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.     

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.     

Meteorological effects in the variations of ionospheric parameters—I. The lower thermosphere

The influence of the troposphere and stratosphere on variations of ionospheric D-region parameters are discussed. It is found that variations of the minimum observable ionosonde frequency (f_min) and horizontal winds are closely related to temperature and pressure fields in the lower atmosphere. Experimental data for the Central Asian and East Siberian U.S.S.R. regions have been used.     

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.     

The winter anomaly in the ionospheric D-region—some numerical calculations

This paper reports on results of numerical calculations for conditions under which the anomalously high absorption of radio waves appears and disappears at middle atmosphere heights, having characteristic time constants of about 24 h. In the calculations we have solved a system of equations which are self-consistent in a one-dimensional approximation and govern the temperature and neutral and ion composition behaviour in the height range 40–150 km. The values of the turbulent transfer coefficient and turbulent energy dissipation rate were calculated for the case of the anomalous absorption commencement, using experimental data pertaining to the vertical structure of the wind and temperature field. The results derived from numerical calculations involving evidence of turbulent processes were used to interpret some characteristic properties of neutral and charged particles, temperature and densities for day-time conditions with anomalously high absorption. The lack of experimental data on winds and temperature throughout the height range under study for the dates with normal absorption forced us to use indirect measurements in making assumptions about the vertical profile of the turbulent transfer coefficient and the values of the turbulent dissipation rate. The results thus obtained agree with measurements of electron density, temperature and hydroxyl emission intensity (the hydrogen content decreases as the conditions change from anomalous to normal). There are, however, discrepancies between calculated and measured atomic oxygen densities.     

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.     

Introduction to ionospheric heating at Tromsø—I. Experimental overview

The HF ionospheric modification (heating) facility at Ramfjordmoen will become a part of the EISCAT association from January 1993. This paper, which is intended for the new user, describes the technical capabilities of the facility and the broad range of geophysical and plasma physical experiments which are possible. An overview is presented of the physical effects that a powerful HF electromagnetic wave incident on the ionosphere can produce on timescales ranging from tens of microseconds to minutes in height regions ranging from 50 to hundreds of km. Emphasis is placed on the practical implementation of ionospheric heating experiments using the EISCAT incoherent scatter radars as the main diagnostic, but other diagnostic techniques using ground-based radars, radio links, radio receivers, photometers, rocket and satellite instrumentation are also described. A companion paper presents in greater depth some of the current scientific issues being addressed in ionospheric modification research.     

The amplitude of auroral backscatter—III. Effect of tilted ionospheric layer

The effect of tilting of the E-layer on auroral backscatter is considered for the conditions applying in the vicinity of an auroral arc. Tilting modifies the degree of refraction, sometimes causing internal reflection, and this can explain a number of previously anomalous phenomena observed in auroral backscatter, including strong backscatter at apparently large aspect angles. As a result of tilts, refraction effects may be relevant over a wider range of frequencies than considered in the past, and the apparent aspect sensitivity and altitude of backscattered signals should be re-examined carefully.     

The investigation of electromagnetic interactions between regions surrounding the Earth—2. Results of the numerical simulation

A numerical simulation of the penetration of the global atmospheric electric field from the troposphere into the ionosphere has been performed. The numerical model, which we use here, has been described in paper 1 (Kuznetsov et al., 1995). Evaluations of the amplitude of the electric mode penetrating into the upper regions, which depends on the frequency, the characteristic spatial scale of the field and the wave electric field orientation relative to the geomagnetic field have been made using realistic models of the anisotropic conductivity. We show that in an inhomogeneous medium the high-altitude behaviour of the electric mode is characterized by the appearance of regions with either vertical or horizontal currents being dominant. The possibility of using an analogy with the laws for calculating the currents in different regions, namely Kirchhoff's laws for circuits with lumped parameters, is demonstrated. Characteristics of the magnetic modes excited in the ionosphere due to the action of the penetrating atmospheric electric fields are also described.     

