Modelling of the electron density profile in the lowest part of ionosphere D-region on the basis of radiowave absorption data: 1. Theoretical model

A simplified full-wave method adapted to the propagation of very obliquely incident LF radio waves is developed. For a selected ionosphere model the wave-field structure is calculated inside a horizontally stratified ionosphere and the peculiarities of the reflected field are clearly described. The penetration of the investigated radio waves in the lower ionosphere at noon-time is found to be restricted to a layer several wavelengths thick. The reflected wave is created entirely by the mechanism of partial reflections and the region responsible for its formation is usually below 70 km. The influence of some typical parameters of the electron density profile, as well as the atmospheric pressure and temperature, on the attenuation of the investigated radio waves is demonstrated. It is also found that the reflection at very oblique incidence depends mainly on the height of the bottom of the ionosphere.     

Indirect phase height measurements of the lower ionosphere compared with rocket measurements of D-region electron density

A comparison between rocket-measured electron density profiles of the lower ionosphere and the results of ground-based indirect phase height measurements in the LF range, carried out near the Soviet rocket sounding station Volgograd over several years, confirms—to a first-order approximation—the height of the level of electron density which according to magnetoionic ray theory is necessary for reflection of the waves. In a second-order approximation, however, an additional phase path change has to be taken into account, which is caused by the ionization below the reflection level. This makes the observed phase height always slightly smaller than the real geometric height, on average by −1 km, but in extreme cases by up to −4 km, depending on the actual height gradient of electron density below the reflection level. Due to systematic diurnal and seasonal variations of this gradient, the amplitude of the diurnal variation of the observed phase height is found to be slightly larger than that of the real geometric height, whereas the reverse is true for the seasonal variation at constant solar zenith angle.     

Shortcomings in our understanding of the lower ionosphere as revealed by an analysis of radiowave absorption measurements

Empirical relationships derived from an extensive series of equatorial ground-based radiowave absorption measurements are analysed in terms of our current understanding of the basic processes that control the formation and behaviour of the lower ionosphere. Multi-frequency absorption and virtual height measurements are combined with the results of a rocket borne experiment to construct an equatorial noontime reference electron density profile corresponding to the conditions of a non-flaring sun at solar cycle maximum. The enhancement in this electron density profile, and hence in the absorption of radio waves, as the solar 1–8 Å flux increases by moderate amounts is then calculated from our knowledge of the ion production and loss processes. In order to bring the calculated increase of absorption into agreement with the empirically established relationship (Gnanalingam, 1974), we find it necessary to reduce by a factor of about 5 the Meira (1971) nitric oxide densities below 90 km.Attention is drawn to the wide disparity between the effective recombination coefficient and the average dissociative recombination coefficient for the known ion composition in both the molecular and cluster ion regions of the lower ionosphere. The factor of 5 reduction in the nitric oxide profile required to explain the solar control of absorption, however, largely eliminates this disparity in the cluster ion region.Our analysis of the diurnal variation of radiowave absorption and virtual height also reveals a gross discrepancy between the calculated and measured variations. In order to resolve this discrepancy, a mechanism is needed which would cause the electron density in the 95–105 km region to decrease with increasing solar zenith angle faster than is predicted by the present theory of ion production and loss processes in this region.Also presented in this work is an ad hoc model of the diurnal variation of the electron density profile which is consistent with the measured diurnal variation of radiowave absorption and virtual height.     

Electron temperature and electron density in the F-region of the ionosphere. I. Observed relationship

Ionospheric data from three incoherent scatter stations over the height range 225–450 km were studied for all daylight hours over a wide range of solar conditions. The relationship between electron temperature T_e, electron density Nand solar flux at 10.7 cm wavelength S_10.7 was expressed as T_e = A−B·(N−5 × 10¹¹) + C·(S_10.7−750), where N is in units of m⁻³ and S_10.7 in kJy.This provided a very satisfactory expression for all data taken at Malvern and St. Santin between 0800 and 1600 LT. For data taken at Arecibo, however, the linearity broke down at low electron densities. The data from all three stations were therefore divided into two sets according to electron density and reexamined.ForN < 5 × 10¹¹ m⁻³ B increased steadily with height and decreased steadily with latitude.For N > 5 × 10¹¹ m⁻³ B did not appear to vary with height, with season or with latitude. C was approximately constant for all sets of data.The different mechanisms involved in the heat balance of the electron population are discussed and a qualitative explanation for the relationship is proposed.     

