赤道區電離E層約95∼110公里的高空，由於大氣潮汐運動經常會形成強烈的電噴流(Electrojet)現象，此電噴流易導致該區域電漿不穩定發展，進而構成電漿不規則體產生電漿回波。根據雷達觀測已確定類型1 (TypeⅠ)與類型2 (TypeⅡ)兩種不同的電漿回波，其中類型1回波係來自於雙流體不穩定(Two-stream instability)所造成的電漿不規則體，故又稱為雙流體電漿波。而本文即利用二維電漿流體模擬程式(其中忽略電子慣性項而保留離子慣性項)進行垂直於地磁方向的赤道區電離E層純雙流體電漿波模擬，以期深入瞭解純雙流體電漿波的二維非線性發展。為了配合3m的VHF雷達觀測尺度，我們採取適切的二維空間模擬尺度(27×54m )且成功激發出近似3m的主要電漿波，並推展二維傅利葉級數(Fourier series)分析方法來表達各模態功率與ω 頻譜分佈隨時間演變的過程。 經由一系列赤道區電離E層純雙流體電漿波發展的模擬分析結果，我們可驗證一些已知理論與現象，例如﹔純雙流體電漿波初始發展符合線性頻散關係; 共振耦合現象的展現; 回波功率隨天頂角變化滿足0.3(dB/度)的分佈以及二維電漿波的非線性發展導致垂直結構產生形變而構成垂直模態分量，最後造成主要電漿波偏離電噴流漂移(E×B )方向發展。然而，更重要的是這些垂直模態的發展具有非對稱結構。由我們的模擬結果發現電漿波尺度大小會影響非對稱的發展，即大尺度模態(波長尺度大於3m)朝k?E<0 方向傳播發展，而小尺度模態(包括3m電漿波)則朝 k?E>0 的方向傳播發展，其中3m電漿波符合VHF雷達非對稱觀測結果。我們認為非對稱結構發展主要導因於赤道電噴流的強大垂直背景極化電場E 。同時，由於強大固定的垂直背景極化電場E 影響，我們發現雙流體電漿波頻率僅與該水平模態Kx有關，以至於電漿波傾斜傳播(Kz≠0 )的相速低於線性理論之預測值，尤其在大尺度電漿波最明顯。 At the altitude range of 95∼110 km in the equatorial E-region ionosphere, a large current in the magnetic equator called the equatorial electrojet is created by a dynamic process involving atmospheric tides. The equatorial electrojet causes the medium to be unstable to generate plasma waves. Radar observations have shown the existence of two classes of plasma waves, called typeⅠand typeⅡ. The typeⅠwave resulting from the two-stream instability is also called two-stream wave. In this paper, the behavior of two-stream wave in the plane perpendicular to the Earth's magnetic field is simulated by a two-dimensional fluid model simulation code in which the electron inertia is neglected while the ion inertia is retained. We consider the suitable two-dimensional simulation space (27×54m ) and successfully excite the dominant 3-meter wave compatible with VHF radar echoes. The analysis methods of two-dimensional Fourier series are derived to express the time variation of mode powers and ω spectra. From the series of simulation results for two-stream waves in the equatorial E-region ionosphere, we have studied the dispersion relations, the resonant coupling, and the echo power variation with the zenith angle rate of approximately 0.3(dB/degree). We have also studied the vertical small-scale waves developed from the distortions of the dominant waves. The vertical waves will cause dominant waves to turn away from the electrojet drift direction (E×B ). The important point about the development of vertical modes is asymmetry. We find that the spectral asymmetry depends on the scale size of plasma waves, that is, the large-scale waves (above 3 meters) propagate in the k?E<0 region, and the small-scale (smaller than 3 meters) propagate in the k?E>0 region. The simulation result of up-down asymmetry for 3-meter wave is consistent with the radar observations. We believe that the background polarization electric field E driving the electrojet is responsible for the spectral asymmetries. In the same time, owning to the strong and fixed background polarization electric field E , the wave frequency depends only on horizontal wave number Kx . Therefore, the phase velocities of many obliquely propagating two-stream waves (Kz≠0 ) are below the value predicted by linear theory, and this characteristic is prominent especially for large-scale waves.
