent in sunlight, an important parameter during the early stages of spring ozone depletion.

Carbon dioxide 4.3‐μm emission in the Earth's atmosphere: A comparison between Nimbus 7 SAMS measurements and non‐local thermodynamic equilibrium radiative transfer calculations

A detailed analysis of CO2 4.3‐μm limb radiances measured by the Stratospheric and Mesospheric Sounder on Nimbus 7 has been conducted using a non‐local thermodynamic equilibrium (non‐LTE) radiative transfer model. The data display diurnal, latitudinal, and seasonal variations which are shown, after the latitudinal variation of the instrument's response has been taken into account, to be caused by different conditions of solar illumination and variations in the kinetic temperature profile. A study of the sensitivity of the populations and limb radiances to spectral, atmospheric, and collisional interactions parameters has been undertaken, and a limited range of values for some of these quantities has been derived. Among other findings, we conclude that the rate at which energy is exchanged (as ν3 quanta) between N2 and CO2 may be approximately twice the initial model value, which was inferred from laboratory data, and that the CO2 volume mixing ratio above 80 km may be significantly less than that predicted by photochemical models.

A comparison of model photochemical ozone formation potential with observed regional scale ozone formation during a photochemical episode over the United Kingdom in April 1987

A photochemical trajectory model has been employed to calculate the maximum potential for ozone generation in air parcels passing over the U.K. during a photochemical pollution episode in April 1987. In all, 11 trajectories have been studied and the model results compared against an objective analysis of the integrated ozone generation based on the observations reported from the U.K. Department of the Environment ground level O 3network. There is apparently good correlation between the observed O 3formation and the model O 3formation potential although the latter overestimates the former by a factor of about 2.8. The solar illumination conditions employed in the photochemical trajectory model may have caused this overestimation, since the model is formulated for O 3control strategy assessment and simulates the ‘worst case’ situation likely to give the maximum potential for secondary pollutant formation. In addition to the model results for O 3, a wide range of primary and secondary pollutant concentrations from the model were examined, together with the influence of precursor pollutant emissions.