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

A nonlinear unified theory of type I and II irregularities is presented that explains their principal observed characteristics. The power spectrum is predicted by using Kolmogoroff-type conservation law for the power flow in cascading eddies.

Unified theory of Type I and Type II irregularities in the equatorial electrojet

: We present, in this paper, an overview of theories of naturally occurring high latitude E and F region ionospheric irregularities. (For a review of artificially-induced irregularities, e.g., from ionospheric 'heating' experiments, see Fejer 1979.) Emphasis is placed on recent results, particularly with regard to sources of high latitude irregularities. Physical mechanisms are stressed with extensive mathematical analysis avoided. Expertise in plasma physics has not been assumed. We summarize the recent and previous theoretical literature dealing with F region irregularities, and we concentrate on E region irregularity phenomena.

Theories of high-latitude ionospheric irregularities: A review

We have examined the relationship between the radial mean Doppler velocities observed with coherent radars, such as in the STARE (Scandinavian Twin Auroral Radar Experiment) system, and the electron drift velocities observed with an incoherent scatter facility. The two sets of measurements were made simultaneously and can with high accuracy be related to a common magnetic flux tube. We find that the Doppler velocities of electron density irregularities produced by the two-stream instability in the E region are limited to a value near the ion acoustic velocity. Moreover, we find that this value is a strong function of the electron drift velocity, in agreement with theoretical predictions. An algorithm has been determined which enables us to use the STARE data to derive estimates of the electron drift velocity which are in excellent agreement with values deduced from EISCAT (European Incoherent Scatter Faciilty) data.

Coherent radar Doppler measurements and their relationship to the ionospheric electron drift velocity

Theories of high latitude ionospheric irregularities: A review

A pair of coupled, nonlinear, partial differential equations which describe the evolution of low‐frequency, large‐scale‐length perturbations propagating parallel, or nearly parallel, to the equilibrium magnetic field in high‐β plasma have been obtained. The equations account for irreversible resonant particle effects. In the regime of small but finite propagation angles, the pair of equations collapses into a single Korteweg‐de Vries equation (neglecting irreversible terms) which agrees with known results.

Parallel Propagation of Nonlinear Low‐Frequency Waves in High‐β Plasma

It is suggested that the stabilization mechanism of type 1 irregularities in the equatorial electrojet has two aspects: (1) the turbulence tends to inhibit the formation of large currents and to maintain the plasma in a state relatively ‘close’ to linear marginal stability, and (2) in this state, energy is transferred by two-dimensional nonlinear wave-coupling processes from the linearly growing modes to linearly damped ones propagating in other directions. Energy transfer processes can thus provide detailed stabilization over the entire spectrum. It is argued that (1) the theory provides a plausible interpretation of the as yet puzzling experimental result that the irregularities appear more or less simultaneously over a substantial range of radar elevation angles and (2) the fact that their phase velocities are close to the sound speed and often completely independent of the direction of propagation is perhaps a consequence of mode coupling, but definite prediction here requires numerical calculations.

two‐dimensional nonlinear processes associated with ‘type 1’ irregularities in the equatorial electrojet

A nonlinear theory of the E × B electron drift instability for Te∼Ti is developed on the basis of the wave kinetic equation for weakly turbulent plasmas in a uniform magnetic field. It is found that nonlinear electron cyclotron resonances stabilize short wavelength—but destabilize long wavelength modes; the total wave energy, however, is stabilized by this mechanism. In a two‐dimensional model (the waves propagate in a narrow fan encompassing the plane orthogonal to B), wave‐wave scattering processes can transfer energy from the unstable small k region of the spectrum toward larger k's where nonlinear electron damping dominates. Either balancing the relevant stabilizing and destabilizing processes for small k's, or considering over‐all energy balance, predicts that the theoretical turbulence level at k = kD (the inverse of the Debye length) is a few hundred times thermal. The estimated effective electron‐ion collision frequency is found to be much too small to explain the anomalous heating observed in col...

Nonlinear theory of the E × B electron drift instability in high‐β plasmas with T i ∼ T e

The nonlinear evolution and stabilization of absolute whistler instabilities driven by electron pressure anisotropy and propagating obliquely to the external magnetic field in finite β plasmas are investigated without introducing the assumption of random phases. Both in the firehose and the mirror instability regimes the pressure anisotropy relaxes quasilinearly toward a more stable state. The nonlinear wave equation has also been obtained. By choosing the parameters in such a way that the plasma is sufficiently close to linear marginal stability, it has been possible to show both numerically and analytically that the (firehose) instability stabilizes nonlinearly at sufficiently low levels of excitation. The fluctuation level does not asymptotically approach a constant value, but oscillates in time, the maximum amplitude being proportional to the linear growth rate. Also, close to marginal stability, finite Larmor radius effects can play a significant role even in the limit kR ≪ 1. It is found that finite...

André Rogister

Papers

Parallel Propagation of Nonlinear Low‐Frequency Waves in High‐β Plasma

two‐dimensional nonlinear processes associated with ‘type 1’ irregularities in the equatorial electrojet

Nonlinear theory of the E × B electron drift instability in high‐β plasmas with T i ∼ T e

Nonlinear theory of absolute whistler instabilities in finite β plasmas