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.     

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.     

Lightning transient fields in the atmosphere—low ionosphere

The complexity of the electric properties of the atmosphere, which influences the excitation and propagation of electromagnetic signals generated by lightning discharges, gives rise to a diversity of the typical signal wave forms and amplitudes in the various space-time domains. This causes difficulties and controversies in the interpretation of the experimental data. In geophysical problems of such a kind, considerations accounting for a broadband spectrum are particularly important.In the present paper, lightning induced ULF-VLF fields in the daytime inhomogeneous atmosphere and D-region of the ionosphere above the thundercloud are analyzed for a standard model of the vertical return stroke. The developed analytical approach makes it possible to unite the canonical model of the electromagnetic radiation into a free-half-space, the discharge (ELF-VLF spectrum) and ‘post-discharge’ (ULF spectrum) low-frequency models and to clarify the temporal scales in the mechanism of lightning coupling with the atmosphere and low ionosphere with respect to the local relaxation time, discharge characteristic time scales and observer location.     

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.     

A numerical study of the lunar tide in the mid-latitude F2-region of the ionosphere—I. Oscillations of the electron density

The continuity equations for four ion species and the momentum equations for the ion, electron and neutral gases are solved numerically. The momentum equations for ions and electrons include the effects of the electric field, and that for the neutral gas includes the effects of the collision between ions and neutrals, Coriolis force, the viscosity and the horizontal pressure force. It is found that the calculated semi-monthly tidal oscillations in NmF2 during the day and night-time are almost out of phase to each other, while the phase of the oscillations in hmF2 is weakly dependent on the solar time. The amplitude and phase of the calculated semi-diurnal lunar tide averaged over the solar times are 1.7% and 9.1 h (in NmF2) and 4.6 km and 8.1'h (in hmF2) respectively, at 45.0° dip. The height distribution of the semi-monthly lunar tide in the electron density is also obtained. The results show that the phase is 2 h for the lower region (below 200 km) and shifts to 9 h above about 250 km in the daytime, whereas it is nearly constant with height in the night-time. The calculated results are in good agreement with observations except for the phase at night. Thus it may be concluded that the major features of the observed lunar tidal variations in the mid-latitude F2-region can be explained by the electromagnetic drifts due to the solar and lunar electric fields.     

VLF radiation generated by a loop antenna in P2 layer of the ionosphere—2. Interpretation of the measurements

The results of an experiment on radiation and reception of VLF waves with a frequency of 5 kHz in the F2 layer of the ionosphere are compared with theoretical calculations made in the framework of the linear theory of a loop antenna in a plane-stratified plasma. The measurements made outside the Storey cone can be explained in the framework of the linear theory by the proper modification of the ionosphere model. The wave polarization differs from circular polarization on the boundary of caustic cone, due to non-linear effects.     

VLF radiation generated by a loop antenna in F2 layer of the ionosphere—1. Theory

In 1987 an experiment on the transmission and reception of VLF electromagnetic waves was carried out in the Earth's ionosphere. The interpretation of the experiment required an elaboration of the theory of a loop antenna in a plane-stratified plasma with parameters changing along the external magnetic field. Formulae for the wave field at arbitrary distances from the antenna have been obtained. Asymptotic expansions for the wave field permit us to construct an effective numeral algorithm for the calculation of the signal when the far zone approximation does not hold and in the presence of the fine structure.     

A numerical study of the lunar tide in the mid-latitude F2-region of the ionosphere—II. Oscillations of the ion and neutral gas velocities

The drift due to the lunar electric field which is communicated from the ionospheric E-region causes a re-distribution of the electron density and a neutral air motion in the F2-region. The lunar tide in the electron density of the F2-region due to the lunar electric field has been obtained and discussed in Part I. The lunar tides in the ion and neutral velocities which are obtained in the same way are dealt with in this paper. It is found that the lunar tidal oscillations in the drifts and neutral winds markedly depend on the solar time, and that the amplitude and phase in the x-component of the drift averaged over the solar times are 10.1 m s⁻¹ and 2.6 h and those in the x-component of the neutral wind are 4.6 m s⁻¹ and 4.3 h, respectively, at 45.0° dip.     

