The lowest regions of the ionosphere are conveniently studied by means of reflection techniques using very low frequency radiowaves (10-30 kHz.).Most observations of this kind have measured only the cross polarised component of the reflected signal yielding the so-called conversion coefficient, R1. In this thesis an attempt is made to measure the in-plane signal, R, in addition to R1. From these two parameters the polarisation of the reflected waves may be determined. The results are interpreted in terms of the D-region electron density profiles by modelling techniques based on the full-wave analysis. Data collection for a period of one year are presented and discussed. A detailed account is given of the construction of equipment suitable for measurement of the normal polarised component of the ionospherically reflected radiowave, using the a modification of the 'loop and aerial' system. Due consideration is given to the experimental difficulties. It is established that the stability, both in phase and amplitude, is an extremely important factor in the success of the experiment. Methods are discussed of ensuring that no ground wave contamination exists, and details are given of a computer program to do this. A large section of the disertation is devoted to a detailed descrimption of the development of the data logging systems as these were responsible, to a large extent, for the success of the experimental programme. Of particular importance is the description of the on-line computing facility, using an advanced programmable calculator, with magnetic tape facility. The experimental data collected over one year is presented, and both seasonal and diurnal variations are presented. The related electron density profiles, derived by modelling the four measured paramenters, are discussed. Disturbed conditions have been considered for times of solar flare, stratospheric warmings and magnetic storm activity. Again modelling of profiles has been undertaken and a representative set is given. The necessity of measuring both, R and R1 for modelling work has been demonstrated and the errors likely to occur from using R1 alone have been fully discussed.

The large diocese of Lichfield and Coventry covered most of the north Midland region and educational development within it has been examined in the context of social, economic and religious changes during the seventeenth century. The study falls into four main sections, corresponding to developments before 1600, from 1600 to 1640, from 1640 to 1660 and from 1660 to 1700. A substantial part of the research in each of the sections 1600 to 1640 and 1660 to 1700 is based on the diocesan records kept at Lichfield. Before 1600, most of the large centres of population in the diocese were provided with an endowed school. This had been achieved partly by the philanthropy of merchants and gentry but also to a large extent by local initiative, particularly that of town corporations. There was widespread use of confiscated church revenues for school endowments, and a substantial measure of lay control. Between 1600 and 1640, the established schools in the diocese flourished. For the most part, they educated the sons of the 'middle classes', many of whom entered the church. New school endowments were made but not on the scale of the pre- 1600 period. Also, most were smaller in scope and were made by men of lower social status than the sixteenth century benefactors. A new feature, however, was the endowment made specifically for reading and writing, and this type of education was also provided by a large number of 'unendowed' schoolmasters, most of whom taught in small villages. During the period 1640 - 1660, when puritan ideas were in the ascendancy, the provision of endowments for reading and writing continued and some were intended for girls' education. The majority of the earlier endowed schools surmounted the difficulties of the time with apparently little disturbance. After 1660, the effects of religious, social and economic influences could be seen in the large increase in endowments for reading and writing, made mostly by men (and women) of the 'middling sort'---the lesser gentry, emergent yeoman families and well-to-do town merchants. The number of 'unendowed' schoolmasters in the diocese also increased and, overall, contributed to a significant advance in the spread of literacy in town and country. The larger endowed schools continued to educate boys from a wide social background for entry to the professions. By 1700, the diocese was provided with a widespread and varied number of schools in the towns and in the countryside. In their different ways, they provided opportunities for social advancement; contributed to social mobility and the spread of literacy, and as yet appeared untouched by the exclusiveness and social divisions of the following century.

