磁鞘快速流引起的磁层顶凹陷事件

Magnetopause Indentation Induced by the Magnetosheath Fast Flow

最近研究表明,磁层顶凹陷对磁层-电离层耦合具有重要作用.但是,磁层顶凹陷现象的确认需要多颗卫星的联合观测,目前为止报道的磁层顶凹陷事例非常少.本文利用THEMIS 5颗卫星的联合观测结果,分析了一例由磁鞘快速流引起的磁层顶凹陷事件.2007年7月21日10:00 UT-10:45 UT,位于日下点磁层顶附近的THEMIS卫星在磁鞘观测到很强的地向流(约400km·s-1),随后THEMIS 5颗卫星相继穿越磁层顶进入磁层.通过最小方差MVA方法确认局部磁层顶法向,与经典磁层顶模型比较发现,磁鞘快速流压缩磁层顶形成局部凹陷.为了探究此磁鞘快速流的起源,对位于L1点的WIND卫星观测到的太阳风数据进行分析发现:在这个时间段内太阳风条件非常稳定,行星际磁场主要为径向,磁场南北向分量非常小.由此推测此磁鞘快速流的产生很可能与径向行星际磁场有关.

Recent studies indicated that the magnetopause indentation plays an important role in magnetosphere-ionosphere coupling.Confirmation of magnetopause indentation requires joint observations with multiple satellites.So far,there have been few magnetopause indentation events reported.In this paper,a case of magnetopause indentation induced by fast magnetosheath flow is reported with multiple spacecraft analysis based on the observations of five THEMIS probes.During the interval from 10:00 UT to 10:45 UT on 21 July 2007,when the five THEMIS probes are located near the subsolar magnetopause,a fast anti-sunward flow(with a velocity of 400 km·s-1) was observed in the magnetosheath just before THEMIS crossed the magnetopause to the magnetosphere.A magnetopause local indentation event was identified by comparing the nominal magnetopause and the tangential magnetopause plane calculated using the MVA method.In order to explore the origin of this magnetosheath fast flow,solar wind data observed by WIND satellite at L1 point were analyzed.It is found that the solar wind is very stable during this period.The Interplanetary Magnetic Field(IMF) is mainly radial and the component of the north-south direction is very small.It is speculated that the generation of this magnetosheath fast anti-sunward flow may be related to the radial IMF.