The theory of radio windows in the ionosphere and magnetosphere

Radio waves in a stratified plasma can sometimes penetrate through a region where, according to a simple ray theory, they would be evanescent. They emerge on the far side in a different magnetoionic mode. This occurs when the incident wave normal is within a small cone of angles, called a radio window. The best known example is the Ellis window, used to explain the Z-trace in ionosonde records. Other phenomena where windows may be important have recently been studied. Simple approximate formulae are given for the transmission coefficient of a window and for its angular widths. These show the dependence on frequency, electron concentration gradient and direction of the ambient magnetic field. Comparison with more accurate calculations shows that these formulae are likely to be reliable in practical applications. The tracing of rays near a window is discussed, and the properties of a second kind of window are described.     

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.     

Passage of a powerful HF radio wave through the lower ionosphere as a function of initial electron density profiles

The results of numerical modelling of powerful HF radio wave propagation through the ionosphere plasma are presented. Comparison of the heating wave parameters with those of a low powerwave gives the possibility to study the self-action of the powerful HF wave. At low altitudes the ‘translucence’ of the ionosphere plasma takes place. At high altitudes the wave absorption sharply increases. Both self-action effects lead to the reduction of the altitude range of the heated region. The dependence of self-action effects and the resulting electron temperature profiles on the initial electron density profiles are studied.     

Determination of the atmospheric refractive index structure parameter from refractivity measurements and amplitude scintillation measurements at 36 GHz

Results are presented of the determination of the atmospheric refractive index structure parameter C_n², deduced from amplitude scintillation measurements made on a 36 GHz radio link and also from the refractive index frequency spectrum computed from refractivity measurements using a radio refractometer. The two methods of measurement give acceptable agreement and provide further experimental evidence of the applicability of Tatarski's wave propagation theory in a turbulent medium to millimetre wavelengths.     

The effect of travelling ionospheric disturbances on the group path,phase path,amplitude and direction of arrival of radio waves reflected from the ionosphere

A fixed frequency amplitude modulated transmission was reflected from the ionosphere and changes in the group path, phase path, signal amplitude and directions of arrival of this transmission caused by travelling ionospheric disturbances were measured. These measurements enabled approximate determinations of the horizontal wavelength, period and horizontal phase velocity of the disturbances, which were compared with the theory of disturbances for atmospheric waves. A simple model is proposed to explain the phase relationships between the TIDs observed in the group and phase paths, and the faster decrease in power of the phase path than the other measured parameters indicated by spectral analysis.     

Numerical simulation of the penetration and reflection of a whistler beam incident on the lower ionosphere at very low latitude

By the full-wave algorithm with Fourier synthesis, 3-D propagation of a whistler beam incident on the pre-dawn lower ionosphere at very low latitude is numerically investigated. Processes of transmission, reflection, and coupling with the Earth-ionosphere waveguide are discussed via the wave energy and polarisation distributions and their dependence on the wave parameters and the ionospheric profile (such as the E_s-layer). It is shown that the dominant wave above 90 km altitude has the propagation characteristics of the magneto-ionic whistler mode, and absorption, spreading, reflection and mode conversion mainly occur at, and are greatly affected by, the bottom of the ionosphere. It is found that the transmitted energy density along the Earth's surface is reduced by 20 dB or more. Beam transmission loss varies asymmetrically with the incident angle, but changes little with the frequency. In the region 150 km (for 5 kHz) away from the ‘exit area’ where whistlers emerge, the bearing measurements using ground-based VLF direction-finders may be in error because direction-finding algorithms assume plane wave propagation. Only a small portion (about −25 dB at 5 kHz) of the incident energy is reflected up to an altitude of 150 km, and major reflection takes place in a small range of altitude at the bottom of the ionosphere with little spreading and lateral shift with respect to the incident beam. Reflection is enhanced considerably at lower frequency. Our results also suggest that an E_s-layer or an ionospheric gradient refracting waves to higher latitudes would be favorable factors for multi-hop echoes to be received on the ground.     

Morphology of low-frequency waves in the solar wind and their relation to ground pulsations

Three classes of low frequency waves (period range 20–80 s) were identified using data from the UCLA fluxgate magnetometer experiment on board the ISEE 2 spacecraft. These are continuous pulsations similar in type to Pc 3, band-limited oscillations distinguished by mixed period fluctuations, and relatively isolated wave bundles. The waves were preferentially observed when the interplanetary magnetic field (IMF) direction was sunward and were most common when the cone angle, i.e. the angle between IMF and the Sun-Earth line (θ_xb) was often between 15° and 45°. Their frequency is proportional to the IMF magnitude.Comparison between the waves observed on board the ISEE 2 spacecraft and the Pc 3–4 recorded simultaneously at a mid-latitude ground station, Oulu (L = 4.5), showed that similarity of spectra of the waves in the spacecraft and on the ground was very rare and that correspondence between the events in space and on the ground was extremely low.     