Dynamics of the Antarctic and Arctic mesosphere and lower thermosphere regions—I. The prevailing wind

The dynamics of the Antarctic and Arctic mesopause regions (ca. 95 ±15 km) are investigated through comparative analyses of winds measured by radars at the Scott Base (78°S), Molodezhnaya (68°S), and Mawson (67°S) stations in the Antarctic, and the near-conjugate stations of Heiss I. (81°N) and Poker Flat (65°N) in the Arctic region. The data were analyzed specifically to delineate hemispheric differences in mean monthly prevailing wind climatologies, and show the circulation systems in the Arctic and Antarctic mesosphere and lower thermospheres to exhibit significant asymmetries. These asymmetries may be attributable to hemispheric differences in dynamical forcing due to one or more of the following: insolation absorption by ozone, other mesospheric heat sources such as exothermic chemical reactions, tropospheric forcing of vertically or obliquely propagating gravity waves which engage in mesospheric mean-flow interactions, and dissipation of planetary waves which find ducting channels through the middle atmosphere.Interannual variability is also examined in the Molodezhnaya and Heiss I. data, which cover the periods 1967–1986 and 1968–1985, respectively. Accompanying significant year-to-year variability, eastward winds at 95 km over the Antarctic (Molodezhnaya station) exhibit a trend of decreasing amplitude from 1968 to 1977 that is not reflected in the Arctic data (Heiss I.); for instance, the annual mean wind decreases in a monotonie sense from 20–25 to 5 m s⁻¹ during this period. It cannot be unambiguously established whether this trend represents a decrease in intensity accompanying secular changes in thermal forcing, or a latitudinal contraction or shifting of the mesospheric jet system. The annual mean winds at Molodezhnaya remain at the 4–8 m s⁻¹ level from 1977 to 1986.In addition, existing empirical models are evaluated against the data, and are shown to be deficient in reproducing some salient characteristics of the high-latitude circulation systems. This latter result especially questions the common practice of deriving winds based on the geostrophic approximation in this altitude/latitude regime.     

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.     

Inheritance and Vicissitude of the Tibetan Diet——A Case of A Tibetan Rural Community

The dietary structure and customs of the Tibetans1 include both simple and complex elements.The transition from a diet composed mainly of tsampa and butter tea to the integration of alien and industrialized foodstuffs into the Tibetan dietary culture reflects,in a way,the processes of adaptation,integration and vicissitude of the Tibetan rural community during modernization.This article attempts to discuss the relationship between the traditional culture and modernization in Tibetan rural communities during the process of cultural interaction and modernization by studying the dietary structure and customs of Tibetan farmers in Nyang-re Township located in the north outskirts of Lhasa,Tibet Autonomous Region (TAR).     

Effects of ionospheric irregularities on radio waves—II. Group path changes

The effects of ionospheric irregularities on the group path of radio waves reflected at vertical incidence are studied using model irregularities. The basic approach used is quite general in that irregularities and background ionospheres of arbitrary shape may be considered. The method is demonstrated by the use of simple triangular shaped irregularities. For the ionospheric conditions considered irregularities below X ∼ 0.85 have relatively little effect on the group path of ordinary rays. Irregularities extending above this level produce large effects partially due to the rapid variation of the group refractive index near reflection. High gradients produced by irregularities, particularly near reflection, can also be regarded as causing large group path effects. The effect of irregularities near the reflection level is not as dramatic in the extraordinary ray case because the refractive index does not change as rapidly near reflection as it does in the ordinary ray case. When the reflection height is changed by the irregularity the situation is complex because different effects produce opposing changes in the group path. The result is that an irregularity of a given percentage increase in intensity has a maximum positive effect on the group path when it is positioned so that it extends up to reflection without causing reflection height variations. Its maximum negative effect occurs when maximum reflection height lowering takes place. The group path change produced by an irregularity is dependent on irregularity intensity, thickness and background gradient but is least sensitive to intensity. This is in contrast to the phase path changes which are very dependent on background gradient and irregularity intensity but not on thickness. Possible applications of the results to swept frequency group path experiments are discussed and some examples are given.     

J.G.A. Pocock and the idea of the ‘Cambridge School’ in the history of political thought

This article offers a reinterpretation of the origins and character of the so-called ‘Cambridge School’ in the history of political thought by reconstructing the intellectual background to J.G.A. Pocock's 1962 essay ‘The History of Political Thought: A Methodological Enquiry’, typically regarded as the first statement of a ‘Cambridge’ approach. I argue that neither linguistic philosophy nor the celebrated work of Peter Laslett exerted a major influence on Pocock's work between 1948 and 1962. Instead, I emphasise the importance of Pocock's interest in the history of historiography and of his doctoral supervisor, Herbert Butterfield. By placing Pocock's intellectual development in these contexts, I suggest, the autonomy of diverse versions of the ‘Cambridge’ approach can more readily be perceived.     

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.     

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.