磁層頂是一個介於磁鞘與磁層間分開上下游不同電漿特性之邊界。磁層頂可以分為兩種一維之結構：旋轉不連續（RD）以及切線不連續（TD）；前者與磁場重聯有關，有明顯之法線磁場分量以及與阿爾文速度相當之電漿流，後者則為沒有磁場重聯之狀態，沒有法線磁場分量且沒有與阿爾文速度相當之電漿流，但是觀測結果顯示切線不連續也會有與RD相同的磁場特性。在此篇研究中，除了使用 2007年Themis衛星通過磁層頂之磁場與電漿資料，並利用Minimum Variance Analysis以及DeHoffmann-Teller frame 分析518個磁層頂穿越事件，並且同時使用考慮壓力均向之Walen relation與考慮壓力非均向之 Walen relation來檢測符合阿爾文速度之電漿流。加了壓力非均向之檢驗對於分辨旋轉不連續與切線不連續有些微之影響；值得注意的是有一類似RD之案例，可能存在於α>1之電漿環境中。 Magnetopause is a boundary lying between magnetosphere and magnetosheath which separates different plasma states in the upstream and downstream. Magnetopause structures can be classified into two types of one-dimensional models: the rotational discontinuity (RD) and the tangential discontinuity (TD). The former is associated with reconnection event with significant normal component of magnetic field and Alfvenic plasma flow, while the latter is associated with non-reconnection events without normal component of magnetic field and Alfvenic plasma flow. Although the existence of normal component of magnetic field and rotation in tangential component of magnetic field are the characteristics for RD in ideal Magnetohydrodynamics (MHD) theory, sometimes TDs have the same properties in observation. In this study, by using plasma and magnetic field data observed by Themis satellite in 2007, we analyze 518 magnetopause crossing events through the analysis methods of Minimum Variance Analysis, DeHoffmann-Teller frame and Walen relation. Both Walen relations in an isotropic plasma and in anisotropic plasma are used to test whether the flow is approaching the Alfevn velocity. We have found that the Walen relation with pressure anisotropy assumption may slightly modify the identification of the event as a RD or TD and also found that a RD event may exist in a plasma condition, α>1.
