OI 630 nm imaging observations of equatorial plasma depletions at 16° S dip latitude

Equatorial ionospheric irregularities in the F-layer have been the subject of intensive experimental and theoretical investigations during recent years. The class or irregularities which continues to receive much attention is characterized by large scale plasma depletions, generally referred to as ionospheric plumes and bubbles. The OI 630.0 nm F-region night-glow emissions arising from recombination processes can be used to observe the dynamics of transequatorial ionospheric plasma bubbles and smaller scale plasma irregularities. In a collaborative project between the Center for Space Physics of Boston University and Brazil's National Institute for Space Research (INPE), an all-sky imaging system was operated at Cachoeira Paulista (22.7° S, 45.0° W, dip latitude 15.8° S), between March 1987 and October 1991. In addition to the imager, photometer and VHP polarimeter observations were conducted at Cachoeira Paulista, with ionospheric soundings carried out at both C. Paulista and Fortaleza, the latter at 3.9° S, 38.4° W, 3.7° S dip latitude. For this longitude, the observed seasonal variation of the airglow depletions shows a maximum from October through March and a very low occurrence of airglow depletions from April through September. This long series of OI 630.0 nm imaging observations has permitted us to determine that when there are extended plumes, the altitudes affected over the magnetic equator often exceed 1500 km and probably exceed 2500 km at times, the maximum projection that can be seen from Cachoeira Paulista. This holds true even during years of low solar flux.     

A technique for the analysis of two-dimensional data: application to OI 5577Å airglow

A method to determine whether a two-dimensional data set can be represented by two independent separable functions is demonstrated using both synthetic data and satellite observations of OI 5577Å airglow. For such cases when the functions are separable it is possible to identify the function in the presence of noise using an iterative procedure based on the properties of an equitable matrix. In the specific case of the airglow, the latitudinal and temporal variations are separable for the data set studied. The spatial structure shows a minimum near the equator, with maxima at 35° in the winter hemisphere and 25° in the summer hemisphere. The long-term temporal function shows maxima after the equinoxes, with minima near the solstice. A 12 month component has a maximum near day 300.     

Thermospheric temperatures and winds derived from OI λ630 nm night airglow line profiles

Thermospheric temperatures and winds over Mt. Torrens (near Adelaide) have been determined from the [OI] λ630 nm night airglow line profile width and displacement recorded with a high resolution scanning Fabry-Perot spectrometer. The results are compared with other measurements and with atmospheric models. During geomagnetically quiet periods the observed temperatures are reasonably well represented by the empirical model of Hedin et al. (1974). The nightly variations of the zonal and meridional winds follow similar patterns to the predictions of Hernandez and Roble (1977) which are based on the temperature and composition models of Hedin et al. (1977a, b).Observed perturbations of temperature and wind following geomagnetic disturbance are qualitatively consistent with heat input to the auroral oval. The coefficient of increase of thermospheric temperature with Kp is about twice that reported by most others.     

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.     

In-situ studies of equatorial spread-F over SHAR—steep gradients in the bottomside F-region and transitional wavelength results

RH-560 rockets instrumented with Langmuir probes were launched from SHAR, India (dip 11°N) for in-situ studies of electron density irregularities associated with equatorial spread-F (ESF) when the F-region plasma was drifting down and strong range spread-F was observed with an ionosonde at SHAR. A high variability was observed in the steepness of the base of the F-region. The bases were found to be steeper during the periods when the F-region plasma was drifting down. On one of the flights irregularities were observed in the region around 280 km where the gradients in electron density were downwards, indicating that the gradient drift instability is the main mechanism for their generation. Assuming a power law of the type P_k∝ kⁿ for irregularities of transitional scale (20–200 m), it was found that the spectral index n ranges between −1.5 and −4.6, when the mean integrated spectral power P_T of the irregularities in the above scale size range varied from −45 to −12 db. A relationship between n and P_T was observed and can be represented by a Gaussian function using the above expression; the altitude variation of n normalized for a P_T value of −10 db showed that the nature of spectral index remains the same between 230 km and the apogee of the rocket. This is at variance with the observations of Kelley et al. [(1982), J. geophys. Res. 87, 1575] that 280 km is the threshold altitude for the steep drift wave type of spectra to a shallower spectra.     

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.     

How Capitalism Was Built. The Transformation of Central and Eastern Europe,Russia, and Central Asia. Anders Åslund

An American political geographer and noted specialist in the electoral geography of the post-Soviet states explores the extent to which underlying social, political, and economic conditions in North African countries experiencing regime change prompted by mass political unrest (Egypt, Tunisia) resemble those prevailing in the five Central Asian states. The author compares the countries' rankings on a number of relevant indicators (e.g., Human Development Index, Corruption Perceptions Index, Freedom House indices of political rights and civil liberties) before undertaking a more qualitative assessment of human rights, institutional control, and external support for current Central Asian regimes. Although Uzbekistan, the most populous state with the most repressive regime in the region, is a focus of attention, the same abuses and challenges are evident, albeit in varying degrees, in other vulnerable post-Soviet countries of Central Asia.     

