本文利用信使号飞船2011-2015年期间在轨磁场数据对水星磁尾电流片的磁场结构分布特征进行了统计分析.为探究磁场结构分布随水星径向距离的变化,电流片划分为近磁尾(-1.5RM＞X＞-2.0RM)和远磁尾(-2.0RM＞X＞-2.5RM)两个区域.所得结果表明:(1)无论是近磁尾还是远磁尾,电流片中的磁场都以+Bz分量为主,磁场方向几乎与磁赤道面垂直.(2)相比近磁尾,远磁尾电流片中磁场强度、Bz分量较弱,By分量较强,而且-Bz信号出现概率相对较大,这表明电流片中磁活动相对容易在远磁尾中发生.(3)磁场强度以及Bz分量在晨昏方向上的分布存在晨昏不对称性——在方位角120°～190°范围内相对较弱.弱Bz数据点(Bz＜5 nT)也在昏侧(Y＞0)发生较为频繁.(4)与Bz分布相反,磁场强By分量(|By|＞5 nT)倾向于在晨侧(Y＜0)发生.统计分析还表明,磁场By分量与行星际磁场By分量并无明显的相关性.对比地球磁尾电流片,我们对水星磁尾电流片Bz分量、强By分量的晨昏不对称起源机制作了探讨分析.%In this study,we use the magnetic field data measured by MESSENGER from 2011 to 2015 to survey the average magnetic field distribution in Mercury's magnetotail current sheet.It is found that the tail current sheet coincidently lies in the magnetic equator plane due to the ignorable dipole tilt angle (＜3°).The magnetic field Bz component is much larger than the By component at the current sheet center,which suggests that the field structure is almost vertical to the equatorial plane.In comparison with the region closer to Mercury (-1.5RM＞X＞-2.0RM),at the distant region (-2.0RM＞X＞-2.5RM) the magnetic field strength Bmin as well as the Bz component are weaker,By component is stronger,while the signature of negative Bz tends to appear there.Thus,the distant current sheet (-2.0RM＞X＞-2.5RM) is more dynamic than the closer current sheet.The distributions of Bmin as well as the Bz show evident dawn-dusk asymmetry,i.e.Bmin and Bz are relatively weaker duskward within the azimuthal angle 120°-190° where the data points of weaker Bz(Bz＜5 nT) also show higher occurrence probability.In contrast,the strong By component (| By | ＞5 nT) at the current sheet tends to appear at the dawnward region (Y＜0).In addition,the statistical study shows that the By component does not have a significant correlation with the By component of interplanetary magnetic field.By comparison with the Earth's magnetotail current sheet,the mechanisms to control the dawn-dusk asymmetry of magnetic field at Mercury's tail current sheet are discussed.
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