Compensation-theorem analyses of ground wave propagation over irregular,inhomogeneous terrains

Work during the last 40 years on various aspects of ground wave propagation is surveyed, including dipole propagation over flat homogeneous and inhomogeneous terrains, smooth homogeneous earth, and irregular homogeneous and inhomogeneous terrains. The Ott-Volterra integral equation for TM_p ground wave field over irregular homogeneous paths is discussed and an occasional troublesome behavior of the modified Sommerfeld attenuation function explained. Several representations of spatially distributed transmitter current are presented. A Compensation-theorem Volterra for the ground wave over inhomogeneous terrain is derived, in the parabolic wave equation approximation. A recent Compensation-theorem integral equation for general terrain satisfying the hyperbolic wave equation is reviewed. Computations for several very rough terrains indicate the ‘parabolic’ integral equations may often be more accurate.     

ELF fields for an inhomogeneous ionosphere with diffusive lower boundary

An ELF field equation is derived for an inhomogeneous ionosphere with diffusive lower boundary. Numerical results are presented to show that consistent ELF fields independent of the stratification of the ionosphere can be obtained by using this field equation.     

Learning to teach geography for primary education: results of an experimental programme

Students training to become primary school teachers appear to have little awareness of the core concepts of geography (teaching). To ensure that future primary school teachers are able to develop their pupils’ geographical awareness, a six weeks programme was developed. The characteristics of this programme – named Consciously Teaching Geography (CTG) – are: principles of good geography teaching, conjunction and a recurrent structure during training, modelling and reflection. In a quasi-experimental research design the question is answered what the effects are of CTG on the development of pedagogical content knowledge (PCK) for the subject of geography of first year primary student teachers. The results indicated that the programme has a positive effect on the domain-specific PCK development in the short term.     

Non-relativistic and relativistic approaches to the problem of plasma wave propagation

When using the approximate expression for different wave refractive indices derived by Sazhin we outlined those values of plasma parameters and wave frequencies for which cold and non-relativistic plasma approximations are valid. For R and O waves the latter approximation is valid when the wave frequency is close to the electron gyrofrequency; for the X wave it is valid when the wave frequency is close to the doubled electron gyrofrequency.     

Waveguide model analysis of VLF wave propagation at 16 kHz

Preliminary results of measurement of phase and amplitude of 16 kHz VLF signals propagated in the Earth-ionosphere waveguide are presented. From the measured values of diurnal phase, amplitude changes and sunrise fadings the relevant waveguide parameters are estimated.     

Comparisons between EISCAT observations and model calculations of the high latitude ionosphere

Calculations using a numerical model of the convection dominated high latitude ionosphere are compared with observations made by EISCAT as part of the UK-POLAR Special Programme. The data used were for 24–25 October 1984, which was characterized by an unusually steady IMF, with Bz < 0 and By > 0; in the calculations it was assumed that a steady IMF implies steady convection conditions. Using the electric field models of Heppner and Maynard (1983) appropriate to By > 0 and precipitation data taken from Spiroet al. (1982), we calculated the velocities and electron densities appropriate to the EISCAT observations. Many of the general features of the velocity data were reproduced by the model. In particular, the phasing of the change from eastward to westward flow in the vicinity of the Harang discontinuity, flows near the dayside throat and a region of slow flow at higher latitudes near dusk were well reproduced. In the afternoon sector modelled velocity values were significantly less than those observed. Electron density calculations showed good agreement with EISCAT observations near the F-peak, but compared poorly with observations near 211 km. In both cases, the greatest disagreement occurred in the early part of the observations, where the convection pattern was poorly known and showed some evidence of long term temporal change. Possible causes for the disagreement between observations and calculations are discussed and shown to raise interesting and, as yet, unresolved questions concerning the interpretation of the data. For the data set used, the late afternoon dip in electron density observed near the F-peak and interpreted as the signature of the mid-latitude trough is well reproduced by the calculations. Calculations indicate that it does not arise from long residence times of plasma on the nightside, but is the signature of a gap between two major ionization sources, viz. photoionization and particle precipitation.     

Elements of an inclusive evolutionary model for archaeology

In the following pages it is argued that the transformational, progressive framework of Cultural Evolution often employed in archaeology is limited to the extent that it can but rarely function in an explanatory capacity. It is proposed that the goals of a processualist archaeology can be met more fully by the scientific, inclusive evolutionary paradigm discussed here. Components of this new paradigm, based on a selectionist perspective of change, are introduced. In sum, it is suggested that the construction of an evolutionary theory applicable in archaeology requires that artifacts at any scale be interpreted as part of the human phenotype, and that fitness be assessed through the concept of replicative success. In this fashion, selective mechanisms may be identified and processual explanations of change generated.     

