用单流体和双流体ＭＨＤ近似，研究了近磁尾位形不稳定性（ＮＥＴＣ）．分析表明，ＮＥＴＣ可能存在两种漂移不稳定情况Ｃ１和Ｃ２与卫星观测资料对比显示，Ｃ２较容易在亚暴膨胀相前夕出现，它可以解释亚暴膨胀相期间磁场和等离子体扰动的特征周期、尾向传播速度、磁场扰动和等离子体压强扰动之间的位相关系，场向电流的周期性结构，西向涌浪头部的电子沉降和极光隆起等观测特性和现象．薄电流片的极端情况（Ｒｃ≈ｒｉ）不在本文的讨论范围之内． Using the single-fluid and two-fluid MHD approximations, the near-magnetic tail-shape instability (NETC) was investigated. The analysis shows that there are two drift instability cases in NETC. The comparison of C1 and C2 data with satellite observations shows that C2 is easier to appear on the eve of the subduction phase. It can explain the characteristic period of magnetic field and plasma perturbation during the subduction phase, The propagation velocity, the phase relationship between magnetic field disturbance and plasma pressure disturbance, the periodic structure of field current, the electronic sedimentation and the auroral uplift of the westerly surge. The thin case (Rc≈ri) is beyond the scope of this article.