Middle atmospheric electrodynamic modification by particle precipitation at the South Atlantic Magnetic Anomaly

Evidence for a localized middle atmospheric electrodynamic modification at low latitudes (southern Brazilian coast) of the South Atlantic Magnetic Anomaly (SAMA), in association with enhanced geomagnetic activity, are presented in a unified way combining recent observational efforts and related numerical studies. They involve a distortion effect in the fair weather electric field at balloon altitudes. This effect is attributed to a local intensification of energetic electron precipitation through a related middle atmospheric ionization enhancement and is elucidated by numeric simulation. From the electric field measurements and the numeric simulation, the intensification of precipitation is considered to occur in fairly narrow regions at the observed low L values (around L = 1.13) of the SAMA, with horizontal extensions of the order of a few hundred kilometers. A physical mechanism that could be responsible for this sort of intensification is suggested. Furthermore, a comparison of the phenomenon of middle atmospheric electrodynamic modification at the SAMA with a similar one at auroral latitudes, in response to enhanced solar and geomagnetic activity, is also given.     

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.     

South American problems: A review

ABSTRACT

Tourism can be potentially disruptive for the recipient communities; particularly for residents who do not directly benefit from the spending of tourists. To this end in the 1970s George Doxey derived an ‘irridex’, and this is assessed in this paper in the Manx context. The Isle of man Tourist Trophy Motorcycle Races involve road closures and are potentially disruptive to the lives of many residents. Survey results show that Manx residents do not, however, find the races unacceptably disruptive or intrusive. As such, questions are raised on the nature of environmentally disruptive tourism.     

The effect of mid-latitude electron precipitation on the geoelectric field

A simple model is outlined to describe electron precipitation from the population of charged particles trapped in the Earth's magnetic field; almost all of the precipitation is shown to occur in the region of the South Atlantic Anomaly (SAA). When the effect of a dawn-to-dusk electric field across the magnetosphere is included in the model, a diurnal modulation of the precipitated electron flux is predicted. Experimental evidence which supports the diurnal modulation model is described; the measurements which are discussed are principally those from University of Houston rocket payloads flown in the region dominated by the SAA. The resulting diurnal variation in atmospheric conductivity in the SAA is shown to be such that it could account for part of the daily UT variation in the geoelectric field. The observed seasonal variation in the pattern of geoelectric field modulation is also shown to be consistent with the proposed source of the daily variation. The difficulty of altering global conductivity by intense ionization of the mesosphere is discussed and the point is made that further in situ investigation of the Antarctic mesosphere is needed.     

On the effect of electron precipitation on the fair-weather electric field

In this short paper we have estimated the influence of the diurnal modulation of the electron precipitation at low and middle latitudes of the South Atlantic Magnetic Anomaly (SAMA) on the fair-weather electric field. We have used simple exponential atmospheric conductivity models, together with the ion production rates determined from balloon and rocket measurements in the SAMA. An upper limit to this influence was also calculated and compared with the normal diurnal variation of the fair-weather electric field due to the diurnal variation of the global thunderstorm activity.     

Controlled electron beam experiments in space and supporting laboratory experiments: a review

The injection of energetic charged-particle beams in space represents an exciting area for controlled experiments in the Earth's ionosphere and magnetosphere. Under the influence of a multitude of controlling parameters, a mono-energetic charged-particle beam might follow a well-defined singleparticle trajectory or it might undergo collective beam-plasma effects that substantially modify its energy and density distributions. In the latter case, one finds a multitude of possibilities for unstable plasma modes. If on the other hand the beam maintains single-particle characteristics, it can be a useful diagnostic tool for such applications as geomagnetic conjugate mapping and measurement of magneticfield-aligned potentials. The evolution of these concepts, along with the developing technology of spaceborne charged-particle beam accelerators, has ushered in an area of basic plasma and geoplasma physics previously not available to the scientific community. Over the past five years this activity has brought about important improvements in our fundamental understanding of beams and beam plasma processes. This progress has been brought about not by space experiments alone, but by the synergistic efforts of computational physicists, and the exchange of ideas born from laboratory simulations of space plasma processes involving energetic particle beams. It is the aim of this review paper to develop a synoptic perspective on recent accomplishments, and to summarize plans and recommendations for future experiments which will contribute to continued growth in the field.     