On the influence of a plasma hot component on whistler propagation beyond the plasmapause

Theoretical spectrograms are computed for whistlers propagating beyond the plasmapause. The electron distribution function was modelled as consisting of a hot plus a cold component and an appropriate dispersion equation is used. A collisionless (CL) model is used for the cold electron concentration and for the hot electron component the derived model assumes a bi-maxwellian distribution function with a loss cone at the equator. The results indicate limits on the use of the cold plasma approximation (c.p.a.) in the study of magnetospheric whistler propagation beyond the plasmapause and show that whistler analysis with the c.p.a. may under or overestimate the L value of the path deduced from ground spectrograms, depending on the anisotropy of the hot component.     

Raptors in towns: towards an ecological model

A survey was undertaken of the occurrence of bones of raptorial birds in Roman and medieval nucleated settlements in northwestern Europe, and the results were compared with a series of a priori predictions as to which species were the most likely to have colonized such settings. The comparison showed that Haliaeetus albicilla, Buteo buteo and Milvus milvus were recovered relatively frequently, and that these three were amongst the species predicted to be the most successful urban scavengers. It is suggested that Buteo lagopus and Milvus migrans are underrecorded, and that records of Aquila pomarina might be expected in the circum-Baltic region. Accipiter species are inherently unlikely urban birds because of their hunting and feeding behaviour, and the numerous records, especially of Accipiter gentilis are argued to be birds kept for hawking. This would be consistent with the observation that Accipiter spp. are commonly found as whole or partial skeletons. The general agreement of observations with expectations is taken to validate the underlying premise that refuse constituted a major environmental factor in Roman and medieval towns. © 1997 John Wiley & Sons, Ltd.     

Seasonal variation in mesospheric semi-diurnal tides. Comparison of meteor radar observations and results from an excitation source model

The seasonal variation of the semi-diurnal tide is well established in the upper mesosphere from meteor radar observations, such as those made at Garchy (France). A classical propagation model, using a realistic excitation source from ozone and water vapour solar heating, can account for most of the seasonal variation characteristics, and in particular the strong difference between summer and winter features.     

The lenses of nationhood: an optical model of identity

ABSTRACT. This paper tries to make the case for a model of political identity based on an optical metaphor, which is especially applicable to nations. Human vision can be separated into sentient object, lenses and inbuilt mental ideas. This corresponds well to identity processes in which ‘light’ from a bounded territorial referent is refracted through various lenses (ideological, material, psychological) to focus in certain ways on particular symbolic resources like genealogy, history, culture or political institutions. Distinguishing between referent, lenses and resources helps us more precisely situate many hitherto disparate problems of national identity. These include the ‘ethnic‐civic’ dilemma, the mystery of national identity before nationalism, and the relationship between local and national, and individual and collective, identities. The model also clarifies the place of universalist ideology, which currently fits poorly within the leading culturalist and materialist theories of nationalism.     

Application of a simplified theory of ELF propagation to a simplified worldwide model of the ionosphere

The approximate theory of ELF propagation in the Earth-ionosphere transmission line described by Booker (1980) is applied to a simplified worldwide model of the D- and E-regions, and of the Earth's magnetic field. At 1000 Hz by day, reflection is primarily from the gradient on the underside of the D-region. At 300 Hz by day, reflection is primarily from the D-region at low latitudes, but it is from the E-region at high latitudes. Below 100 Hz by day, reflection is primarily from the gradient on the underside of the E-region at all latitudes. By night, reflection from the gradient on the topside of the E-region is important. There is then a resonant frequency (~300 Hz) at which the optical thickness of the E-region for the whistler mode is half a wavelength. At the Schumann resonant frequency in the Earth-ionosphere cavity (~8Hz) the nocturnal E-region is almost completely transparent for the whistler mode and is semi-transparent for the Alfvén mode. Reflection then takes place from the F-region. ELF propagation in the Earth-ionosphere transmission line by night is quite dependent on the magnitude of the drop in ionization density between the E- and F-regions. Nocturnal propagation at ELF therefore depends significantly on an ionospheric feature whose magnitude and variability are not well understood. A comparison is made with results based on the computer program of the United States Naval Ocean Systems Center.     

Toward an ecological model for chalcolithic cultures of central and western India

The culture area is a construct of behavioral uniformities which occur within an area of environmental uniformities. It is assumed that cultural and natural areas are generally coterminous because the culture represents an adjustment to the particular environment. (Steward 1955:35)     

The triangular model as an instrument for visualising and analysing residuality

Over the last thirty years, residuality has developed into a major research topic in archaeology. In these studies, several ways of visualising residuality have been proposed, facilitating the analyses of archaeological contexts. These all started from linear temporal concepts, though. In the present article, a new approach is proposed, which makes use of an extra dimension. This enables the inclusion of extra variables and, hence, a more balanced analysis of the contexts through their graphic representations. Archaeological contexts from recent excavations in Carthage are used to illustrate the potentials of the model and its application in the archaeological practise.     

