磁层顶穿越事件自动识别算法

磁层顶是太阳风与磁层进行质量、动量、能量交换的关键区域.磁层顶穿越事件（MCEs）可通过对卫星探测到的粒子能谱和磁场数据图进行人工分析的方式来识别.因太阳风动压和行星际磁场的易变性，位于磁层顶附近的卫星经过长期观测可能会经历成千上万次的磁层顶穿越.人工分析的方法工作量巨大，而且识别速度慢.本文发展了一种新的日下点附近MCEs自动识别算法.此算法综合分析卫星探测到的粒子和磁场数据，能有效地减少误判的发生.为了验证算法的有效性，采用单CPU计算机对THEMIS卫星在2007—2018年靠近日下点附近观测到的数据进行MCEs自动识别，最终在约6h共识别出16758个MCEs.这些自动识别出来的MCEs样本可用于统计研究磁层顶相关的诸多物理问题，如凹陷磁层顶、太阳风与磁层相互作用，磁层顶磁场重联等.同时还分析了算法的精确性和局限性.      

TC-1和Cluster对向阳侧磁层顶通量传输事件的联合观测研究

2004年2至4月期间,探测一号(TC-1)卫星和Cluster卫星有25次同时处在向阳侧磁层顶附近的磁鞘内,TC-1卫星在低纬区,Cluster卫星在中高纬区.利用这一期间两卫星探测到的27个通量传输事件(FTE),分析行星际磁场(IMF)横向分量BT={By,Bz}对磁层顶重联发生位置的影响,以及分量重联的观测事实,得到如下主要结果.(1)当IMF南向分量Bz占优势(|Bz|＞|By|)时,FTE大多(约占87.5%)能在低纬观测到,而当IMF By分量占优势(|Bz|＜|By|)时,则FTE大部分能在中高纬观测到(占84.2%);(2)很少观测到相关联的事件(关联事件指在低纬生成的FTE,向高纬运动中先后被TC-1卫星和Cluster卫星探测到的事件),表明在低纬形成的FTE可能大多沿磁层顶两侧滑向磁尾,只有少数可能运动到高纬地区;(3)中纬地区探测到的FTE大多是以分量重联方式产生于该区,而非来自磁赤道附近成对形成的FTE.      

磁层顶通量传输事件轴向统计分析

磁层顶通量传输事件（Flux Transfer Event,FTE）与磁重联相关,其典型特征为磁场法向分量的双极变化.在不同FTE模型里,FTE结构可能为重联的通量管、由多X线重联形成的闭合磁通量绳或者由单X线重联形成的开放磁场环,从而在磁层顶有不同的整体位形.使用一种新的轴向分析方法,对Cluster在一个向阳面磁层顶穿越季观测到的505个FTE进行统计研究.结果表明:在磁层顶中低纬度的侧翼,大多数FTE轴向均为沿磁层磁力线方向即南北方向,少数FTE轴向沿着不同于磁层磁力线方向的东西方向;在高纬磁层顶,大多数FTE轴向沿东西方向,少数FTE轴向沿着磁层磁力线方向即南北方向.这些统计特征有助于重新认识FTE的全球形态.      

磁鞘快速流引起的磁层顶凹陷事件

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THEMIS5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·-1），随后THEMIS5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.      

太阳风向磁层输入电磁能量研究

利用全球磁流体力学（MHD）模拟结果,通过确立包含磁层顶的太阳风流线内边界来识别三维磁层顶位形,并以极尖区位置作为磁层顶日侧与夜侧的分界线,在此基础上定量研究了不同条件下穿过磁层顶向磁层内输入的电磁能量. 研究发现,磁层顶的能量传输与太阳风条件密切相关,磁重联是控制电磁能量传输的重要机制. 结果表明,当IMF（行星际磁场）南向时,极尖区后方的磁尾附近存在电磁能输入最大值,当IMF北向时,电磁能输入最大值发生在极尖区附近;南向IMF条件下,在IMF强度增大或太阳风密度增大时,磁层顶电磁能传输的电磁能量比北向IMF条件时增加更显著. 太阳风通过调节磁层顶面积间接影响到磁层顶能量传输大小. 研究还发现,北向IMF与南向IMF条件下穿过磁层顶的电磁能输入的比值范围约为10%～30%,此比值一定程度上反映了北、南方向IMF与地磁场磁重联效率的比值.      