Measurements have been made of the resistivity of liquid nickel-cobalt and silver-palladium alloys as a function of composition and temperature together with some measurements on these systems in the solid state and a measurement of the thermoelectric power of a single liquid silver-palladium alloy as a function of temperature. Experiments have also been performed to determine the resistivity and thermoelectric power of liquid antimony as a function of temperature, and of liquid nickel-tellurium alloys as a function of temperature and composition. Apparatus has been developed to enable such experiments to be undertaken. In those cases where information is available, the results obtained here are generally in agreement with other experimental data. Approximate expressions can be derived from a single site resonant scattering model which give the resistivity of suitable liquid transition metal alloys in terms of the resistivities of the pure components, and give the thermoelectric power of the alloy in terms of the resistivities and thermoelectric powers of the pure components and the resistivity of the alloy. Predictions derived using these expressions have been compared with the experimental data: this comparison indicates that the theory in its present form provides, at best, only a qualitative account of the observed transport properties of liquid transition metal alloys. Calculations based on a percolation model of electronic transport in inhomogeneous liquids have been performed in an attempt to understand the experimentally observed variation of resistivity with concentration in the nickel-tellurium alloy system. Such a model appears to provide a reasonable qualitative description of the experimental data for the tellurium-rich alloys.

Spectral analysis of Ginga observations of Seyfert galaxies suggests a departure from the simple power law model which adequately described the HEAO-1 and EXOSAT data for these sources. In particular, compelling evidence is found for an Fe K-emission line, a K-absorption edge and a spectral flattening above about 10 keV. Analysis of a sample of AGN spectra shows that these features are extremely common. The mean energy of the emission feature suggests an origin in Fe K fluorescence from material in a low state of ionization, with a mean equivalent width ~ 150 eV. In contrast, highly ionized gas is required to reproduce the absorption edge energy. Monte Carlo modelling, carried out in various geometries, suggest that the line arises via fluorescence in cold, optically thick material out of the line of sight to the X-ray source, which is accompanied by continuum photons 'reflected' from the material. The characteristic shape of the reflected spectrum, when combined with the primary power law, leads to the observed emission line and 'hard tail'. The reflecting material must subtend a large solid angle at the X-ray source to explain the strength of the emission line and hard tail. Such a geometry is suggestive of an accretion disc. Observations of spectral variability in two Seyfert galaxies, MCG-6-30-15 and NGC 5548, show that such variability can be explained by varying proportions of directly viewed and reflected continua, constraining the extent of the reflecting material. Detailed analysis of the spectra of these two objects show that they are consistent with X-ray reflection from an accretion disc, with additional absorption by a highly ionized 'warm absorber' in the line of sight, as found for the bulk of the sample sources. In the case of NGC 5548, an extra component to the line emission from the warm absorber is also inferred. Concluding, the accretion disc reflection model provides an excellent description of the observed spectral features and variability in Seyfert galaxies, but a complex of emission lines around 6.5 keV seems highly likely. Future observations of these objects, with improved spectral resolution, should therefore be of great value.

The first IUE SWP high-resolution spectral atlas of central stars of planetary nebulae and hot white dwarfs is presented. Its primary aim is as a set of comparison spectra, with which it is shown that LS V +46 21, the postulated central star of S 216, has a temperature Teff = 80, 000K + 10,000K, and log g ~ 7. These are the characteristics that had been predicted from studies of the nebula itself. The claim of Feibelmann and Bruhweiler (1990) that only those central stars with Fe VII absorption lines show N I has been refuted. Many of the spectra listed by these authors as lacking both ions have continua that are either non-existent or severely under-exposed. Of the remainder, all show N I, and the presence of Fe VII correlates with photospheric temperature. Subsequently, it was found that there is a correlation between the equivalent widths of the N I and Si II 1260A lines and the surface gravity - and thus the age - of the star. This turned out to be a selection effect, with the younger stars both more luminous and sparser, and thus seen behind a larger interstellar column. EUV/soft X-ray data from five central stars and hot white dwarfs has been analysed. For the three with the largest number of observations, opacity sources used in current model atmospheres programs are insufficient. With both H1504+65 and NGC 246, the last two ionisation stages of C and O need to be included, as does Fe V for the hot DA white dwarf G191-B2B. Over twenty lines of this species were detected when five IUE SWP high-resolution spectra of G191-B2B were co-added. The excitation potential of these lines requires a photospheric origin. Consequently, if an observed blueshift of the doublets of N V and C IV with respect to Fe V by ~ 5km s-1 is real, it would suggest that both ions may be formed in a slow wind rather than in the photosphere.