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.     

Preservation Status and Priorities for In Situ Monitoring of the Weapon Sacrifice in Illerup Ådal,Denmark

Abstract

Excavations of the southern part of a very rich sacri?cial bog in Illerup Ådal, Denmark between 1950 and 1985 recovered approximately 15,000 Iron Age artefacts. At the time, 60 per cent of the area was left unexcavated and thousands of objects are now preserved in situ, but the present preservation status has not been investigated for approximately twenty-?ve years. Extensive in situ monitoring was carried out for one year in order to present a prioritized plan for further monitoring of the unexcavated areas, producing documentation of groundwater table variations, water quality and vegetation in the area. Results show that the remaining artefacts are generally well preserved in a waterlogged and anaerobic environment. However, in the north-eastern part of the bog, the groundwater table is too low even in a year when net precipitation and hydrological conditions were near normal. In the centre of the bog elevated salt concentrations have been measured in the groundwater.     

Correlations among O2(0–1) atmospheric band,OH(8–3) band and [OI] 5577 Å line and among P1 (2), P1 (3) and P1 (4) lines of OH(8–3) band

Using a photometer equipped with a cooled photomultiplier RCA-C31034A and an optics consisting of three aspheric lenses, the O₂(0–1) Atmospheric band at 8645 Å and the OH(8-3) band at 7274 Å are observed simultaneously from the ground level along with [OI] 5577 Å line, to investigate the instantaneous intensity changes. The P and R branches of O₂(0–1) Atmospheric band and the P₁(4), P₂(4), [P₁(3) + P₂(3)] and [P₁(2) + P₂(2)] lines of the OH(8-3) band are spectrally clearly resolved. It is found that the O₂(0–1) Atmospheric band and the OH(8-3) band covary fairly well in intensity; it seems that the correlation coefficient during the night lies around 0.500. Interrsity correlations between the OH(8-3) band and [OI] 5577 Å line are also investigated. It is found that the correlation coefficient during a night lies around −0.200 and thus the correlation between the OH(8-3) band and [OI] 5577 A line is weaker than that between the O₂(0–1) Atmospheric band and the OH(8-3) band. Correlations among P₁(4), [P₁(3) + P₂(3)] and [P₁(2) + P₂(2)] lines of the OH(8–3) band are studied, to find that their correlations are complete within an error of experiment; the correlation coefficient during a night ranges from 0.980 to 0.997. A midnight maximum of intensity of the OH(8-3) band is hardly found. During night the OH intensity always fairly fluctuates.     

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.     

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.     

Electric fields and currents in the equatorial electrojet deduced from VHF radar observations—III. Comparison of observed ΔH values with those estimated from measured electric fields

From VHF backscatter radar measurements at Thumba (dip: 56′S) of the phase velocities of type II irregularities in the equatorial electrojet (EEJ), electric field (E_y) values are estimated for different times of the day. Using the electric field values thus deduced and the Pedersen and Hall conductivities calculated using model values of electron densities and the collision frequencies of ions and electrons, the height integrated current intensity in the EEJ is estimated. The surface level geomagnetic field perturbation ΔH produced by this ionospheric current is then calculated. The calculated values of ΔH are compared with observed values of ΔH (after subtracting the magnetospheric contribution of D_st) for a number of days. The comparisons show good agreement between observed ΔH values and those calculated from measured electric fields. The agreement is found to be good even when type I irregularities are present at higher altitudes in the EEJ. This comparative study demonstrates the validity of estimating electric field values from VHF radar measurements and it indicates the possibility of deducing electric field values from ground level ΔH values, at least for statistical studies.     

Integration of geophysical surveys,ground hyperspectral measurements,aerial and satellite imagery for archaeological prospection of prehistoric sites: the case study of Vésztő-Mágor Tell,Hungary

An integration of geophysical surveys, ground hyperspectral data, aerial photographs and high resolution satellite imagery for supporting archaeological investigations at the multi-component Vészt?-Mágor Tell, located in the southeastern Great Hungarian Plain, is presented in this study. This is one of the first times that all these techniques have been combined and evaluated for retrieving archaeological information. Geophysical explorations, specifically magnetic gradiometry and ground penetrating radar methods, have revealed shallow linear anomalies and curvilinear rings at the Tell. The use of remote sensing images has confirmed the diverse anomalies with respect to geophysics through photointerpretation, radiometric and spatial enhancements. Moreover, several indices from ground hyperspectral data also have revealed stress vegetation anomalies. These integrated results were used to map the main areas of archaeological interest at the Vészt?-Mágor Tell and plan future excavations. It was found that these multiscalar data can be used efficiently for detecting buried archaeological features.