磁层顶位置的卫星观测

本文用了自1963年到1979年卫星穿越磁层顶的1024个观测资料计算了向阳侧磁层顶椭球面的参数。企图从这些资料得出星际磁场和太阳风热压强对向阳侧磁层顶位置和形状的影响大小。但数值计算和理论分析都表明,目前所具有的观测资料的精度不足以获得星际磁场和太阳风热压强影响的数值。本文分析了各种因素所造成的在计算该椭球面参数时的不确定性。进而提出了对进一步工作的建议。该椭球面的平均大小、形状和方位的观测值与理论预计值是非常一致的。      

远磁尾磁层顶位形的偏转

利用WIND和ARTEMIS卫星观测数据，分析远磁尾磁层顶对行星际和太阳风变化的响应，尤其是偏离日地连线的太阳风速度改变对远磁尾磁层顶的影响.研究发现在2011年9月13日的事件中，P2卫星观测到高速且高密度的磁鞘流.利用最小变量法进行分析发现，磁层顶沿着偏离日地连线的太阳风速度方向发生偏转.根据相似三角形定理，推断出本次事件中磁层顶在y方向和z方向上的偏转幅度分别达到10R_e和6R_e.P1和P2卫星的相对位置也证实了这一观点.因此，偏离日地连线的太阳风速度对远磁尾磁层顶的位形影响很大.研究结果可为建立包含太阳风速度v_y和v_z效应的磁层顶模型提供观测证据.      

磁层顶附近流场剪切度的确定

磁层顶附近的流场剪切度与磁层顶附近能量转换的程度有关.很多磁层顶数值模拟用到流场剪切度这个输入参数,但一直是假设的.本文利用Cluster多卫星同时观测数据及独特的时空分辨功能,采用线性插值和重心坐标的方法确定了磁层顶附近流场的剪切度.通过对晨侧和昏侧磁层顶及附近磁层磁鞘流场剪切度的真实空间分布的研究结果表明,在平静的太阳风条件和地磁条件下磁层顶附近流场剪切度有时也很大,可达每百公里330 km/s的相对速度差.但在很多情况下流场是弱剪切的,在上千公里的距离上只有每秒几十公里的相对速度差.本文确定流场剪切度的方法可以推广用来确定任一位置的流场剪切度.      

极向运动极光结构和喉区极光光学观测特征的对比分析

利用中国北极黄河站高时间分辨率的三波段全天空成像仪极光观测数据，联合太阳风和行星际磁场等观测，分析了极向运动极光结构（PMAFs）和喉区极光的形成及演化特征.研究发现:一系列PMAFs与喉区极光事件同时出现在观测视野中，其中PMAFs主要发生在日侧极隙区极光卵赤道向边界的极向一侧，沿东西方向分布，点亮后向高纬运动；喉区极光紧靠PMAF一侧发生，从极光卵赤道向边界向低纬延伸，沿南北方向分布，点亮后向高纬偏西方向运动；观测期间PMAFs发生频率高于喉区极光；当PMAFs与喉区极光同时出现时，PMAFs可以与喉区极光几乎同时出现或略晚于喉区极光出现，持续时间较喉区极光短.观测结果表明:与PMAF相对应的磁层顶重联过程和与喉区极光对应的磁层顶凹陷导致的磁重联过程在日侧磁层顶上的相邻区域分别发生，两种极光事件的形成过程相对独立，可能不存在相互触发关系.      

磁尾等离子体片对流对亚暴的影响

采用2(1/2)维全粒子电磁模拟方法研究了等离子体片中稳态对流及局地爆发高速流对磁层亚暴触发过程的影响.研究发现,地向瞬时局地高速流可触发磁场重联,导致储存于磁尾磁场能量的快速释放.但是,等离子体片稳态对流可抑制磁尾磁场重联过程.此项研究结果表明,局地爆发高速流能够触发磁层亚暴;而行星际磁场(IMF)持续南向时的稳态磁层对流期间,不易发生亚暴.      