D-region electron density diurnal changes observed in association with the PCA event of July 1974

Well defined diurnal phase and amplitude deviations of very low frequency radio signals propagation were detected during the solar proton event that occurred from 3 to 8 July 1974. Phase advances observed on signals transmitted from GBR (U.K.), received at Atibaia (Brazil) and at Inubo Radio Observatory (Japan), were compared with simultaneous NWC (Australia) transmissions received at Atibaia, and at Syowa Station (Antarctica). Emphasis was given to the propagation paths NWC-Atibaia and NWC-Syowa, because both propagate completely in the southern hemisphere, crossing regions where the Earth's magnetic field behaves anomalously. The comparison allowed the determination of parameters typical of the D-region at a given height in the lower ionosphere, for geomagnetic regions defined through the McLlwain parameter(L). An exponential model was adopted to fit the vertical distribution of the diurnal electron density. The experimental results showed a delayed contribution to the total ionization observed which was attributed to a slow precipitation of energetic particles in the South Atlantic Geomagnetic Anomaly region.     

The height of electron content changes in the ionosphere from ATS6 beacon data

A technique is described which uses relative changes in Faraday rotation and modulation phase of satellite radio signals to determine the median height of the enhancement (or depletion) in the electron density of the ionosphere. During the post sunrise formation of the F mlayer the incremental layers have a median height of around 210 km (±40) and in the afternoon the decremental median is above the peak at 340 km (±40) on a winter day. A winter night-time enhancement just after midnight appears as a thick layer extending upwards from the peak, with a median height at about 730 km. The method applies to large scale irregularities but not to small, dense, scintillation-causing irregularities for which Faraday and modulation phases do not represent the total electron content.     

Dependence of seasonal variation of absorption on solar activity in the equatorial ionosphere

From data for the absorption of radio waves at oblique incidence in Lagos, and at vertical incidence at Colombo, the seasonal variation of absorption at the two sites are examined. It is shown that, if subsolar absorption be assumed to depend upon sunspot number, the cos X law gives the same index for both the diurnal and the seasonal variation of absorption.     

Night-time electron density of the lower ionosphere in the South Atlantic Geomagnetic Anomaly region obtained with a LF/VLF oblique ionosonde

Studies of the reflectivity of the night-time lower ionosphere in the region of the South Atlantic Geomagnetic Anomaly have been carried out for the period August 1980 to July 1981, using data collected by an oblique ionosonde at low/very low frequency, in the southern part of Brazil, near the centre of the Anomaly. From these studies monthly average behaviour of heights and reflection coefficients of the lower ionosphere between 15 and 60 kHz have been deduced. Assuming an exponential model of the electron density distribution in the lower ionosphere the appropriate numerical parameters were calculated using a trial-and-error approach with ‘full wave’ calculations and iterative computational techniques. For the period considered three sets of parameters could satisfactorily represent the lower ionosphere in the anomalous region and in the period analysed: one valid from August to November 1980, one valid from December 1980 to March 1981 and the other valid from May to July 1981. April 1981 seemed to present anomalous behaviour.     

Seasonal and latitudinal variations of the lower thermospheric tidal winds from meteor radar measurements in the U.S.S.R.

Meteor wind results obtained at 93–95 km altitude at seven sites, six in the Soviet Union and one in Antarctica, between 1965 and 1985 are reported. Attention is focussed on the amplitudes and phases of the semi-diurnal tide, showing a 22-yr oscillation, and of the generally weaker diurnal tide. The measurement results are compared with the results of theoretical models.     