The investigation of electromagnetic interactions between regions surrounding the Earth—1. Initial equations,the boundary conditions,the distribution of external currents and the methods of numerical solution

A numerical model is presented suitable for the study of the penetration of the global atmospheric electric field from the troposphere into the ionosphere. Equations are obtained in the plane-stratified approximation which permit us to use the same mathematical techniques in the atmosphere, the ionosphere and the magnetospheres. We take into account the anisotropy in the general case, the variation of conductivity of the medium and the temporal changes of the fields. The relationship between the electric and magnetic modes is taken into account by means of Ohm's law in anisotropc media, using the electrical conductivity sensor. The feasibility of the plane-stratified approximation is analysed by means of the analogous equations taking into account the spherical symmetry of the global electric field. Models of the external current distribution are discussed. The boundary conditions take into account the field attenuation into the conducting Earth and the field propagation into the magnetosphere as Alfven and magneto-acoustic waves. The methods of numerical solution of the equations obtained and their accuracy are described in brief.     

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.     

The origin and construction of pre-Hispanic mounds in the Upper Delta of the Paraná River (Argentina)

Pre-Hispanic mounds, known as “cerritos,” “cerritos de indios,” or “aterros” across southeastern South America, are one of the most conspicuous and well-studied cultural manifestations in lowlands archaeology. Nevertheless, in the Upper Delta of the Paraná River, mounds are rarely studied, and even their anthropic origin is under debate. This could be related to the fact that anthropogenic mounds are located on a floodplain where other “mound-like” natural geoforms (generated by fluvial processes) are also present. In addition to this, the natural geoforms also contain evidence of Holocene human occupation (sherds, bones, charcoal, humans burials, etc.), which can lead to interpretive errors of their origin and formation. Thus, this project set out to determine the genesis and evolution of these mounds and also to identify the cultural occupation and transformation of natural landforms found in the area. In this article, natural and anthropogenic systems and processes were identified and characterized through the application of proxy record analysis (i.e., sediment composition, stratigraphy, micromorphology, silica bodies and chronological analysis) at the Los Tres Cerros archaeological locality in the Upper Delta of the Paraná River of Victoria County, Entre Ríos Province, Argentina. This analysis allowed for the recognition of natural anthropogenic interfaces, such as the “pre-mound” occupation as well as evidence of cultural activities such as mound construction, between 1,000 and 500 ¹⁴C years bp. These findings were integrated into current research on the variability of mound construction during the Late Holocene in the lowlands of South America.     

On the distribution of certain forest trees in Scotland,as shown by the investigation of post‐glacial deposits

An attempt is made to identify the main trends in the development of physical geography as a distinctive discipline. In particular the relationship between physical and human geography is examined with a view to identifying possible paths for the future development of physical geography as an integrated discipline.     

On tidal variability and the existence of planetary wave-like oscillations in the upper thermosphere—I. Observations of tidal variability

The evidence for the existence of tidal variability as observed in the meridional thermospheric wind (approx. 300 km height) is presented for a set of eight ionosonde locations (three in the northern hemisphere and five in the southern hemisphere). The data set corresponds to a full year (1984) of hourly values. The detected variability can be seen in the tidal components of the meridional wind. The diurnal and semidiurnal components are spectrally analysed. The quarterly spectra show that the tidal amplitudes oscillate with periods between 2 and 60 days. The more important oscillations have periods from 15 to 3 days. No direct link between solar and magnetic activity indices was detected. Possible reasons for the observed tidal variability are discussed in the light of the current theory developed for the mesosphere and lower thermosphere.     

Foucault, fields of governability, and the population–family–economy nexus in China

It was only in the early twentieth century that China discovered that it had a population, at least if a population is understood not as a simple number of people but instead in terms of such features as variable levels of health, birth and death rates, age, sex, dependency ratios, and so on—as an object with a distinct rationality and intrinsic dynamics that can be made the target of a specific kind of direct intervention. In 1900, such a developmentalist conception of the population simply did not exist in China; by the 1930s, it pervaded the entire social and political field from top to bottom. Through a reading of a series of foundational texts in population and family reformism in China, this paper argues that this birth of the Chinese population occurred as a result of a general transformation of practices of governing, one that necessarily also involved a reconceptualization of the family and a new logic of overall social rationalization; in short, the isolation of a population–family–economy nexus as a central field of modern governing. This process is captured by elaborating and extending Foucault's studies of the historical emergence of apparatuses (dispositifs) into a notion of fields of governability. Finally, this paper argues that the one-child policy, launched in the late 1970s, should be understood not in isolation from the imposition of the “family-responsibility system” in agriculture and market reforms in exactly that period, but as part—mutatis mutandis—of a return to a form of governing that was developed in the first half of the twentieth century.     

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.