Simulations of Wind Field Effect on Two-Stream Waves in the Equatorial Electrojet

Abstract

The wind field effect on the phase velocities of 3- to 10-meter Farley-Buneman two-stream waves in the equatorial E region ionosphere at altitudes in the range of 95 - 110 km is studied by numerical simulation. The behavior of this two-stream wave in the uniform wind field Un in a plane perpendicular to the Earth’s magnetic field is simulated with a two-dimensional two-fluid code in which electron inertia is neglected while ion inertia is retained. It is confirmed that, the threshold condition for the appearance of two-stream waves is VthD ≈  (1 + ψ0)Cs / cos θ + Un ; and the phase velocity of the two-stream wave at the threshold condition is Vp ≈ Cs + Un cos θ, where θ is the elevation angle of the wave propagation in a limited range and Ψ0 = νinνen / ΩiΩe. The first formula in dicates that the wind field parallel (anti-parallel) to the electron drift velocity will raise (lower) the threshold drift velocity by the amount of the wind speed. This means that parallel wind is a stable factor, while anti-parallel wind is an unstable factor of two-stream waves. This may explain why high speed (larger than acoustic speed) two-stream waves were rarely observed, since larger threshold drift velocity demands larger polarization electric field. The result of the simulations at the saturation stage show that when VD was only slightly larger than VD th , the horizontal phase velocity of the two-stream wave would gradually down-shift to the thresh old phase ve loc ity Cs + Un. The physical implications of which are discussed.

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