Evidence and effects of a wave-driven nonlinear current in the equatorial electrojet

Ionospheric two-stream waves and gradient-drift waves nonlinearly drive a large-scale (D.C.) current in the E-region ionosphere. This current flows parallel to, and with a comparable magnitude to, the fundamental Pedersen current. Evidence for the existence and magnitude of wave-driven currents derives from a theoretical understanding of E region waves, supported by a series of nonlinear 2-D simulations of two-stream waves and by data collected by rocket instruments in the equatorial electrojet. Wave-driven currents will modify the large scale dynamics of the equatorial electrojet during highly active periods. A simple model shows how a wave-driven current appreciably reduces the horizontally flowing electron current of the electrojet. This reduction may account for the observation that type I radar echoes almost always have a Doppler velocity close to the acoustic speed and also for the rocket observation that electrojet regions containing gradient-drift waves do not appear to also contain horizontally propagating two-stream waves. Additionally, a simple model of a gradient-drift instability shows that wave-driven currents can cause non-sinusoidal electric fields similar to those measured in-situ. Introduction As early as the late middle ages, navigators observed that magnetic compass readings taken near the equator often varied by a few degrees in the day. In 1839, Friedrich Gauss speculated that these fluctuations resulted from the presence of large currents in the atmosphere. Balfour Stewart proposed in 1882 that these currents resulted from a solar-driven dynamo in an ionized region of the upper atmosphere which he called the ionosphere. At the beginning of this century, Schuster [1908] and Chapman [1919] developed a mathematical description of the dynamo which drives the equatorial electrojet. Not long after the development of radar in the 1940s, Bowles et al. [1960] reported observing strong coherent radar echoes from the equatorial electrojet indicating the presence of plasma density irregularities. A number of years later, Farley [1963] and Buneman [1963] applied linear kinetic and fluid theories to describe the origin of these echoes, now called the Farley-Buneman or two-stream instability. Both Maeda et al. [1963] and Simon [1963] extended this theory to describe a second E region instability, the gradientdrift instability. However, linear theories cannot fully describe the behavior of these nonlinearly saturated waves. This paper describes a nonlinear process resulting from E region waves, its relative importance and effects on a number of electrojet phenomena. We first described wave-driven currents in a letter [Oppenheim, 1996]. This paper elaborates on the theory of wave-driven currents, extends the theory to describe longer wavelengths and discusses the effects of wave-driven currents on the large-scale equatorial electrojet. A wave-driven current results from two fundamental features of E region plasma waves. First, electrons travel mostly perpendicular to the electric fields due to the geomagnetic field while ions travel mostly parallel to the fields because ion-neutral collisions make magnetic field effects inconsequential. Second, gradient-drift and two-stream instabilities cause compressional waves where the plasma density enhancements and the perturbed electric fields remain largely in phase. At the plasma density maxima of the propagating wave fronts, electrons move perpendicular to the wave direction and the the geomagnetic field. At the density minima, electrons move in the opposite direction with an equal velocity. However, more electrons exist at the maxima than at the minima causing a greater current in one direction than the other, resulting in a net (direct) current.