利用ISEE1& 2 ,AMPTE/IRM和IMP8的高分辨率数据 ,以及MV和TD分析法首次给出了在同一辨认原则下找到的所有磁层顶穿越的法线方向 .在此基础上 ,用分段拟合法研究了磁层顶的平均位形 .结果表明 :(ⅰ )大多数磁层顶穿越是切向间断面 ;(ⅱ )赤道面内的磁层顶存在着某种不对称性 ;(ⅲ )无论行星际磁场南向还是北向 ,夜间一侧的磁层顶截面的张角明显大于白天一侧 ,现有经验模型采用的简单单一曲线可能难以完整描述整个磁层顶的位形 . Using the high-resolution data of ISEE1 & 2, AMPTE / IRM and IMP8, and the MV and TD analysis, the normal directions of all the magnetospheric crossings found under the same recognition principle are given for the first time. On this basis, The results show that: (i) most of the magnetospheric crossings are tangential discontinuous planes; (ii) there is some asymmetry in the top of the magnetosphere in the equatorial plane; (iii) No matter whether the interplanetary magnetic field is south or north, the angle of the top of the magnetosphere on the nighttime side is obviously wider than that on the daytime. The simple single curve used in the current empirical model may not be able to fully describe the top of the magnetosphere.