The Seasonal Mobility of Prehistoric Gazelle Herds in the Azraq Basin,Jordan: Modelling Alternative Strategies Using Stable Isotopes

The hunting of Gazella subgutturosa was a dominant practice for Epipalaeolithic and early Neolithic hunter–gatherers in the east Jordan steppe. The seasonal mobility of this taxon in the Levant is poorly understood, especially for early prehistory when herd movements would have influenced hunter-gatherer use of the steppes. This paper proposes four patterns of seasonal herd mobility for G. subgutturosa centred on Jordan’s Azraq Basin. The four patterns are modelled using oxygen, carbon and strontium stable isotopes. Seasonal environmental signatures of each are understood through carbon and strontium isotopic variation in sixty modern plant specimens collected from twelve selected locations in north Jordan, published data on oxygen isotopes in local precipitation, and the adaptive behaviour of G. subgutturosa. The integrated isotopic datasets provide clear discriminatory markers for each proposed mobility pattern. Results will be applied in future to isotopic data from archaeological gazelle teeth from the Azraq Basin.     

A comparison of the height of the maximum electron density of the F2-layer from real height analysis and estimates based on M(3000)F2

Estimates of the height of the F-layer peak based on formulations using the ionospheric transmission factor M(3000)F2 are compared with hmF2 derived from the real height analysis of digital ionograms acquired at a mid-latitude station. Based on the analysis of 27 hours of quiet data, our result shows that the M(3000)F2 methods are highly accurate and that the formulation developed by Bradley P. A. and Dudeney J. R., (1973, J. atmos. terr. Phys. 35, 2131) is most accurate.     

A theoretical study of the distribution of ionization in the high-latitude ionosphere and the plasmasphere: first results on the mid-latitude trough and the light-ion trough

A model of the O⁺ and H⁺ distributions in the plasmasphere and high-latitude ionosphere is described and first results are presented. The O⁺ and H⁺ continuity and momentum equations are solved from the F-region to the equatorial plane in the inner plasmasphere, and to an altitude of 1400 km in the outer plasmasphere and high-latitude ionosphere. Account is taken of high-latitude convection, departure from corotation inside the plasmasphere, and neutral air winds. The neutral air winds are consistent with the assumed convection pattern. For equinox and magnetically quiet conditions the calculations show that a mid-latitude trough in F-layer electron concentration is present from 1600 to 0600 LT and the trough may occur either inside or outside the plasmasphere. The movement of the trough in this period is from higher to lower latitudes and is in qualitative agreement with AE-C and ESRO-4 data. A light-ion trough feature is apparent in the H⁺ distribution in the topside ionosphere at all local times. During the day the upward H⁺ flow increases with latitude to produce the light-ion trough. At night the H⁺ trough may be directly produced by the occurrence of the mid-latitude O⁺ trough. The relationships between the position of the plasmapause and the trough are discussed. Also discussed are the influence of particle ionization in the auroral zone and the effect of the neutral air wind.     

Secular variations in seawater chemistry as a control on the chemistry of basinal brines: test of the hypothesis

The hypothesis that basinal brines inherited their major ion chemistries and elevated salinities from evaporated paleoseawaters is tested by comparing the compositions of basinal brines in Silurian (Michigan basin, Illinois basin, Appalachian basin in eastern Ohio) and Jurassic/Cretaceous (Central Mississippi Salt Dome basin, Arkansas shelf, and south‐central Texas) host rocks, when the world oceans were ‘CaCl₂ seas’, with those from Permian and Pennsylvanian rocks (Palo Duro basin, Central Basin Platform, and Delaware basin, Texas and New Mexico) when the world oceans were ‘MgSO₄ seas’. Basinal brines examined are assumed to have originally formed from evaporation of the same seawaters that produced major evaporites. Sulfate, Mg and K levels in basinal brines are below the concentrations expected from evaporation of seawater of any type, which emphasizes the importance of diagenetic mineral–brine interactions in controlling basinal brine chemistry. There are no major differences in SO₄, Mg and K concentrations between basinal brines hosted by rocks originally formed during ages when the world oceans were MgSO₄ seas versus CaCl₂ seas. Basinal brines in Pennsylvanian–Permian rocks are compositionally distinct (relatively high Na and low Ca) from basinal brines in Silurian, Jurassic and Cretaceous host rocks, which may reflect original differences in seawater chemistry. Basinal brines enriched in Ca and depleted in Na relative to evaporated seawater of any type have traditionally been interpreted to form by albitization of plagioclase feldspar. A new explanation for Ca enrichment and Na depletion of basinal brines is the mixing of evaporated CaCl₂‐type seawater with more dilute water. Some basinal brines are similar in major ion composition to evaporated seawater of a particular age, for example basinal brines in the Cretaceous Edwards Group carbonates, Texas, where dolomitization is the only reaction required to convert evaporated Mesozoic CaCl₂ seawater into Edwards Group brine.     

