Contours of equatorial electrojet current density

A set of 17 rocket measured altitude profiles of the equatorial electrojet current density have been used to determine the parameters of a two-dimensional model of the equatorial electrojet with which the contours of equal current density of the electrojet have been constructed. The contours are in full agreement with contours by other workers constructed from wind models of the electrojet. They show the existence of return (westward) currents of the equatorial electrojet, on both flanks of the dip equator, extending from about 250 km to about 1000 km or more, with a peak at about 500 km–600 km from the dip equator, whose peak intensity is about 30% of the peak intensity of the eastward current at the dip equator. Other evidences of the westward current, the location and intensity of its peak have been mentioned.     

Inter-relations between current and electron density profiles in the equatorial electrojet and effects of neutral density changes

A simple model of the equatorial electrojet is used to try to reproduce observed current density profiles and it is found that an increase in neutral density is required. The effects of neutral density changes of various kinds are investigated. Changes in the electron density profile due to the j × B force are found to be fairly small and, in the cases studied here, are decreases at all heights.     

Ion-neutral collisions and the equatorial electrojet

There are various theoretical models of ion-neutral collision and the resultant collision frequencies (v_in) differ significantly in their values. Also there is a range of uncertainty associated with each of the theoretical values of v_in. The effects of the differing theoretical values and the uncertainty in the v_in on the estimates of the equatorial electrojet (EEJ) current strength are examined numerically. It is found that the differences in various v_in-models affect the amplitude of the EEJ, but leave the peak altitude of the EEJ nearly unchanged. However, modification to v_in by the order of its uncertainty does change the peak altitude of the EEJ by at least 2km.     

Distortions in the height structure of the equatorial electrojet during counter electrojet events

This paper presents simultaneous observations made near the magnetic equator during counter electrojet events using a coherent VHF backscattcr radar, magnetometer and digital ionosonde to understand the physical processes that generate the counter electrojet conditions. The VHF backscatter radar gives the height structure of the drift velocity or the ionization irregularities, the equatorial electrojet current variations are obtained from the magnetometer and the digital ionosonde provides the presence of blanketing E-layers at the F-region heights which give rise to the backscatter signals. These observations have provided direct experimental evidence for the theoretically predicted distortions in the height structure of the polarization electric field in the equatorial electrojet due to the local effects of shearing zonal neutral winds.     

Lunar modulations of the equatorial electrojet

An attempt has been made to reproduce the counter electrojet (CE) in the equatorial dynamo by considering neutral winds with solar (1,–2), (2, 4), (2, 2) and lunar (2, 2) tidal modes as well as a constant electrostatic field (E_y). The daily variation of conductivity (σ) is assumed to consist of steady (average), diurnal and semi-diurnal components. An equation governing the relationship between j_y (jetcurrent), E_y, σ and wind is given, and this equation is then used to describe diurnal, semi- and ter-diurnal variations of j_y separately. It is found that: (1) the lunar tide is relatively powerful in affecting semi- and ter-diurnal components of j_y; (2) such a possibility is a maximum for the afternoon CE near new and full moon and (3) the morning CE is likely to occur at lunar age between the new and full moons. From this theory, the seasonal characteristics and the solar activity dependence of CE are demonstrated to be predictable.     

Topside irregularities in the equatorial ionosphere

This paper is a brief review of ionospheric irregularities in the equatorial topside ionosphere. Results from topside sounders, direct measurement satellites, and the Jicamarca incoherent scatter radar are discussed. Scintillation observations and theories of irregularities are not discussed in detail as these are the subject of other review papers. Many of the phenomena detected in the topside ionosphere are related to bottomside irregularities, commonly known as spread-F. These include aspect-sensitive scattering observed on topside sounders, significant concentrations of Fe⁺, electrostatic turbulence and the topside irregularities detected by the Jicamarca radar. Satellite measurements show that the irregularities in electron concentration have amplitudes which increase almost linearly with wave-length over the range 70m to 3km. Duct irregularities detected by the topside sounders and some wavelike irregularity structures detected occasionally by direct measurement satellites may be separate from the general spread-F phenomenon although this has not definitely been established.     

Absolute electron density measurements in the equatorial ionosphere

Accurate measurement of the electron density profile and its variations is crucial to further progress in understanding the physics of the disturbed equatorial ionosphere. To accomplish this, a plasma frequency probe was included in the payload complement of two rockets flown during the CONDOR rocket campaign conducted from Peru in March 1983. In this paper we present density profiles of the disturbed equatorial ionosphere from a night-time flight in which spread-F conditions were present and from a day-time flight during strong electrojet conditions. Results from both flights are in excellent agreement with simultaneous radar data in that the regions of highly disturbed plasma coincide with the radar signatures. The spread-F rocket penetrated a topside depletion during both the upleg and downleg. The electrojet measurements showed a profile peaking at 1.3 × 10⁵cm⁻³ at 106 km, with large scale fluctuations having amplitudes of roughly 10 % seen only on the upward gradient in electron density. This is in agreement with plasma instability theory. We further show that simultaneous measurements by fixed-bias Langmuir probes, when normalized at a single point to the altitude profile of electron density, are inadequate to correctly parameterize the observed enhancements and depletions.     

