Drifting patches of equatorial spread-F irregularities—experimental support for the spatial resonance mechanism in the ionosphere

The spatial resonance mechanism which sets in when the plasma drift velocity matches the phase velocity of atmospheric gravity waves in the ionosphere is applied to explain large-scale structures of the equatorial spread-F. The resonance condition is examined for reasonable parameters of travelling ionospheric disturbances, plasma drift and neutral wind velocities. It is shown that the resonance condition can be fulfilled in equatorial regions during the post-sunset hours. The measured drift velocities of patches of range-type equatorial spread-F are similar to measured plasma drift velocities. Some spread-F structures observed with the RTI-technique at Jicamarca, range-type spread-F observed on ionograms of Huancayo as well as the occurrence of spread-F patches observed with transequatorial propagation experiments do occur after the vertical plasma drift reverses from upward to downward. The periods, wavelengths and velocities of the large-scale structure of the spread-F patches appear to be similar to those parameters for medium-scale TID's. It is argued that a non-linear break-up of resonance-steepened TID's gives rise to the quasi-periodic structures which are typical for the range-type equatorial spread-F.     

Multi-satellite scintillations,spread-F and sporadic-E over Brisbane — 2

Amplitude scintillations of radio transmissions from six U.S. Navy Navigational Satellites at a frequency of 150 MHz have been recorded at a southern mid-latitude station over a number of years. Scintillations were most pronounced in winter daytime and summer night-time. The winter maximum coincided with maxima of range spread (Se) and critical reflection frequency (foEs) of sporadic-E (Es) and range spread of spread-F (Sr) was negligible or absent. The summer scintillation maximum was associated with simultaneous maxima of nocturnal Se, Sr and high values of foEs.The well-known occurrence of intense Es in summer daytime, characterised by the diurnal maxima of Se and foEs, coincided with small scintillation activity. This appears to be due to the fact that the Es which occurs at this time is of the “sequential” type (Ess) the structure of which is too uniform to cause scintillations. On the other hand, the summer night-time and winter daytime maxima in scintillation activity coincided with the occurrence of so-called “constant height” Es (Esc) which is believed to have a small scale irregular structure. It is evident that Esc is responsible for daytime scintillations even in the UHF range of satellite transmissions.     

Variabilities in the thermospheric temperatures in the region of the crest of the equatorial ionization anomaly—a case study

Results on spectroscopic measurements of thermospheric temperatures made from a low latitude station, Abu (24.6°N, 72.7°E, geographic; 18°N dip latitude), India, situated in the crest region of the equatorial ionization anomaly (EIA), are presented. On many occasions, these measurements reveal large deviations frorn the predictions of the neutral atmospheric model, MSIS-86, bringing out its limitations as applied to the equatorial and low latitude thermosphere. The role played by large-scale geophysical processes like the EIA, equatorial spread F (ESF) and the midnight temperature maximum (MTM), all of which influence the thermal structure of the upper atmosphere, is examined in the context of explaining the differences between the measured temperatures and model predictions. It has been conclusively shown that Joule heating associated with ESF irregular electric fields is not solely responsible for the observed deviations, and the possibility of a significant role by the EIA related processes is indicated.     

Vertical structure of AGW associated ionospheric fluctuations in the E- and lower F-region observed with EISCAT—a case study

The vertical structure of AGW (atmospheric gravity wave) associated fluctuations of ionospheric plasma parameters for the 100–240 km altitude range in the daytime of 7 September 1988 has been investigated by making use of the data provided by the Tromsø measurements in the EISCAT CP1 observation mode.The wave power profile vs height has been studied by integrating the power spectral density in each altitude. The essential feature of the power variation can be explained in terms of the energy conservation of AGWs propagating in a dissipative thermosphere. Intrinsic propagation parameters of the dominant AGW have been successfully estimated with a method based on the retrieval of the Doppler effect due to the horizontal prevailing wind. From the fluctuation structure analysis in a time-altitude frame, a downcoming AGW has been clearly identified. This downcoming wave might have been reflected from a wind shear at the altitude around 200 km, which is inferred from the meridional prevailing wind profile.     