Using airborne small-footprint laser scanner data for detection of cultural remains in forests: an experimental study of the effects of pulse density and DTM smoothing

Cultural remains are laborious to register by means of field surveys. Thus, in recent years several trials using remote sensing data to detect cultural remains have been carried out. The most promising remote sensing technique for such purposes is airborne laser scanning (ALS) from which digital terrain models (DTM) that enable visual interpretations of anomalies on the ground are generated. Although it has been documented previously that ALS is a powerful data source for detection of cultural remains, it has not yet been carried out studies that focus on the effects of the point density of the ALS data and properties of the subsequent generation of the DTM on the detection success of cultural remains. Thus, this study aimed at analyzing these effects by letting four experienced archeologists interpret DTMs generated from ALS data with point densities of 1 p m⁻², 5 p m⁻², and 10 p m⁻², each with three different levels of DTM smoothing. The experiment was conducted according to a randomized complete block design and the analyses were carried out using analysis of variance. The results showed that there was a significant improvement in the detection success when the point density was increased from 1 p m⁻² to 5 p m⁻². There were also some improvements when increasing the point density further to 10 p m⁻², but they were less pronounced. Furthermore, the results showed that the DTM smoothing did not have any significant effect on the detection success.     

Deformation bands and their impact on fluid flow in sandstone reservoirs: the role of natural thickness variations

Cataclastic deformation bands, which are common in sandstone reservoirs and which may negatively affect fluid flow, are generally associated with notable thickness variations. It has been suggested previously that such thickness variations represent an important control on how deformation bands affect fluid flow. The effects of such thickness variations are tested in this study though statistical analysis and fluid flow simulation of an array of cataclastic deformation bands in Cretaceous sandstones in in the Bassin de Sud‐Est in Provence, France. Spatial outcrop data are statistically analyzed for incorporation in flow simulation models, and numerical simulations are used to investigate the effects of notable thickness variations on how the deformation bands influence effective permeability and flow dynamics. A suite of simulations is performed using a combination of fine‐scale and coarse‐scale grids, revealing that the effective permeability of the simulated reservoir is reduced by a factor of 15–25. More interestingly, the simulations further demonstrated that, as compared to the overall effect of the deformation band array on fluid flow, thickness variations along the bands proved to have negligible effects only. Thus, our simulations indicate that the configuration and connectivity of the deformation bands, together with the permeability contrast between the bands and the host rock and the mean band thickness, are the most important controls on the effective permeability. Our findings represent new insight into the influence of deformation bands on fluid flow in subsurface aquifers and reservoirs, indicating that thickness variations of individual deformation bands are of less significance than previously thought.     

New density data for unfused and fused sheep bones,and a preliminary discussion on the modelling of taphonomic bias in archaeofaunal age profiles

The analysis of mortality profiles forms an important part of zooarchaeological analysis. It has long been assumed that the bones of immature animals are more prone to taphonomic destruction than those of mature animals, potentially distorting mortality profiles. This study measures the DPD (a proxy for taphonomic strength) of the postcranial skeletons of 91 modern sheep of known age. The data produced are used to demonstrate that the DPD of unfused bones is indeed generally lower than that of their fused equivalents, although considerable inter-individual variation exists. It is further suggested that bone density increases throughout the life of an animal and occurs independently of the fusion of its bones. Consequently, taphonomic bias is partly dependant on the age structure and not the fusion status of the deposited assemblage. Relative levels of bias can only be assessed if this age structure is constant for all elements being considered (e.g. if the material were deposited as entire carcasses). Finally, because potential age-related bias is not equal for all elements, the degree of bias associated with a particular fusion group may partly depend on the numbers of certain elements within that group.