Broad-band auroral VLF hiss and inverted-V electron precipitation in the polar magnetosphere

A polar map of the occurrence rate of broad-band auroral VLF hiss in the topside ionosphere was made by a criterion of simultaneous intensity increases more than 5 dB above the quiet level at 5, 8, 16 and 20 kHz bands, using narrow-band intensity data processed from VLF electric field (50 Hz–30 kHz) tapes of 347 ISIS passes received at Syowa Station, Antarctica, between June 1976 and January 1983.The low-latitude contour of occurrence rate of 0.3 is approximately symmetric with respect to the 10–22 MLT (geomagnetic local time) meridian. It lies at 74° around 10 MLT, and extends down to 67° around 22 MLT. The high-latitude contour of 0.3 lies at invariant latitude of about 82° for all geomagnetic local times. The polar occurrence map of broad-band auroral VLF hiss is qualitatively similar to that of inverted-V electron precipitation observed by Atmospheric Explorer.(AE-D) (Huffman and Lin, 1981, American Geophys. Union, Geophysics Monograph, No. 25, p. 80), especially concerning the low-latitude boundary and axial symmetry of the 10–22 h MLT meridian.The frequency range of the broad-band auroral VLF hiss is discussed in terms of whistler Aode Cerenkov radiation by inverted-V electrons (1–30 keV) precipitated from the boundary plasma sheet. High-frequency components, above 12 kHz of whistler mode Cerenkov radiation from inverted-V electrons with energy below 40 keV, may be generated at altitudes below 3200 km along geomagnetic field lines at invariant latitudes between 70 and 77°. Low-frequency components below 2 kHz may be generated over a wide region at altitudes below 6400 km along the same field lines. Thus, the frequency range of the downgoing broad-band auroral hiss seems to be explained by the whistler mode Cerenkov radiation generated from inverted-V electrons at geocentric distances below about 2 R_E (Earth's radius) along polar geomagnetic field lines of invariant latitude from 70 to 77°, since the whistler mode condition for all frequencies above 1 kHz of the downgoing hiss is not satisfied at geocentric distance of 3 r_e on the same field lines.     

Maritime Archaeology and the Early Atlantic Trade: research at Elmina,Ghana

This article details recent maritime archaeological research off the town of Elmina in coastal Ghana. Founded in 1482, Elmina Castle was the first and largest European outpost in sub‐Saharan West Africa, and remained a centre of maritime trade for almost four centuries. Survey and diver investigations led to the discovery of several sites, including a shipwreck dating to the mid 17th century, which is characterized by a mass of trade goods, cannon and buried hull remains. A second Dutch vessel dating to c.1700 was documented after its discovery during dredging operations in the Benya Lagoon adjacent to Elmina. These sites present an opportunity to study ships and cargoes involved in the West African trade.     

The mid-latitude trough in the electron concentration of the ionospheric F-layer: a review of observations and modelling

Experimental observations and theoretical modelling of the terrestrial mid-latitude trough are reviewed. The mid-latitude trough is considered as an F-layer phenomenon, and its relationships to the lightion trough in the topside ionosphere and to the plasmapause are discussed. The observed morphology of the mid-latitude trough is summarised. Recent evidence on plasma temperatures in the trough is examined. The physical processes that may be important in the trough region are listed. Large-scale computational models that include some of those processes are described and the results compared with observations. Deficiencies in the models and possible future developments are mentioned.     

