| 摘要: | 電離層的電子密度不規則體，經由各種不穩定擾動過程形成，主要都是沿著地球磁力線方向排列，形成這種非均向特徵排列的原因，是來自於電子平行與離子垂直於地球磁力線方向的移動率相差很大。雷達觀測也受限於這種非均向的特徵影響，在波束指向垂直於磁力線的方位有最強的散射回波，當偏離垂直的角度增加，回波強度即迅速降低，這個偏離的角度稱為方位角。利用雷達干涉法，由南北方向排列兩組接收陣列的歸一化互相關頻譜，已經可以有效的準確測量不規則體回波的方位角［Kudeki and Farley,1989; Huang et al., 1995; Lu et al., 2008]，一般而言，3米尺度Es場列不規則體的方向角度平均約0.1-0.5。Wang el al. (2011) 利用中壢雷達的三維空間干涉技術，統計1997~2000年52Ｍ雷達觀測Es的夜間回波，首次得到層狀與團狀不規則體的平均方位角分別為0.25與0.3。我們的方法本質上與 Kudeki and Farley （1989）相同。中壢雷達52M系統在更新之後，不但回波的質、量遽增也能接收到許多有效的日間Es訊號；本計畫目的為利用此方法，統計更多長時間的資料，並且比較日間與夜間Es場列不規則體，沿磁力線排列的方向角度差異，同時藉以討論兩者形成的機制。
The plasma irregularities in sporadic E region driven by various instability processes in ionosphere are highly elongated in the ambient geomagnetic field direction. The reason of this anisotropy characteristic is due to the large difference of parallel and cross mobility of electrons and ions. Backscatter signal from such irregularities is highly aspect sensitive. Therefore, the strongest signal is from the beam perpendicular to the geomagnetic field direction. The angle between the deviated from the perpendicular direction is called the aspect angle. By using interferometry technique, from the normalized cross-spectrum (or coherence) of antennas spaced in the magnetic north-south direction, it has been shown the averaged aspect angle is in a range of 0.1-0.5 [Kudeki and Farley,1989; Huang et al., 1995; Lu et al., 2008]. The radar interferometry has been applied to investigate 3-meter field-aligned irregularities for more than one decade. Wang et al.( 2011) develop a measurement aspect angle method from statistic the nighttime Es echoes from 1997~2000. They first conclude the average aspect angles of layer and clump types plasma structures are 0.25 and 0.3, respectively. Basically, our method is the same as Kudeki and Farley used in 1989. The new 52 M system has accumulated much Es echoes including daytime signals. The goal of this proposal is to compare the difference of nighttime and daytime Es aspect angles from more data base. The knowledge of aspect angle will benefit to increase the understand of the mechanism of Es layer. |
