中文摘要：

2004年10月12日，在01：30--04：30UT期间，位于向阳侧磁层顶附近的Geotail卫星探测到行星际磁场为持续南向．此太阳风条件驱动了一个小磁暴，Sym—H指数在04：12UT达到最小值-33nT．在磁暴主相期间，AE指数维持在较高的水平，其最大值达400nT．02：00--03：00uT期间，TC-1卫星在近地磁尾（-10．6，3．2，-0．1）R。处观测到明显的亚暴膨胀相特征和磁场偶极化过程．在偶极化前1min，有较强的（vx〈-100km／s）持续时间超过3min的尾向流发生．分析发现该尾向流具有低温、高密度和沿磁场流动的特点，这说明尾向流具有来源于电离层风的特征．尾向流期间，TC-1观测的磁场分量Bx和总的磁场强度增加，磁倾角减小，磁场结构变成非偶极型，说明尾向流对磁场结构有一定的影响，文中尝试给出了相应的物理解释．观测表明，该事例中的近地磁尾尾向流可能对磁场偶极化过程的发生有重要意义．

英文摘要：

On Oct. 12, 2004, the Geotail spacecraft was located in the solar wind near the dayside magnetopause and observed sustaining southward IMF from 01：30UT to 04：30UT. During the interval, a weak storm is driven with minimum Sym-H index of -33 nT at 04：12UT. In the main phase of the storm, AE index had been kept at a high level with a maximum value of about 400 nT. The TC-1 satellite located at （-10.6, 3.2,-0.1） Re observed evident characteristics of substorm expansion phase and the process of magnetic field dipolarization at the near-Earth magnetotail during 02：00--03：00 UT. Prior to the dipolarization, tailward flow （vx 〈 -100 km/s） lasting over 3 min with the characteristics of high hot ion temperature, low density and flowing along the magnetic field occurred. The features of the tailward flow indicated that it was originated from the ionosphere.During the interval of tailward flow, both the Bx component and the intensity of the magnetic field had been enhanced which suggested that the tailward flow had certain effects on the magnetic field configuration. According to the TC-1 observations, near-Earth tailward flow may be in close relation with the process of the magnetic field dipolarization.