A strange result in the measurement of the angles of arrival of the first and second echoes from the ionosphere at high radio frequencies

Angles of arrival of first echoes (those directly reflected from the ionosphere) and second echoes (those twice reflected from the ionosphere with an intermediate reflection from the ground) were measured. It is easy to show that under specified conditions the off-vertical angle of arrival of the second echo ought to be twice that of the first echo. It is consistently found to be less than this for much of the time. Several possibilities are canvassed, but none provide a convincing explanation. The place on the Earth from which the second echo was reflected was nearly always the sea or flat ground. Apparently, rapid phase variations, as the tilt of the ionosphere changed, prevented recognition of the second echo by this particular radar system for echoes reflected from rough terrain.     

Modification of the strato-mesospheric temperature and wind structure resulting from the 26 February 1979 solar eclipse

Temperature and wind behavior observed during the February 1979 solar eclipse shows significant change immediately following and up to one hour after totality. Statospheric and mesospheric data obtained from Fort Churchill, Manitoba, indicate quite clearly a cooling trend between 50–60 kilometers with the maximum temperature decrease of approximately 10°C evident above 52 kilometers. This temperature perturbation was accompanied by an amplification of the meridional wind speed of 20–30 mps near 60 kilometers. These results are essentially in agreement with those obtained at Wallops Island during the March 1970 solar eclipse. Although the stratosphere was under the large-scale influence of a stratospheric warming, the short-term perturbations caused by radiative changes as a result of the solar eclipse did not appear to be masked.     

Transcending the academic/public divide in the transmission of theory: Raglan,diffusionism, and mid-century anthropology

Anthropology is plural, not singular, and only a section of its history is decided within universities. A critical re-examination of the work of Lord Raglan demonstrates that retaining an overly academic conception of anthropology impoverishes our understanding of its pasts and its futures. The last of the gentleman-scholars in British anthropology, Raglan was a prominent polemicist of the mid-century, who persistently kept anthropological approaches to contemporary concerns within the public eye. Though a postwar President of the Royal Anthropological Institute and praised by scholars in neighbouring disciplines, Raglan’s diffusionism was sharply criticized by standard-bearers of structural-functionalism. Adopting a broader perspective, Raglan can be viewed as both a sharp-eyed scholar and a successful public intellectual; re-assessment of his work and its effects leads to a re-consideration of the historiography of mid-century UK anthropology: particular theories, though denigrated by mainstream anthropologists, may continue to flourish in other disciplines or extra-academic arenas.     

There was No More Sea: the supersession of the ocean,from the bible to cyberspace

In considering the particular nature of the ocean as an object of western thought, sources to consider include the Hebrew Bible and some of its Eastern Mediterranean and Semitic antecedents, all of which posit the ocean as an antithetical element or object. This relationship to the ocean—and comparisons with other traditions show that the West's is distinctive—appears to inform a range of visions of the ocean, up to the present. Philosophical texts suggest that there may be particular difficulties inherent in the ocean as an object of embodied knowledge.     

Simulations of the seasonal variations of the thermosphere and ionosphere using a coupled,three-dimensional,global model,including variations of the interplanetary magnetic field

The University College London Thermospheric Model and the Sheffield University Ionospheric Convection Model have been integrated and improved to produce a self-consistent coupled global thermospheric/high latitude ionospheric model. The neutral thermospheric equations for wind velocity, composition, density and energy are solved, including their full interactions with the evolution of high latitude ion drift and plasma density, as these respond to convection, precipitation, solar photoionisation and changes of the thermosphere, particularly composition and wind velocity. Four 24 h Universal Time (UT) simulations have been performed. These correspond to positive and negative values of the IMF BY component at high solar activity, for a level of moderate geomagnetic activity, for each of the June and December solstices. In this paper we will describe the seasonal and IMF reponses of the coupled ionosphere/thermosphere system, as depicted by these simulations. In the winter polar region the diurnal migration of the polar convection pattern into and out of sunlight, together with ion transport, plays a major role in the plasma density structure at F-region altitudes. In the summer polar region an increase in the proportion of molecular to atomic species, created by the global seasonal thermospheric circulation and augmented by the geomagnetic forcing, controls the plasma densities at all Universal Times. The increased destruction of F-region ions in the summer polar region reduces the mean level of ionization to similar mean levels seen in winter, despite the increased level of solar insolation. In the upper thermosphere in winter for BY negative, a tongue of plasma is transported anti-sunward over the dusk side of the polar cap. To effect this transport, co-rotation and plasma convection work in the same sense. For IMF BY positive, plasma convection and co-rotation tend to oppose so that, despite similar cross-polar cap electric fields, a smaller polar cap plasma tongue is produced, distributed more centrally across the polar cap. In the summer polar cap, the enhanced plasma destruction due to enhancement of neutral molecular species and thus a changed ionospheric composition, causes F-region plasma minima at the same locations where the polar cap plasma maxima are produced in winter.     

