Geomagnetically quiet day ionospheric currents over the Indian sector—III. Counter equatorial electrojet currents

Counter equatorial electrojet (CEJ) occurring at all hours from 0700 to 1700 h LT in the Indian sector have been studied. The percentage occurrence of morning and afternoon CEJ in each hour and each season are given and discussed. The first quantitative determinations of eight landmark parameters that depict the structures of hourly latitudinal profiles of CEJ current have been presented and discussed.These are the peak height integrated forward current density (or intensity) at the centre of the current, the peak return current intensity, the ratio of the peak return to the peak forward current intensity, the total forward current, the dip latitude of the current centre, the distance of the focus from the current centre, the distance of the peak return current intensity from the centre, and the latitudinal extent of the current. They reveal new and interesting features of diurnal and seasonal variations, and marked contrasts between morning and afternoon CEJ. Other evidences support our focal distance w of CEJ. Comparison with equatorial electrojet (EEJ) shows that five of the parameters are similar for EEJ and morning CEJ but substantially different for afternoon CEJ. The counter worldwide part of Sq currents have also been compared with CEJ.     

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.     

Introduction to ionospheric heating experiments at Tromsø—II. Scientific problems

In the first section the theory of parametric decay instabilities (PDI) is briefly described. Then results of joint heating/incoherent scatter observations at Tromsø are presented. It turns out that most of the observed features are in good agreement with that theory, while some others remain still unexplained. Among the latter features the most striking is the existence of a ‘space-time blob structure’, which means that the time variation of scattered power from adjacent altitudes seems to be correlated. Experiments at Arecibo often lead to results different from ours. Some scientists in the field explain these observations in terms of a ‘strong turbulence’ in which also caviton formation is involved. We think that most of the Tromsø results can be adequately explained by the parametric decay process.     

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.     

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.     

Legends of the Lincolnshire Cars.—Part II

The Jataka, or Stories of the Buddha's Former Births. Translated from the Pali by various hands under the Editorship of Professor E. B. Cowell. Vol. II. translated by W. H. D. Rouse, M.A., 1895. Vol. III. translated by H. T. Francis, M.A., and R. A. Neil, M.A., 1897. Cambridge: University Press. By Joseph Jacobs.

The Cults of the Greek States. By L. R. Farnell. In Three Volumes. Clarendon Press. Vols. I. and II. 1896.

Transactions and Proceedings of the Japan Society, London. Supplement I. Nihongi, Chronicles of Japan from the Earliest Times to A.D. 697. Translated from the original Chinese and Japanese by W. G. Aston, C.M.G. Vol. II. London: Kegan Paul, Trench, Trübner, &; Co., Limited. 1896.

The Book of Wonder Voyages. Edited by Joseph Jacobs. Illustrated by John D. Batten. London: D. Nutt, 1896.

The North-Western Provinces of India, their History, Ethnology, and Administration. By W. Crooke. London : Methuen &; Co., 1897.

The Popular Religion and Folklore of Northern India. By W. Crooke, B.A. 2 vols. New Edition. Westminster: Archibald Constable &; Co., 1896.

Greek Folk-Poesy. Annotated Translations from the whole Cycle of Romaic Folk-Verse and Folk-Prose. By Lucy M. J. Garnett. Edited, with Essays on the Science of Folklore, Greek Folkspeech, and the Survival of Paganism, by J. S. Stuart-Glennie, M.A. 2 vols. London : David Nutt, 1896.     

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 model of the ionospheric dynamo—I. Formulation and numerical technique

A new method of numerically solving a suitably formulated ionospheric wind dynamo equation for electrostatic potential and field is developed. Unlike in many other dynamo models, the upper boundary does not exist and the formulation asymptotically approaches the equatorial boundary condition. Therefore, it naturally incorporates the symmetric, asymmetric E- and F-region dynamo actions in any given ionosphere and any given global or local wind field. It also enables the equation to be posed as an initial value problem and solved numerically using an efficient, accurate, stable and fast integration method of ordinary differential equations. The numerical technique can be extended to compute three dimensional dynamo-generated electric currents in the ionosphere.     

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.     

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.     

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.     

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.     

Dynamics of the Antarctic and Arctic mesosphere and lower thermosphere regions—II. The semidiurnal tide

The semidiurnal tidal dynamics of the Antarctic and Arctic mesopause regions (95 ± 15 km) are investigated through comparative analyses of monthly mean tidal wind fields determined from radar measurements 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°) in the Arctic region. The main feature common to all stations is the fall equinoctial maximum in amplitude (10–20 m s⁻¹), which is also reproduced by the most recent numerical tidal model. However, the wintertime amplitude growth with height and the shorter vertical wavelengths characterizing the model are features not reflected in the data. There is also a spring equinoctial maximum in the Antarctic data which the model does not reproduce.Examination of interannual variability reveals characteristics similar to those noted in Part I for the mean zonal wind; namely, some degree of year-to-year variability superimposed on apparent long-term decreases of order 0.3–0.5 m s⁻¹ yr⁻¹ (depending on month) in the Southern Hemisphere semidiurnal tidal amplitudes. Numerical simulations presented herein indicate that changes of this magnitude cannot even be induced (via mode coupling) by a change in the mean zonal wind field of order 30%, and are more plausibly explained by a secular change in the tidal forcing by ozone insolation absorption. However, contrary to Part I, the annual mean tidal amplitude is not characterized by any significant secular trend, remaining within the 10.0 ± 2.5 m s⁻¹ range throughout the 1970–1986 period. Analyses of other data sets are required to ascertain confidence in the apparent trend reported here.     

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.     

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.     

On tidal variability and the existence of planetary wave-like oscillations in the upper thermosphere—II. Non-linear interactions and global scale oscillations

In Part 1 of this paper the variability of diurnal and semidiurnal components of the meridional thermospheric wind was discussed. The observed variability is discussed in the light of the non-linear theory of wave-wave interactions. It is shown that it is possible to explain, at least partially, the variability through the non-linear interactions of tides with long period oscillations having periods between 2 and 15 days. Given that the periods obtained both from the quarterly spectra of the tidal amplitudes and the non-linear interaction analysis coincide with periods reported in the literature for planetary waves in the lower and middle atmosphere, the existence of global scale oscillations are sought in the upper atmosphere. Wave events are observed, coincident with the above-mentioned periods, for a set of five longitudinally distributed locations between 30 and 35 S throughout the yearly samples. The wave events have basically westward phase displacements though eastward travelling phases are observed at times.