Photoelectron spectroscopy, low energy electron diffraction (LEED), Auger electron spectroscopy (AES), secondary electron emission crystal current (SEECC) and work function change measurements have been used to correlate the electronic and geometric structures of well characterised overlayers of Tm and Gd on Mo(110) and Ag(001). Tm deposited on Mo(110) is shown to form two ordered structures with (10x2) and c(8x4) symmetry before a hexagonal monolayer is formed. Mixed valence, caused by low co-ordinated adatoms, has been observed for sub-monolayer coverages of this system at room temperature. Tm alloys with Ag(001) at room temperature and above, and is found to be trivalent. At 130K the diffusion is inhibited and low co-ordinated atoms are again found to be the cause of mixed valence. Resonant photoemission measurements of thick, mixed valent Tm layers at the 4d-4f absorption threshold indicate that the 4d hole, caused by three strong absorption processes, decays in all cases by direct recombination. Strong enhancement of the trivalent 5s and 5p photoemission are noted for the lowest energy absorption peak and the second two peaks result in enhanced trivalent 4f features. There is also evidence of divalent 4f level enhancement. Gd grows on Mo(110) in a layer by layer mode, and forms six different sub-monolayer structures. The symmetry of the overlayers is similar to Gd/W(110) but with an extra (4x2) phase occurring. The initial growth is characterised by a constant ordering in the [001] direction and gradual contraction in the [110] direction, before isotropic growth of hexagonally based structures. The 3d-4f absorption process for thick Gd layers, reveals two strong peaks due to absorption from the 3d3/2 and 3d5/2 electron energy levels. The 3d hole decays by direct recombination leading to enhanced photoemission features, although a significant Auger intensity is observed on the high energy sides of the absorption peaks.

The aim of this work is to investigate some of the properties of one- and two-dimensional systems, in particular, those concerned with electrical conduction. In the quantum mechanical regime such systems have exhibited a range of interesting and potentially useful characteristics. The Landauer-Buttiker formalism has shown that the quantity which determines the resistance in mesoscopic systems is the quantum-mechanical transmission. For a strictly one-dimensional system the transmission is easily evaluated for different arrays of model potentials. Results show that the resistance increases exponentially with length as the system becomes long. For shorter systems the degree of localisation of electronic states is investigated. A time-dependent approach is adopted to calculate the transmission in the case of quasi-one-dimensional quantum wires. This requires a modification of the simple (zero temperature) Landauer conductance formula to accommodate a wave packet of finite width in momentum space. The method developed is fast, efficient and is easily adapted to study quantum wires of differing geometry, an important factor when transport is ballistic. The effect of the finite width of the wave packet is investigated. The time-dependent approach is also extended to model the Coulomb repulsion between electrons; this is applied to electron tunnelling through a quantum dot. In addition, a suitable propagation scheme for the conductance calculation allows the inclusion of a magnetic field. The effect of a transverse field on point contact conductance is considered. As a further example, the question of transport in a model lateral surface superlattice potential is addressed. Again, the finite width of the wave packet is important.

No description available.