Aberglasney: the theory,history and archaeology of a post-medieval landscape

Abstract

Printed and documentary sources, archaeological excavation, dendrochronology and geophysical survey are employed to investigate the history of Aberglasney, a small country house near Llandeilo, Carmarthenshire (SN 5815 2213). Traditions about its garden layout, parapet walkway, a gatehouse and a yew ‘tunnel’ are examined. Circa 1600, Bishop Anthony Rudd (1549–1614) probably built a ‘cwrt’ enclosure aligned north-south. The gatehouse belonged to this or a later house. In 1770 or later, the Dyer family rebuilt the house, probably redesigned and rewalled the entire estate layout, when an earlier farm building was converted into the parapet walkway and stock pens, in an area later known as the ‘cloistered court’. In Victorian times this feature, originally a farmyard, became a pleasure garden, part of a typical Georgian-Victorian complex including a kitchen garden, glasshouses, orchards and fishpond. A yew grove was established, most likely c. 1805, when the Philipps family began planting in a Picturesque style. Abandoned c. 1950, house and garden became dilapidated. The site is now the object of a radical development programme, involving inter alia the stabilization of all buildings fabric, and imposing 16th-/17th-century style formal gardens over the 18th-/19th-century kitchen garden and orchard.     

Analysis of the connections between solar wind parameters and day-side geomagnetic pulsations based on data from the observatories Nagycenk (Hungary) and Uzur (U.S.S.R.)

A comparison is made of results obtained at two widely separated observatories, Nagycenk in Hungary and Uzur in Soviet Siberia (both at geomagnetic latitude 48°) and the connection is studied between pulsation parameters and parameters of the interplanetary medium. It is concluded that, in spite of differences in recording and processing, the results are similar and confirm earlier suppositions about these connections. Hypotheses about the location of the excitation clearly favour the pre-shock interplanetary medium with reflected protons counterstreaming to the solar wind. Propagation through the shock wave and the magnetopause, as well as infra-magnetospheric propagation, may later alter certain parameters of the pulsations.     

Healing,Entertaining, and Accumulating Merit: The Circulation of Medical Recipes from the Late Ming through the Qing

This article examines the circulation of medical recipes through vernacular literature and personal networks from the late Ming through the Qing. During this period, vernacular texts played a leading role in circulating practical instructions for everyday healing techniques, especially in the form of recipes. Recipes became a versatile textual form for recording and transmitting experience in quotidian practice. They moved among different genres of texts, providing information about healing, offering advice for entertainment, and delivering moral lessons. Literati sociability as well as philanthropic and religious commitments motivated people of varied social means to distribute vernacular texts bearing healing information to a broad audience. Recipes acquired legitimacy and authority by clearly marking their provenance and thus its relationship to particular social networks and, sometimes, a religious purpose as well.     

Living through,living with and living on from breast cancer in the UK: creative cathartic methodologies,cancerous spaces and a politics of compassion

This paper contributes to a growing literature exploring the embodied emotions involved in death studies. It does so through a creative cathartic autobiographical account of living through and on from breast cancer. In presenting this ‘storifying experience’, this UK-based paper has three key aims: first, it attempts to counter the disjuncture between the fleshy and emotional cancer journey I have travelled through and the sometimes abstract, disembodied accounts of cancer circulating in some geographical texts; second, it reveals some geographical insights that are uncovered through the use of creative cathartic methodologies which unsettle commonly held discourses about dying and surviving; and third, it poses some troubling questions for geographers working in this field with respect to the methodologies, politics and emotions of such research. In the paper, I argue that employing a creative cathartic methodology gestures towards ‘an opening into learning’ that provokes emotional enquiries about what it means to be taught by the experience of (traumatised) others. In particular, I advocate for a politicised compassion that both cares for those who are living through, with or living on from life-threatening illnesses and also cares about the complex conditions that shape their experiences, both within and beyond the academy.     

Application of GIS methods to retrieve orientation patterns from imagery; a case study from Beds I and II,Olduvai Gorge (Tanzania)

The role of natural processes in the formation of the Bed I and II sites at Olduvai Gorge (Tanzania) remains the subject of much debate, five decades after their original excavation. Preferred orientation of items is indicative of water disturbance, but compass bearings were not available in Mary Leakey's (1971) study. Using GIS methods, we have vectorized maps from 1960s excavations at Olduvai, and applied a range of statistical techniques to calculate items strike. The GIS analysis suggests strongly preferred orientation patterns in Bed I, and more heterogeneous angular histograms in Bed II. Two complementary lines of evidence support these results. Firstly, modelling of potential mapping errors provides a large interval of confidence for preferred orientation patterns. Second, the GIS study was extended to photographs of the earlier excavations and to maps from recent fieldwork at Olduvai; both yielded patterned arrangement of lithics and bones, and are consistent with results based on the analysis of the 1960s maps, i.e. a large number of the Bed I and II assemblages are preferentially oriented. These results highlight the potential of GIS applications to the analysis of imagery in Stone Age studies, and bear important implications for the understanding of the role of natural agents in site formation processes at Olduvai.