Energetic Electrons as a Field Line Topology Tracer in the High Latitude Boundary/CUSP Region: Cluster Rapid Observations

Energetic electrons (e.g., 50 keV) travel along field lines with a high speed of around 20 R_Es⁻¹. These swift electrons trace out field lines in the magnetosphere in a rather short time, and therefore can provide nearly instantaneous information about the changes in the field configuration in regions of geospace. The energetic electrons in the high latitude boundary regions (including the cusp) have been examined in detail by using Cluster/RAPID data for four consecutive high latitude/cusp crossings between 16 March and 19 March 2001. Energetic electrons with high and stable fluxes were observed in the time interval when the IMF had a predominately positive B_z component. These electrons appeared to be associated with a lower plasma density exhibiting no obvious tailward plasma flow (<20 keV). On the other hand, no electrons or only spike-like electron events have been observed in the cusp region during southward IMF. At that time, the plasma density was as high as that in the magnetosheath and was associated with a clear tailward flow. The fact that no stable energetic electron fluxes were observed during southward IMF indicates that the cusp has an open field line geometry. The observations indicate that both the South and North high latitude magnetospheric boundary regions (including both North and South cusp) can be energetic particle trapping regions. The energetic electron observations provide new ways to investigate the dynamic cusp processes. Finally, trajectory tracing of test particles has been performed using the Tsyganenko 96 model; this demonstrates that energetic particles (both ions and electrons) may be indeed trapped in the high latitude magnetosphere.     

2001年1月26日高纬磁层顶通量管事件的观测研究

2001年1月26日11:10～11:40UT, ClusterⅡ卫星簇位于午后高纬磁鞘边界层和磁鞘区,此 时行星际磁场Bz为南向. 本文对在此期间观测到的多次磁通量管事件作了详细的研究 ,获得一系列的新发现：（1）高纬磁鞘边界层磁通量管的出现具有准周期性,周期约为78s ,比目前已知的磁层顶向阳面FTE的平均周期（8～11min）小得多. （2）这些通量管都具有 强的核心磁场;其主轴多数在磁场最小变化方向,少数在中间变化方向,有些无法用PAA判 定其方向（需要用电流管PAA确定）,这与卫星穿越通量管的相对路径有关. （3）每个事件 都存在很好的HT参考系,在HT参考系中这些通量管是准定常态结构;所有通量管都沿磁层顶 表面运动,速度方向大体相同,都来自晨侧下方. 通量管的径向尺度为1～2RE, 与通 常的FTE通量管相当. （4）起源于磁层的强能离子大体上沿着管轴方向由磁层向磁鞘运动; 起源于太阳风的热等离子体沿管轴向磁层传输. 通量管为太阳风等离子体向磁层输运和磁层 粒子向行星际空间逃逸提供了通道. （5）每个通量管事件都伴随有晨昏电场的反转,该电 场为对流电场.     

2004年3月18日高纬磁层顶多重通量管事件分析

2004-03-18 23:10～23:50 UT期间,“双星（Double Star）”探测一号卫星（TC-1）在向阳面磁层顶高纬晨侧由内向外穿越磁层顶,其时TC_1的GSM坐标为 (7.5RE, -5.5RE, -5.4RE), RE为地球半径.穿越过程中TC_1观测到了8个通量管和1个磁通量传输事件（FTEs）.在此期间Cluster星簇位于向阳面太阳风内,其GSM坐标为(18.0RE, -3.1RE, -6.2RE),其4颗卫星监测到行星际磁场（IMF）的BZ分量持续南向,BＹ有较大的负值.本文的研究表明：TC_1观测到的前7个通量管具有准周期重现性,周期大约是1～4 min,明显小于以前所观测到的FTEs的平均周期(8～11 min);所有的通量管都具有较强的核心场.本文分别使用最小方差分析法（MVA）和Grad_Shafranov反演方法（GSR）对通量管的轴向进行了分析和对比,发现所有的通量管主轴基本沿晨昏向,结果显示GSR方法在轴向分析上比MVA优越.本文使用GSR方法对通量管的磁场结构进行了分析,恢复出了通量管的磁场在卫星穿越面的结构图;此外,本文还对这次多重通量管事件进行了deHoffmann-Teller(HT)分析,结果表明,所有通量管大致朝南极方向运动,均来源于向日面低纬区域.这说明它们可能起源于向日面低纬区,由该区的磁场分量重联产生.