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.     

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.     

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.     

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.     

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.     

Temporal variations of POGO equatorial electrojet parameters

An analysis of the POGO satellites observations of the magnetic field of the equatorial electroject for the September equinoctial months of the years 1967, 1968 and 1969, provided about 500 values each of the electrojet half-width w, its peak current intensity J₀, its total eastward current I₊ at 11, 12, 13, 14 and 15 h LT. The all-sector daytime average values of the parameters for the three years are 232 ± 47 A km⁻¹ for J₀, 234 ± 6 km for w and (55± 8)×10³ A for I₊.This first coverage of all sectors of the globe gives the first study of the diurnal variation of the total current and shows that all the three parameters vary substantially with local time; that w has minimal values around local noon; that J₀ has a pronounced peak around local noon as may be expected from the diurnal variation of H; and that I₊ has a broad maximum around 11 h and 12 h LT.     

Electrodynamic effects of metal ions in the noon equatorial E-region

The observations of metal ions in significant concentrations and in layer formation in the lower E-region are briefly reviewed. It is expected that a metal ion layer may alter the electro-conductivity of the ionosphere and modify the distribution of ionospheric dynamo current. The variation in the electroconductivity is theoretically calculated, and it is shown that the Pedersen conductivity is reduced by less than 14%. In the equatorial ionosphere, field line integration of metal ion effects is likely to be large. A metal ion layer, similar to those observed, is numerically modelled in the equatorial region and its electrodynamic effects on the equatorial electrojet (EEJ) are numerically examined using our self-consistent model of the ionospheric dynamo. The effects are found to be significant on the amplitude of the EEJ, but not effectively large on its peak altitude.     

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.     

Electric fields and electron density irregularities in the equatorial electrojet

Two Centaure rockets were launched from Thumba (0 47′S dip). India, with a new arrangement of double probe sensors for the simultaneous measurements of the irregularities in the electron density and the electric field along and perpendicular to the spin axis of the rocket. These experiments were carried out during the period when type I irregularities were observed with the VHF backscatter radar at Thumba. Irregularities with scale sizes ranging from a few meters to a few kilometers in the electron density and in the electric field components both in the east-west and the vertical direction could be studied with these experiments. Irregularities in the electric field in the medium scale size range (30–300 m) were observed with peak to peak amplitudes up to 20 mV m⁻¹ and in the small scale (⩽ 15 m) with peak to peak amplitudes up to 5 mV m⁻¹. Horizontally propagating waves with horizontal scale sizes up to 2.5 km were observed in the region below 105.5 km. Using linear theory for the electrojet irregularities, it was found that for 5 % perturbations in the electron density, the amplitude of the electric field can be as large as 20–30 mV m⁻¹. The spectrum of the irregularities in the vertical electric field in the rocket frame of reference was calculated and it was found that for the range of scale sizes between 10 and 70 m, the mean spectral index was −2.7 and −2.6. while in the scale size range 2–10 m it was −4.0 and −5.1 for the flights C-77 and C-73, respectively.     

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.     

Magnetic variations at other latitudes during reverse equatorial electrojet

Several reverse equatorial electrojet events are examined. Magnetic variations from a sample of worldwide stations on the same days are compared with the quiet daily average for the same month. In most cases departures can be found elsewhere at the same time as the reverse jet. Various wind systems in the dynamo region might produce the observed results but different systems are required on different occasions.     

Solar quiet geomagnetic variations and E-region neutral winds at equatorial latitudes

A model of the ionospheric current system valid at zones close to the geomagnetic equator, taking into account the contribution of neutral winds, is proposed. From this, the external magnetic field at ground is calculated. Also, ground records of the geomagnetic field variations at the Peruvian equatorial zone were separated into their external and internal contributions. Using an iterative process a local particular fitting was found by comparing the separated external field to the one calculated with the proposed model.     

Diurnal variations of equatorial electrojet parameters derived from POGO solstitial data

The POGO electrojet data have been analysed for the winter and summer solstitial seasons of the two years, 1968 and 1969, respectively. Our analysis yielded a very large number of values (about 432), each of the electrojet half width, w, its peak current intensity, J₀, and its total eastward current, I₊, at 0900–1400 LT in December, and at 1000–1500 LT in June solstitial seasons, respectively. The all-longitude daytime values of the parameters are 246 ± 48 km for w,216 ± 60 A km⁻¹ for J₀, and (58 ± 8) × 10³A for I₊, in December of 1968 and 218 ± 19 km for w, 187 ± 20 A km⁻¹ for J₀, and (45 ± 3) × 10³ A for I₊, in June of 1969, respectively. We therefore present a diurnal study covering the entire Earth, in which for the first time, morning data earlier than 1000 LT are incorporated in the analysis. This has enabled us to chart a clearer picture of the temporal variations of electrojet parameters at two different solstices. This shows that all of the three parameters vary substantially with local time, in such a manner that J₀ and I₊ attain maximal values around local noon, while w is a minimum then, and therefore confirms the finding of Agu and Onwumechili.     

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.     

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.     

HF scattering induced by field-aligned irregularities in the equatorial E-region during total solar eclipses

In this paper, results and analyses of solar eclipse effects on the lower ionosphere are presented. After the first contact of the total eclipse on 16 February 1980, an absorption increment of 12 dB was observed. At the same time, the frequency of amplitude fading increased largely and Doppler frequency shift disturbances appeared. The calculation of signal strength is carried out by means of Booker's scattering theory, supposing an outer scale T_o = 1000 m and an inner scale T_i = 5 m, of space scale spectrum of field-aligned irregularities in the equatorial E-region. The calculated results agree fairly well with observations. Results showed that, because of the formation of lower ionospheric field-aligned irregularities in the course of the obscuration of solar local ionization source, radio wave scattering was strengthened.     

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.