Meteor scatter (MS) multipath and diversity is investigated using 37 MHz continuous wave transmissions. A cross-correlation technique and a modelled MS communications system are used to analyse diversity-data collected at temperate latitudes at different antenna separations and season. Cross-correlation studies are first performed at an antenna separation of 102 by investigating the crosscorrelation variation, with time, of signals which have been categorized as "underdense," "overdense," or "not known" (NK). The cross-correlation coefficients of signals from underdense and overdense trails are shown to be high when correlated over their total signal-envelopes. This is not true, however, when the cross-correlations are performed on signal envelopes which have been segmented in time. NK signals are observed to be more decorrelated than underdense or overdense signals and, therefore, are likely to be the most advantageous to a MS communications system incorporating space-diversity. Combining the data from all three signal categories it is shown that ~40% of signals exhibiting a duration 0.75s, have correlation values of less than 0.6 after the first 0.25s of signal decay. The correlation-time dependency observed for NK and underdense signals is not identified for the overdense signal category. It is proposed that for underdense and NK signals the correlation-time dependency is due to the vector addition of other weak signal-modes. These weak signal-modes, however, have little effect on overdense signals which, typically, exhibit higher signal-powers during the early stages of signal decay. Cross correlation analysis of signals received at antennas separated by 5°, 10°, and 20° shows that no spatial variation, and in particular no decrease, in average cross-correlation coefficient is observed for underdense or NK signals. At each antenna separation the cross-correlation coefficients of these two signal categories were strongly dependant on time. Overdense signals, however, show no cross-correlation time-dependency at 5° and 10°, but a strong time-dependency is observed at 20°. The measurements support the view of previous workers who have suggested that a 4° antenna separation may be useful in a MS diversity communications system. At an antenna separation of 10? a modelled broadcast scan-diversity and maximal-ratio diversity systems are investigated. The results show that the optimum broadcast data-block duration, in February, for a non-diversity system is ~90ms assuming a ~10ms preamble. The diversity-gain results, at a signal-to-noise (SNR) ratio of 3 dB in a 3 kHz bandwidth, show that the broadcast throughput scan-diversity gain is ~1.4 in June and ~1.18 in February. These gains are equivalent to transmitter power improvements of ~3 dB in June, and ~1.4 dB in February, assuming a simplified relationship between transmitter power and data throughput. The broadcast throughput gain achieved by a maximal-ratio diversity system in June is ~1.9 and ~1.6 in February. At the 3 dB SNR these gains are equivalent to transmitter power increases of ~5.8 dB and ~4.1 dB in June and February respectively. The summer-winter diversity gains observed are attributed to the reception of more decorrelated NK signals (e.g. sporadic-E) in June than February. Frequency shift keying (FSK) error-probability analysis is investigated for a selection-diversity system and a maximal-ratio diversity system with antennas separated by 10°. The results show that by using signals 3 dB SNR, ~62% fewer errors are observed in February using selection diversity than a non-diversity system, and that in June this improvement decreases to ~50%. Results obtained from the maximal-ratio diversity system show that in both February and June ~90% fewer errors are observed than a non-diversity system. By assuming 216 bit broadcast data-blocks, the results show that a non-diversity system, using signals 3 dB SNR in February, will experience an error every 8th data-block. A selection-diversity system and a maximal-ratio diversity system, however, will experience errors every 24th and 78th data-block respectively. In June every 6th, 16th and 52nd data-block will be received in error by the non-diversity, selection-diversity and maximal-ratio diversity systems respectively.

This thesis studies the phenomenon of the magnetic storm, which is a global disturbance of the Earth's magnetic field and is defined in this thesis as an interval during which the Dst index falls below -50 nT for four consecutive hours. Storms are classified as either storm sudden commencements (SSCs; all storms which are initiated by a sudden impulse recorded by ground magnetometer stations) and storm gradual commencements (SGCs; all other storms). A superposed epoch analysis of solar wind plasma, interplanetary magnetic field (IMF) and magnetic indices AL, Kp and Dst has been undertaken for 538 storms identified between 1963 and 1991. The superposed epoch signature of the interplanetary medium during SSCs is similar to the signature of coronal mass ejections, whereas the corresponding superposed epoch signature of SGCs is similar to the signature of a high speed/ low speed coronal stream interface. A statistical study on the occurrence of the 538 identified storms has also been undertaken. The previously observed semi-annual variation in the occurrence of magnetic storms is shown to apply to SGC events only. Universal time (UT) variations in the onset times of SGCs have been observed. A UT variation has also been observed in the time-of-peak activity in the A m and Dst indices during storms. A case study of the ionospheric convection during the magnetic storm of March 20-21, 1990 has been undertaken. The response time of the ionospheric convection to changes of the IMF at the magnetopause associated with the SSC was a factor of two quicker than previous observations under normal solar wind conditions. A latitudinal dependence in the convection response time was also observed. Reconfigurations of the nightside convection pattern in response to substorm expansion phase onsets were observed implying that the nightside convection pattern can be dominated by substorm activity.