Phase and amplitude scintillations due to electrojet irregularities

Features of the power spectra of weak amplitude and phase scintillations on a VHF signal, transmitted from a geostationary satellite and recorded at an equatorial station, are found to be in good agreement with theory. Irregularity drift speeds transverse to the signal path were extracted from the first few pronounced Fresnel minima in the power spectra. For some data intervals, a low frequency peak, associated with an irregularity wavelength greater than the Fresnel dimension, could be identified in the phase spectrum. The dominant wavelength of the large scale waves is found to be ≳ 1.5km.     

Altitude and latitude dependence of the equatorial electrojet

The effects of day-to-day or seasonal variation of altitude and latitude profiles of the Elayer plasma density in the equatorial ionosphere on equatorial electrojet (EEJ) structure are examined numerically using a self-consistent and high resolution dynamo model. It is found that variations in the E-layer peak altitude and amplitude and its gradient below significantly affect EEJ structure. For any realistic shape, the EEJ peak appears at or below the E-layer peak altitude. Distinct double peaks appear in the EEJ structure, such as revealed by rocket measurements, if the E-layer peak is above 105 km or the gradient is large, as when sporadic-E is present. The influence of the latitudinal variation of ionospheric field line integrated conductivities upon the amplitude and altitude of the EEJ peak is demonstrated.     

The equatorial electrojet and the day-to-day variability of Sq

The day-to-day variability of Sq(H) at equatorial stations has been studied, using the correlation between daily ranges for the IQSY at a large number of pairs of stations. It is found that the daily ranges at stations under the equatorial electrojet are significantly less well correlated with those at other equatorial stations than is the case for pairs of equatorial stations not involving an electrojet station, for comparable distances of separation of the pairs of stations.     

On Joule heating of the equatorial electrojet E-region

Simultaneous data on electron density, electron temperature and current density obtained from a rocket borne Langmuir probe, a glass-sealed Langmuir probe and a proton precession magnetometer flown from Thumba (geomag. lat. 0.99°S, geomag. long. 146.79°E, magnetic dip 0°47'S) have been used to calculate the Joule heating in order to assess whether it contributes significantly to the thermal imbalance in the E-region. It is envisaged that the changes in electron temperature are partially brought about by changes in collision frequency and the energy loss factor. It is found that the Joule heating alone is not sufficient to explain the observed differences in electron and neutral gas temperatures. The inclusion of photoelectron heating and adjustments of profiles of the collision frequency and the energy loss factor bring the computed temperature differences closer to the observed differences.     

New results on the dip equator and the equatorial electrojet in India

The geophysical implications are examined of the continuing southward migration of the magnetic dip equator in India since 1965, its precise ground location in 1971, and thereafter its drift at 1–6 km/yr accelerating to 7 km/yr in the mid-1980s near its mean central position in the 80-yr secular oscillation, estimated to be about 10 km south of Trivandrum. Simultaneously its drift northwards near the antipodal point at Huancayo Observatory, in Peru (South America), is also observed.The ground projection of the mean axis of the equatorial electrojet for 1980 is clearly delineated about 55 km to the north of the dip equator in India, with positive Sq(Z) values of 25 nT recorded right on the dip equator, based on the ground geomagnetic survey 1971 and the magnetometer array experiment of 1980. The half-width and midday peak total current intensity of the Indian electrojet are determined from the H data recorded at Trivandrum, Annamalainagar and Hyderabad for the solar minimum year 1976 (146 ± 46 km, 137 ± 25 Amp/km) and the maximum year 1980 (169 ± 39 km, 203 ±49 Amp/km), assuming a uniform west-east current band model at a height of 107 km centred on its newly discovered axis. These new results are quite different from those of earlier determinations. Severe induction anomalies observed in the region due to subsurface geological bodies are also appropriately incorporated.     

On the equatorial electrojet influence on geomagnetic pulsation amplitudes

It is well known that several types of geomagnetic pulsations show a significant amplitude enhancement near the dip equator due to the daytime equatorial electrojet. In the present study, the dependence of this enhancement on the period and type of geomagnetic variations is examined. The results show that, in general, the amplitude enhancement appears to be more or less uniform, amounting to a factor of 2.0–2.5, over a wide range of periods. However, for pulsations, there is a fairly sharp cut-off of the equatorial enhancement around a 20 s period, the shorter period end of Pc3 pulsations. Further, shorter period pulsations (<20 s) sometimes suffer an attenuation at the dip equator near noon. These results are discussed in the light of the transmission characteristics of the ionosphere, including the possible relation to the equatorial anomaly in the ionospheric F-region.     

The equatorial electrojet and associated currents as seen in Magsat data

Magsat data are re-examined with regard to the presence and character of fields due to the equatorial electrojet and meridional currents at dawn and dusk local times. Dip-latitude organized field variations at dawn are:

• 1.(1) extremely weak,
• 2.(2) extremely variable with longitude,
• 3.(3) inconsistent with the pattern expected from a line or narrow sheet current.

