Petschek 於1964年提出一磁力線重連理論模式，其中兩對的慢速震波扮演著將磁能轉換為電漿動能的重要角色。許多衛星觀測資料顯示，在地球磁尾發現之慢速震波都是屬於電漿 β 值較小處。在這樣的太空電漿環境下，帶電粒子易受磁場束縛做迴旋運動，因此電漿的壓力或溫度可具有垂直磁場與平行磁場的兩個分量。在本研究中，我們使用非線性磁流體力學理論探討壓力非均向電漿中慢速震波之形成。在一個初始條件為一切線不連續面結構中加上一垂直方向的磁場，可以發展出許多的不連續面結構，其中即包含著慢速震波。研究顯示，具密度與磁場正相關或傳播速度較中速波快之異常慢速震波，在壓力非均向的電漿中可以穩定態存在。此結果與 Hau and Sonnerup [1993] 之線性理論所預測相符。 The Petschek’s reconnection model involves two pairs of slow shocks that play the role of accelerating plasma by reducing the magnetic field. There have been some observational evidences for the existence of slow shocks in the Earth’s magnetotail where the plasma beta is usually low. For such plasma conditions, the thermal pressure is found to display the gyrotropic form with two distinct pressure components parallel and perpendicular to the local magnetic field. In this study, the structure of slow shocks is examined based on the anisotropic MHD model for which the energy closure is of the double-polytropic laws. In particular, slow shocks are formed through the evolution of a tangential discontinuity current sheet initiated by the presence of normal magnetic field component. It is shown that anomalous slow shocks with positive density-magnetic field correlation or moving faster than the intermediate mode may form, as predicted by Hau and Sonnerup [1993] based on the linear anisotropic MHD theory.
