An integrated treatment of observations and models of the undisturbed equatorial electrojet is presented. Previous reviews have concentrated on particular aspects of the electrojet phenomenon, such as ground magnetic variations, the counterelectrojet, or E region electron density irregularities and associated plasma instabilities. Here accomplishments during the past 15 years involving all aspects of the phenomenon are covered and, where possible, interrelated. Observational and theoretical studies during recent years have emphasized the local time, longitude, vertical, and latitudinal structure of electric fields and currents in the electrojet; this review similarly places its emphasis along these lines and by necessity involves a detailed examination of electrodynamic coupling with the neutral atmosphere tidal oscillations in temperature, composition, and winds. The review concludes with an assessment of current knowledge concerning the equatorial electrojet and suggestions for the future direction of observational and theoretical research on electrodynamical processes in the equatorial ionosphere.

The equatorial electrojet

The typical quiet day variations of the equatorial electrojet (EEJ) current intensity with time of the day, season, sunspot number, and geomagnetic latitude are presented in terms of the corresponding variations of ΔH which is the deviation of the horizontal component (H) of the geomagnetic field from its steady nighttime level. The observed height structure of the current density in the EEJ as measured in rocket flights is presented, along with the theoretically computed structure. Theoretical model results on the polarization electric fields and east-west currents as generated by the local interactions of height-varying winds in the EEJ show large height gradients and reversals for both currents and electric fields; experimental evidence for the reality of such height structures is also shown. The characteristics of the counter-electrojet events are presented and the possible causative mechanisms are discussed critically.

The Equatorial Electrojet

Using the equivalent electrical circuit method for the theoretical treatment of the local wind effects on the equatorial electrojet, the height structures of electric fields and currents that are generated by height-varying east-west winds in the electrojet region have been calculated for theoretical model wind sturctures and for a variety of experimentally observed wind structures. The results bring out clearly the nature and extent of the local wind effects on the height and latitude structures of the equatorial electrojet.

The more important conclusions of the present study are: (a) The vertical wind shears associated with tidal winds and gravity waves can generate significantly large electric fields and east-west currents in the equatorial electrojet (EEJ). (b) The wind-generated electric fields and currents are characterized by large height gradients, latitudinal gradients, and reversals of direction. (c) At the magnetic equator, wind-generated electric fields and currents are often small (10%–30%) in comparison with the eastward electric-field-generated polarization field and east-west currents around noon hours. (d) At geomagnetic latitudes of 3° and beyond the wind-generated currents and electric fields often exceed those caused by the eastward electric field. (e) Beyond 2° the neutral winds can drastically alter the intensity and latitudinal structure of the EEJ, depending upon the magnitude and height location of the wind shear. (f) The observed variabilities in the ‘width’ and latitude structure of the EEJ can arise from the effects of such east-west winds that have been observed experimentally.

Height and latitude structure of electric fields and currents due to local east-west winds in the equatorial electrojet

Global scale electrodynamic coupling of the auroral and equatorial dynamo regions

[1] The effects of large winds on the low-latitude E region ionosphere and the equatorial electrojet in particular are analyzed theoretically, computationally, and experimentally. The principles that govern the relationship between electric fields, currents, and winds in steady flows in the ionosphere are reviewed formally. A three-dimensional numerical model of low-latitude ionospheric electrostatic potential is then described. Scaled wind profiles generated by the National Center for Atmospheric Research (NCAR) thermosphere/ionosphere/mesosphere electrodynamics general circulation model (TIME-GCM) are used as inputs for the potential model. The model shows that the horizontal wind component drastically modifies the vertical polarization electric field in the electrojet and drives strong zonal and meridional currents at higher dip latitudes outside the electrojet region. Comparison between the model output and coherent scatter radar observations of plasma irregularities in the electrojet indicate that strong winds and wind shears are present in the E region over Jicamarca that are roughly consistent with NCAR model wind predictions if the amplitudes of the latter are increased by about 50%.

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Effects of large horizontal winds on the equatorial electrojet

THE signal strength and Doppler frequency spectra of v.h.f. backscatter radar echoes from the equatorial electrojet are being recorded at Thumba near Trivandrum (dip ∼ 1 °S). The v.h.f. radar operates on 54.95 MHz, with identical arrays of Yagi antennae for transmission and reception; the beam axis is orientated either vertically or obliquely at 30° to the vertical. Analysis of the radar backscatter signals and the magnetograms at Thumba has revealed that quasi-periodic fluctuations of the electrojet on time scales of a few tens of minutes are very frequent. We report here some typical experimental results on this phenomenon.

Short period fluctuations of the equatorial electrojet

The generation of electric fields and currents by local winds in the equatorial electrojet is treated by an equivalent electric circuit method. Apart from its mathematical simplicity, the method provides a full and clear view of the physical processes involved in the generation of currents and electric fields by shearing winds, and thus permits a straightforward interpretation of the main effects of winds. The method is valuable for the theoretical analysis of the wind-related electrodynamics of the equatorial electrojet.

C. V. Devasia

Papers

Short period fluctuations of the equatorial electrojet

Equivalent circuit analysis of neutral wind effects on equatorial electrojet