前艏震波區域充滿著不同特性的粒子分布和波動現象，波動產生機制之線性理論分析顯示波動僅可在波動傳播方向與背景磁場方向夾角呈有限角度時傳播，然而衛星資料觀測報告卻顯示波動傳播方向與背景磁場方向夾角可達20至40度。另外，研究報告顯示前艏震波區域波動現象會受到太陽風對流效應的影響，波動經歷兩個粒子分布特性不同的區域，波動傳播方向因受到折射效應而傳播角度改變，產生波動斜向傳播現象，但此一提議無法符合在徑向行星際磁場環境下(單一粒子分布特性區域無折射效應發生)仍可觀察到斜向傳播現象的觀測結果。目前為止，對於波動斜向傳播現象的了解和解釋尚未完全清楚，而西蜜斯衛星任務擁有相同酬載的衛星群以及異地同步的觀測機會，可幫助我們研究波動斜向傳播現象。我們將選用兩地同步觀測前艏震波區域超低頻波動斜向傳播事件，進行波動歷經時空傳播後其波動特性變化及其受影響因素的分析討論，以增進對波動斜向傳播現象的了解。 Various ion distributions and ULF (ultra-low-frequency) waves are observed in the foreshock region. The theoretical linear analysis of wave generation predicts that the waves can propagate in a limited angle with respect to the magnetic field. However, observations show that the waves can obliquely propagate at a much larger angle than the one predicted by the theory, ranging from 20 to 40 deg. The theory of refraction for waves states that when a wave is carried by the solar wind convection into a different foreshock region, refraction occurred in between the two different media results in a change in the propagating direction of the wave. But this interpretation fails to explain the occurrence of oblique-propagating ULF waves under the radial interplanetary magnetic field condition because two different media cannot be found in such a condition. Thus far, the oblique propagation is still not fully understood. The THEMIS mission, which consists of five identical probes, has some conjunction opportunities to observe wave activities at different places in the foreshock region simultaneously. Taking advantage of simultaneous observations from THEMIS, we will analyze the property of foreshock waves and their oblique propagation, and determine the factors that affect variations of the waves. This study can help advance our understanding of oblique propagation of ULF waves in the foreshock region. 研究期間：10008 ~ 10107