A numerical model of the ionospheric dynamo—III electric current at equatorial and low latitudes

Using the general dynamo model and its special cases derived in a previous paper, the distributions of three dimensional electric current density in a magnetic meridional plane in the equatorial and low latitude ionosphere are computed. The winds generating the ionospheric dynamo are tide-like and locally periodic, similar to those in an internal gravity wave. Very large (several μA m⁻²) field-aligned current density is obtained in the equatorial region at places of sharp vertical gradients of the wind velocity. The currents generated by locally periodic winds of latitudinal wavelength less than several hundred kilometers do not significantly affect the normal equatorial electrojet.     

Mesopleistocene loess deposits in the Mamalyha 2 profile of Ukraine — interlaboratory comparison of the thermoluminescence dating results

The history of the interlaboratory comparison of TL dating results in Poland started in the 1980s. At that time the Lublin, Warsaw and Silesian laboratories made the first attempts at TL dating of the same loess samples from the Odonów profile. However, the cooperation ceased for many years due to great differences in the obtained TL age estimates. The next interlaboratory comparisons were made in the years 2000–2009 for the loess samples from several Polish (Dybawka, Tarnawce, Dankowice, Biały Kościół) and Ukrainian (Boyanychi, Halych, Velykyj Hlybochok, Yezupil) profiles. Most of the compared dates, obtained for the loess deposits from the Upper Pleistocene and younger part of the Middle Pleistocene, were consistent. This encouraged us to undertake the Gdańsk-Lublin interlaboratory cooperation in dating of 200-700 ka old loess deposits. Nine samples were taken from the Ukrainian profile Mamalyha 2 in 2009 for this purpose. The TL dating results indicate that comparable dates are obtained in two laboratories for loess deposits younger than 300 ka BP. The TL signal obtained in the Gdańsk laboratory for the samples older than 300 ka BP was saturated so such samples should not be dated by the multi-aliquot regeneration method. The results obtained in the Lublin laboratory for these deposits (489–682 ka) confirm that it is possible to date loess deposits older than 500 ka. It probably results from the use of total-bleach method with preheating at 160°C for the equivalent dose determination.     

Positive and negative ion densities and mobilities in the middle atmosphere over India—rocket measurements

This paper discusses the results from four rocket experiments conducted from Thumba, India, during the Indian Middle Atmosphere programme (IMAP). These rockets carried instrumented Gerdien Condenser payloads to measure ion densities and their mobilities. In the first two flights only positive ion measurements were attempted while the other two measured both positive and negative ion values. The results show that the positive ion density profiles go through a minimum around 62 km, as expected from the ion production models for this region. The ion density distribution is a function of solar zenith angle. An asymmetry with respect to noon is seen in these measurements, which is not expected theoretically. The positive ion mobilities indicate the ions to be water clusters, of the type H⁺ (H₂O)_n with n = 2 or 3, similar to the earlier reported ones. The negative ion density profile exhibits a maximum around 85 km, which is not predicted by the currently available ion density models and theories of D-region ionisation processes. The negative ion mobility measurements show the ions to have a mass range of 30–60 amu, which is within the range of mass spectrometric measurements.     

Electric fields and currents in the equatorial electrojet deduced from VHF radar observations—II. Characteristics of electric fields on quiet and disturbed days

Using the Doppler spectra of VHF radar signals, the height profiles of the phase velocity (V_p) of 2.7 m irregularities in the equatorial electrojet (EEJ) over Thumba (dip: 56′S) are obtained. The day-time east-west electric fields (E_y) are deduced by matching experimentally observed V_p profiles with theoretically deduced ones for a number of quiet and disturbed days. The experimental E_y values show: (i) a large day-to-day variability; (ii) a large decrease in the afternoon hours on some days (quiet and disturbed); (iii) the frequent presence of short period fluctuations with amplitudes of 30–50% of the background value and with typical time scales of 30–60 min, on moderately disturbed days (9 ⩽ A_p ⩽ 30); (iv) a significant decrease of the average E_y on disturbed days compared to that on quiet days during 0900–1200 h L.T.     

