Abstract
The nonlinear features of type-1 radar echoes were studied by a two-dimensional (2D) simulation of saturation the Farley-Buneman (FB) wave. The behavior of this FB wave in a plane perpendicular to the Earth's magnetic field was simulated with a two-fluid code in which electron inertia was discounted while ion inertia was retained. It showed that the appearance of secondary waves propagating vertically and obliquely as the primary horizontal FB wave saturates. The secondary waves originating from nonlinear saturation process will construct the evolution of 2D modes which can be observed by oblique radar beams. We carried out the statistical analysis of projection phase velocities of 2D modes along oblique radar beam at different radar elevation angles. The result revealed that a likely density gradient effect of type-1 radar echoes for the wavelength dependence of phase velocity would appear at a larger radar elevation angle while short wavelength waves would approach isotropic speeds close to ion acoustic speed. This interesting result is primarily attributed to the spectral features of 2D modes.
Highlights
Type-1 radar echoes revealed the presence of plasma wave in the earth’s equatorial and auroral electrojets
The result of 2D saturation simulations of pure FB waves (Oppenheim and Otani 1996; Fern and Kuo 2001) revealed that, vertically propagating secondary waves emerged as horizontally traveling primary Farley-Buneman wave saturated; The secondary waves evolved from mode-mode coupling; The spectral asymmetry was closely related to the ambient electric field, and the phase velocity of these longer wavelength waves was below the prediction of linear theory, and so on
In order to understand the features of type-1 radar echoes such as 2D wave motions from oblique radar beams, a detailed analysis of 2D mode evolution derived from the saturation process of pure FB waves is necessary
Summary
Type-1 radar echoes revealed the presence of plasma wave in the earth’s equatorial and auroral electrojets. In the lower electrojet, plasma density gradients can develop kilometer large-scale gradient-drift (GD) waves at an electron drift speed below the threshold of FB instability wherein the large electric field originating from these horizontally propagating large scale primary waves could excite vertically traveling FB waves (Sudan et al 1973; Kudeki et al 1987; Pfaff et al 1987a, b). The result of 2D saturation simulations of pure FB waves (Oppenheim and Otani 1996; Fern and Kuo 2001) revealed that, vertically propagating secondary waves emerged as horizontally traveling primary Farley-Buneman wave saturated; The secondary waves evolved from mode-mode coupling; The spectral asymmetry was closely related to the ambient electric field, and the phase velocity of these longer wavelength waves was below the prediction of linear theory, and so on. Their result showed that the evolution of long wavelength waves and the maximum of turbulent energy at saturated stage always shifted toward the longest wavelength waves allowed by the simulation box size
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