Flake selection and scraper retouch probability: an alternative model for explaining Middle Paleolithic assemblage retouch variability

It has been proposed that the relative abundance of retouched objects in Paleolithic assemblages can serve as a measure for artifact transport and by extension a proxy for site occupation duration. This approach is based on the assumption that retouch represents curatory effort for extending the service time of transported artifacts when raw material access is uncertain or limited, a condition that could arise when groups move frequently over long distances across the landscape. This paper proposes an alternative model that explains retouch as a probabilistic outcome of an expedient, on-site flake selection process. A simulation illustrates that the model is capable of producing assemblage retouch configurations akin to those commonly observed in Paleolithic settings. The simulation also indicates that the threshold applied by past individuals for selecting particular artifacts is an important parameter for explaining assemblage retouch variability. Using artifact weight as a proxy for flake selection criteria, several Middle Paleolithic assemblages exhibit patterns that support predictions made from the model simulation. Findings suggest that variation in scraper frequency among the studied assemblages can be accounted for by an interaction between the abundance of artifact production events and shifting artifact selection criteria, without appealing to higher-level behaviors of technological and mobility strategies.     

Degradation of mud brick houses in an arid environment: a geoarchaeological model

A common assumption in Near Eastern tell archaeology is that the majority of sediments originate from degraded mud bricks. Little is known about the mechanism of mud brick wall degradation. Here we present a detailed macro- and microscopic ethnoarchaeological study of the degradation of a mud brick house and propose a comprehensive mechanism for tell formation processes in arid environments. The study took place in southern Israel by trenching a ca. 60 year old abandoned mud brick house, followed by extensive sediment sampling. Macroscopic observations showed that mud brick walls degrade by collapse of single bricks and/or collapse of intact wall parts, either inwards or outwards. In addition, infill sediments within the house and outside it, in close proximity to its walls, form alternating sedimentary layers of various colors and textures. The degraded mud brick material lost its distinctive macroscopic structure, which makes it impossible to accurately identify this material by field observations alone. Mineralogical and elemental analyses established the sources of the house infill sediments, namely mud bricks and wind blown sediments. Alternating layers mostly originate from mixing between degraded mud brick material and wind blown sediments. Micromorphological observations revealed microscopic mechanisms of mud brick degradation and include processes of mud slurry gravity flows, sediment coatings and infillings, wind abrasion of walls, small-scale puddling, and bioturbation. This study provides a working scheme for site formation of abandoned mud brick structures in arid environments. It provides a set of criteria by which it is possible to differentiate floors from post-abandonment sedimentary features and thus improves the reliability of activity area research.     

Ancient social landscapes of northwestern Argentina: preliminary results of an integrated approach to obsidian and ceramic provenance

This article presents the results of ongoing instrumental neutron activation analyses (INAA) conducted on archaeological artifacts from the Formative period of northwestern Argentina (NWA). These studies are part of a wider archaeological project that seeks to understand the structure of the social landscape of the period by examining domestic and burial evidence from a wide range of villages across the area known as the southern Calchaquí valleys. Elemental data is discussed in the light of its potential contribution to reassess past social interaction strategies in the region.     

Applicability of the two-layer model for simulation of non-linear evolution of a large scale plasma cloud in the ionospheric F-region

The adequacy of the two-layer model of Lloyd and Haerendel for describing the behaviour of an ionospheric irregularity is verified by numerical simulation of large plasma cloud dynamics. The background ionosphere is approximated by a set of conductive layers with ion mobilities and concentrations corresponding to the real ionospheric conditions. Polarization electric field produces positive and negative image clouds, i.e. plasma density enhancements and depletions in each layer. Their intensity, form and orientation turn out to change with height depending on the local conditions. However, the drift and deformation of the released cloud slightly differ from the case when the ionosphere is characterized by constant, height averaged parameters, at least if altitude dependent neutral wind and photochemical processes are ignored.     

A model of ionospheric image structure underneath a braking,cross-field plasma jet

A plasma jetting across the geomagnetic field above the ionosphere tends to brake by ohmic dissipation of Pedersen currents. The braking can affect the ionosphere underneath if the associated Pedersen drifts are intense and prolonged enough to cause cumulative image structuring. We examine such image structuring for the parameter regime of forthcoming CRRES releases, involving photo-ionization of kilograms of barium vapor moving at orbital velocity. The resultant structuring in the upper E-region offers possible diagnostic tell-tales of the braking process.