Electric fields and currents in the equatorial electrojet deduced from VHF radar observations—I. A method of estimating electric fields

Using the measured Doppler spectra of the VHF backscatter radar signals from type II ionization irregularities in the equatorial electrojet (EEJ) at Thumba (dip. 56′S), the height profiles of the phase velocity V_p of the plasma waves in the EEJ are determined. It is shown that the east-west electrostatic field E_y in the EEJ can be deduced from the experimental height profiles of V_p using an appropriate model of ion and electron collision frequencies. The theoretical basis and the practical application of the method for deducing E_y are described. The usefulness of the method even when type I irregularities are present at the higher altitudes of the EEJ is demonstrated.It is shown that the collision frequencies of ions and electrons are likely to have a significant diurnal variation, which may be caused by diurnal variations of the neutral densities and temperatures in the E-region.     

Settlement studies and the archaeological survey of North Norway — A discussion based on the survey of North Troms

By the end of the 1984 field season a systematic archaeological survey of the county of Troms was completed. At the same time a revision of the survey was started. The survey of Troms is part of a nation‐wide archaeological survey, connected with the production of an Economic Map of Norway. Though the validity of the methods and the results of this survey have been questioned, few attempts have been made at evaluating the archaeological data base produced by the survey. The current total revision of the survey makes this task even more urgent. The aim of this paper is to contribute towards such an evaluation, by utilizing the survey data from six North Troms municipalities in an attempt at constructing a history of settlement for this area.     

Early medieval T⊘nsberg — dating problems and the results

According to the 13th century Icelandic Saga writer Snorre Sturlasson, there was a marketplace (in old Norwegian: Kaupstadir) in the T?nsberg area by the end of the 9th century. The origin of the medieval town of T?nsberg has therefore been much discussed in the light of this statement. Viking Age and early medieval settlements often lack material datable by archaeology alone. In T?nsberg radiocarbon dating has been used to distinguish between Viking Age deposits and medieval deposits, when stratigraphy alone does not state the differences. However, this dating method has produced various results and there is obvious need for a local calibration.     

Comparison of equatorial electrojet characteristics at Huancayo and Eusébio (Fortaleza) in the South American region

The H, D and Z average daily variations for five international quiet days are compared for Huancayo (12°S, 75°W, dip +1.9°) and Eusébio near Fortaleza (4°S, 39°W, dip −3.5°) in the South American region, for the 12 successive months, October 1978-September 1979. The H range shows that the electrojet is weaker in the Fortaleza region. Also, the electrojet center has latitudinal excursions from month to month at both the locations, but not in a similar way. The D variations indicate excursions of northern hemisphereS_q current systems into the southern hemisphere (or vice versa) but in a dissimilar way at Eusébio and Huancayo. Also, significant ΔD magnitudes are noticed even at midday, indicating possibility of meridional currents. It seems that the overhead S_q current pattern changes in form, while moving over from the Fortaleza region to the Huancayo region.     

Population change in an urban system — the examples of the republic of Ireland and Scotland

State of the Scottish Environment 1991. By T.C.D. Dargie and D.J. Briggs, 296 × 210 mm, 73 pp, Scottish Wildlife and Countryside Link, Perth, 1992, pbk, ISBN 0–9518582–0–3, £5.00.

Ice Age Earth. By Alastair G. Dawson, 233 × 155 mm, xx + 293 pp, Routledge, London and New York, 1991, pbk, ISBN 0–415–01567–7, £17.99.

Fundamentals of Weather and Climate. By Robin McIlveen, 246 × 190 mm, 497 pp, Chapman and Hall, London, 1992, pbk, ISBN 0–442–31476–0, £19.95.