Magnetic Sourcing of Obsidians in Southern South America: Some Successes and Doubts

Sourcing of archaeological obsidians is of great importance in unravelling the cultural, social and economic development of many ancient societies. Use of magnetic properties of obsidian fragments has been reported as a cheap, fast and versatile tool for these purposes. One hundred and seventy-six obsidians from archaeological sites and sources in Argentina and southern Chile were analysed magnetically using the weight-normalized intensity of JNRM (Intensity of Natural Remanent Magnetization), the intensity of JSIRM (Intensity of Saturation of Isothermal Remanent Magnetization) at 0·35 T and the bulk susceptibility. The method allowed identification of at least two different sources for archaeological materials found respectively to the north and south of the Lago Argentino, in SW Santa Cruz province, Argentina. Comparison of values from most samples in south and central Santa Cruz with those from a known source may lead to the interpretation that most of them belong to that source. In contrast, a critical analysis of the resolution of this using a larger than usual number of samples from three well distant obsidian sources in Argentina showed that two of the sources showed an almost complete overlapping of these three parameters, and all displayed a very large dispersion of values. This different result indicates that the magnetic sourcing of obsidians may not always have the resolution previously portrayed, but is applicable in certain localities.     

High-energy electron drift echoes at the geostationary orbit

Approximately 60 high energy (0.7–2.6 MeV) electron drift echoes, observed with the UCSD omni-directional detectors on board ATS-5, have been analyzed. The period for longitudinal drift around the Earth is a function of energy and verifies the now classical law for relativistic electrons. Empirical relationships have been established, which statistically relate the characteristic period T₀ (the drift period for ~320 keV electrons) with the AE and Kp indices. These results are well interpreted if we adopt a Mead and Williams model (with special coefficients) for the magnetic field, and if we assume that the subsolar distance of the magnetopause R_b is mainly a function of Kp, the tail field B_T being mainly a function of AE. Evidence is given that these functional dependences agree with other experimental data, such as Coleman and McPherron's (1976) measurements of the midnight magnetic field intensity at the geostationary orbit. It is demonstrated that the origin of drift echoes is less likely to be an injection of high-energy particles, but more likely to be the consequence of a redistribution of particles among different L-shells after a sudden compression or expansion of the magnetosphere, as originally proposed by Breweret al. (1969).     

Unequal 'cultures'? Racial integration at a South African university

This article examines the process of racial integration at the University of Pretoria since the demise of apartheid. The university has taken significant steps to achieve fairer representation of the different 'races', but has not succeeded in diminishing student perceptions of the overwhelming importance of racial identities. Some suggest that this is because white, Afrikaans-speaking students continue to display racial prejudice against fellow students who are black. We argue that while this may be so, a more important obstacle lies in the institution's adherence to a discourse involving an easy substitution of 'culture' for 'race' as a means to describe 'difference' in South Africa. This discourse, which has a long history at the university, serves to divert attention from the racialised inequalities that underpin the salience of racial identities.     

Preliminary documentation of a chine-girder barge at Conway, South Carolina

The study of small flat-bottomed plantation craft in South Carolina began in 1983 with the discovery and study of a barge at Mepkin Abbey on the Cooper River. Since that date the author has documented a wide range of craft in contexts ranging from plantations and ferry crossings to marine phosphate mines (Newell. 1986; 1989). The flat which is the focus of this report is the first of its type observed during this 8-year study period. Its construction and hull profile indicate that the vessel is a late 18th century craft. Current research by the author into the origins of the unique design suggests eastern European or French sources for the use of iles or chine-girders, a constructional element of the craft used in conjunction with transverse planking not commonly seen in flats of English design.     

Determination of an Optimal Location for an Iron and Steel Plant Based on Iron Ore of the Kursk Magnetic Anomaly

A locational study of the Iron-Steel Design Institute is criticized on the ground that it selected potential plant sites intuitively and then examined them in terms of proximity to iron ore, water availability and minimal transport requirements. An alternative methodology is proposed. The general region for the new plant is first established on the bases of the interregional balance of iron and steel products. The Central European region is found to have a deficit in the entire product range. A more specific location is then derived by calculating potential transport costs with respect to both markets and sources of raw materials (iron ore, coke). The Rzhev area of Kalinin Oblast is found to be optimal in terms of transport costs alone. But when water and, particularly, labor resources are considered, preference shifts in favor of a plant location in the Gor'kiy-Kostroma area.