It is shown that the use of Magsat dusk data can ‘contaminate’ a main field model, introducing apparent equatorial electrojet effects into the dawn data.Fields due to the equatorial electrojet and (presumably) associated meridional currents are clearly present in the dusk data. They show a variation with longitude which is apparently associated with the longitudinal variation of the strength, or square of the strength, of the main field in the E-region. Also evident is a variation with time of the year, although data are available for only a six month period. The meridional currents are generally minimum during January and February and maximum either during November and December or March and April, depending upon longitude. The E-region horizontal currents are minimum in November and December and maximum in March and April, except for − 30° to −90° longitude when the maximum occurs in January and February.Assuming that field gradients in local time are considerably smaller than field gradients in dip-latitude, current densities are estimated to be 1–3.6μA/m² for the horizontal current at 110km and about 10–20 × 10⁻⁹ A/m² for the vertical currents at 400km altitude. These results confirm and extend earlier results of Takeda and Maeda.Most models of the electrojet system in the literature disagree severely with these measurements either because their scope is inadequate or because of the wind system they assume. Those models which best describe the data invoke an eastward wind and/or an eastward electric field at dusk local time.     

Oblique HF radar studies of plasma instabilities in the equatorial electrojet in Africa

While the equatorial ionosphere was regularly investigated in South America by radar technique and a number of very interesting phenomena detected and associated with plasma instabilities, results in the other part of the world along the magnetic equator were relatively few. The preliminary results of new African experiments are presented. The experimental procedures and equipment and the first results at a.frequency of 21.3 MHz are described. The two types of instabilities previously detected at Jicamarca are regularly observed at 21.3 MHz and their characteristics are described in relation with different theories about plasma instabilities.     

Latitudinal and vertical parameters of the equatorial electrojet from an autonomous data set

The magnetic field expressions from the current ribbon and thick current versions of the continuous distribution of current density model and their merits have been presented. For the first time both the latitudinal and vertical parameters of the equatorial electrojet (EEJ) have been derived from the same set of data. The local noon and daytime means of certain key parameters of the EEJ are shown to be in good agreement with those from other sources. Selected local noon means include: peak current density j_o, 10.58 ± 0.34 A/km²; peak current intensity j_o, 224 ± 9 A/km; total eastward current I₊, 74 ± 5 kA ; EEJ current focal distance w, 300 ± 5 km ; half thickness at half of peak current density p, 7.0 ± 0.1 km; peak westward current location x_m, 5.13 ± 0.08° dip latitude; and EEJ latitudinal extent L₁, 12 ± 1° dip latitude. The problem of model calculated landmark distances of EEJ being consistently shorter than observations, encountered by Onwumechiliet al. [J. geomagn. Geoelecl. 41, 443 (1989)] has been solved.     

HF produced ionospheric electron density irregularities diagnosed by UHF radio star scintillations

High frequency waves incident on an overdense ionosphere (i.e. HF < penetration frequency) are known to produce large-scale irregularities with scale sizes of several hundred meters in the F-region of the ionosphere. Three observations of radio star intensity fluctuations at UHF are reported for HF ionospheric modification experiments performed at the Arecibo Observatory. Two observations at 430 MHz and one observation at 1400 MHz indicate that the thin phase screen theory is a good approximation to the observed power spectra. However, the theory has to be extended to include antenna filtering. Such filtering is important for UHF radio star scintillations since the antenna usually has a narrow beam width. HF power densities of less than 37 μW m⁻² incident on the ionosphere produce electron density irregularities larger than 13% of the ambient density (at 260 km) having scale sizes of ~510 m perpendicular to the geomagnetic field. The irregularities form within 20–25 s after the HF power is turned on. From the observed power spectra driftvelocities of the irregularities can be estimated.     

Retrieval of east-west wind in the equatorial electrojet from the local wind-generated electric field

The paper presents a method to retrieve the height-varying east-west wind U(z) in the equatorial electrojet from the local wind generated electric field E_W(z), or from the radar-measured phase velocity V_p^II(z) of the type II plasma waves. The method is found to be satisfactory when E_w ⪢ E_p, where E_p is the vertical polarization electric field generated by the global scale east-west electric field, EY, and E_y < 0.2 mV m⁻¹. Measurements of V_p^II by a VHF backscatter radar can be inverted to obtain the causative wind profile by this method. The method is tested using a simulation study in which E_w(z) and V_p^II(z) as generated by two different wind models are used. The retrieved winds are compared with the original wind profiles and it is found that the error in the retrieved winds is mostly under 5%, for the case of no errors in the model Pedersen conductivity (σ₁) profile and the E_w(z) or V_p^II(z)(z) profiles used in the inversion. Even with a ±20% error in the above profiles, the errors in the retrieved winds are found to be less than 20% over 75% of the altitude range and 20–30% for the remaining 25% of the altitude range, on the average.