Environmental Hazards: Assessing Risk and Reducing Disaster. By Keith Smith, 233 × 155 mm, xx + 324 pp, Routledge, London and New York, 1991, pbk, ISBN 0–415–01217, £16.99.

Global Shift: the Internationalization of Economic Activity (Second edition). By Peter Dicken, 244 × 172 mm, xv + 492 pp, Paul Chapman, London, 1992, pbk, ISBN 1–85396–142–6, £15.95.

Geographic Information Systems: Developments and Applications. Edited by L. Worrall, 238 × 155 mm, 244pp, London, Belhaven, 1991, hbk, ISBN 1–85293–1620, £45.00.

Grieve on Geddes. By Robert Grieve, 208 × 148 mm, iv + 53 pp, Sir Patrick Geddes Memorial Trust, Edinburgh, 1992, pbk, no ISBN, £5.00.

The Changing Scottish Landscape 1500–1800. By Ian and Kathleen Whyte, 233 × 155 mm, 251 pp, Routledge, London, 1991, hbk, ISBN 0–415–02992–9, £40.00.     

Electrical conductivities,ion densities and mobilities in the middle atmosphere over India—balloon measurements

Polar conductivities, ion densities and mobilities measured in three balloon experiments are reported here. Of these, two were made with self-aspirated and one with pumped Gerdien condensers. These measurements were carried out over the period 1985–1986. Both positive and negative ion data were obtained only in the first flight. Positive ions were not collected in the other two flights. The conductivity profiles obtained from the self-aspirated condensers showed fluctuations around the tropopause. These were not seen when the pumped condenser was used. Reduced mobilities of both positive and negative ions agree well with those reported by other experimenters. The ion density profile from the pumped condenser measurement showed a broad maximum around 15 km, as expected from theory. The values were less than what are obtained using available ion pair production rates and recombination coefficients.     

Teaching and learning in the tropics: an epistemic exploration of “the field” in a development studies field trip

Development studies employs theories, tools and methods often found in geography, including the international field trip to a “developing” country. In 2013 and 2014, I led a two-week trip to Ethiopia. To better comprehend the effects of “the field” on students’ learning, I introduced an assessed reflexive field diary to understand what the field trip experience teaches students about themselves and their relationship to the field. In this paper, I present critical reflections on “the field” that speak to a provocative concept – “the tropics”. These reflections illuminate prevailing challenges in the study and practice of development and suggest a way forward.     

Computer simulations of the ionospheric equatorial anomaly in East Asia for equinoctial,solar minimum conditions. Part I—Formulation of model

A new quasi three-dimensional, time-dependent meridional continuity equation for ionization in the ionospheric F-region, taking into account production, loss, electromagnetic and wind drifts, has been derived. This was coupled to the momentum equations for horizontal neutral winds so as to obtain a self consistent solution for the effects of neutral winds on the low-latitude distribution of ionization throughout East Asia. For simplicity, the non-linear inertia terms have been neglected in the wind calculations. The numerical scheme used for solving the coupled equations is given and the selection of input parameters to the numerical model discussed.     

Palimpsests of Progress: Erasing the Past and Rewriting the Future in Developing Societies—Case Studies of Singapore and Jakarta

The former colonial port cities of Southeast Asia are complex in both their landscapes and their collective memories. Centuries of European imperial domination have left a mark on their townscapes and, more so in some cases than in others, on their contemporary political and social cultures. During the colonial period, the integration of these port cities into global trade networks also fostered inter‐ and intra‐regional migration and, thus, the development of complex cultural mixes in their demographic composition. In recent decades, and following the attainment of political independence, this region has experienced spectacular economic growth and the development of a range of nationalisms, both of which have had a considerable impact on the recent transformation of their (capital) cityscapes. Singapore and Jakarta are presented here as case studies of the ways in which economic, political and cultural forces have interacted to produce cityscapes in which elements of the past are variously eliminated, hidden, privileged, integrated and/or reinvented.