ANGEO

Annales Geophysicae

ANGEO

Ann. Geophys.

1432-0576

Copernicus Publications

Göttingen, Germany

10.5194/angeo-20-1167-2002

Imaging radar observations and nonlocal theory of large-scale plasma waves in the equatorial electrojet

Hysell

D. L.

³ ² ¹ ⁵ Chau

J. L.

⁴

Department of Physics and Astronomy, Clemson University, Clemson, South Carolina, USA

On leave at the Radio Science Center for Space and Atmosphere, Kyoto University, Uji, Kyoto 611–011, Japan

Now at the Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, New York, USA

Radio Observatorio de Jicamarca, Instituto Geof´isico del Per´u, Lima, Peru

Correspondence to: D. L. Hysell; (daveh@geology.cornell.edu)

31 08 2002

20 8 1167 1179

2002

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Large-scale (l ~ 1 km) waves in the daytime and night-time equatorial electrojet are studied using coherent scatter radar data from Jicamarca. Images of plasma irregularities within the main beam of the radar are formed using interferometry with multiple baselines. These images are analyzed according to nonlocal gradient drift instability theory and are also compared to nonlinear computer simulations carried out recently by Ronchi et al. (1991) and Hu and Bhattacharjee (1999). In the daytime, the large-scale waves assume a non-steady dynamical equilibrium state characterized by the straining and destruction of the waves by shear and diffusion followed by spontaneous regeneration as predicted by Ronchi et al. (1991). At night, when steep plasma density gradients emerge, slowly propagating large-scale vertically extended waves predominate. Eikonal analysis suggests that these waves are trapped (absolutely unstable) or are nearly trapped (convectively unstable) and are able to tunnel between altitude regions which are locally unstable. Intermediate-scale waves are mainly transient (convectively stable) but can become absolutely unstable in narrow altitude bands determined by the background density profile. These characteristics are mainly consistent with the simulations presented by Hu and Bhattacharjee (1999). A new class of large-scale primary waves is found to occur along bands that sweep westward and downward from high altitudes through the E-region at twilight. Key words. Ionosphere (equatorial ionosphere; ionospheric irregularities; plasma waves and instabilities)