行星际磁场北向时磁层顶区磁场重联的全球模式

在对背阳面磁层顶区局域磁场重联模拟的基础上提出了一个行星际磁场北向时磁层顶磁场重联的全球模式。行星际磁场北向时碰层顶磁场重联导致近地尾瓣的能量被输送到远磁尾,太阳风能量不在磁尾储存,向阳面磁层顶变厚,磁层受到一系列扰动。      

Two-spacecraft observations of reconnection at the magnetopause: Model results and data comparison

We revisit an example of “quasi-steady” magnetic reconnection at the dayside magnetopause on February 11, 1998, observed by Equator-S and Geotail at the dawnside magnetopause. Phan et al. [Phan, T.D. et al., 2000. Extended magnetic reconnection at the Earth’s magnetopause from detection of bi-directional jets. Nature 404, 848–850.] reported oppositely directed jets at these spacecrafts and inferred a length of the reconnection line of about 38R_E. Pinnock et al. [Pinnock, M., Chisham, G., Coleman, I.J., Freeman, M.P., Hairston, M., Villain, J.-P., 2003. The location and rate of dayside reconnection during an interval of southward interplanetary magnetic field. Ann. Geophys. 21, 1467–1482.] used measurements from SuperDARN radars to show that the reconnection electric field was variable. Here we complement this work by obtaining snapshots of the reconnection electric field from the in situ observations. To do this, we apply a reconstruction method based on a model of compressible Petschek-type magnetic reconnection. This independent method uses magnetic field observations as input data to calculate the reconnection electric field. We obtain average values of E_rec in the range of 0.4–2.4 mV/m. Further we infer a distance perpendicular to the reconnection line of 0.4–0.6R_E. The model results are compared with the two studies mentioned above. It thus appears that while the transfer of momentum for this event is indeed large-scale, the actual rate depends on the time it is measured.     

Interconnection of high-latitude and low-latitude boundary layers when IMF BY is dominant

The antiparallel merging model places the location of the reconnection region for a dominant interplanetary magnetic field (IMF) B_Y at high latitudes at the dayside magnetopause and predicts that the low-latitude boundary layer (LLBL) is located on open field lines of the magnetospheric flanks. Interball-1 data obtained in the wide local time range near the low-latitude magnetopause makes it possible to analyze the LLBL plasma population and to find a link between possible reconnection at high latitudes and LLBL occurrence. We found that no boundary layer was observed in the regions which have no topological connection with the merging site. All cases of LLBL observations are located downstream from a specific boundary. This boundary coincides with the first magnetospheric field line touching the reconnection region and can be located in a wide local time region depending on the instant IMF direction. Even the LLBL on closed field lines shows the tendency to be concentrated in the vicinity of this boundary. Thus we show that all types of observed LLBLs are linked to reconnection sites predicted by the antiparallel merging model.     

Magnetospheric Boundary Layer Structure and Dynamics as Seen From Cluster and Double Star Measurements

In this review, we discuss the structure and dynamics of the magnetospheric Low-Latitude Boundary Layer (LLBL) based on recent results from multi-satellite missions Cluster and Double Star. This boundary layer, adjacent to the magnetopause on the magnetospheric side, usually consists of a mixture of plasma of magnetospheric and magnetosheath origins, and plays an important role in the transfer of mass and energy from the solar wind into the magnetosphere and subsequent magnetospheric dynamics. During southward Interplanetary Magnetic Field (IMF) conditions, this boundary layer is generally considered to be formed as a result of the reconnection process between the IMF and magnetospheric magnetic field lines at the dayside magnetopause, and the structure and plasma properties inside the LLBL can be understood in terms of the time history since the reconnection process. During northward IMF conditions, the LLBL is usually thicker, and has more complex structure and topology. Recent observations confirm that the LLBL observed at the dayside can be formed by single lobe reconnection, dual lobe reconnection, or by sequential dual lobe reconnection, as well as partially by localized cross-field diffusion. The LLBL magnetic topology and plasma signatures inside the different sub-layers formed by these processes are discussed in this review. The role of the Kelvin-Helmholtz instability in the formation of the LLBL at the flank magnetopause is also discussed. Overall, we conclude that the LLBL observed at the flanks can be formed by the combination of processes, (dual) lobe reconnection and plasma mixing due to non-linear Kelvin-Helmholtz waves.      

Effect of the Interplanetary Electric Field on the Magnetopause From Global MHD Simulations

The north-south component B_z of the Interplanetary Magnetic Field (IMF) and solar wind dynamic pressure P_d are generally treated as the two main factors in the solar wind that determine the geometry of the magnetosphere. By using the 3D global MHD simulations, we investigate the effect of the Interplanetary Electric Field (IEF) on the size and shape of magnetopause quantitatively. Our numerical experiments confirm that the geometry of the magnetopause are mainly determined by P_dB_z, as expected. However, the dawn-dusk IEFs have great impact on the magnetopause erosion because of the magnetic reconnection, thus affecting the size and shape of the magnetopause. Higher solar wind speed with the same B_z will lead to bigger dawn-dusk IEFs, which means the higher reconnection rate, and then results in more magnetic flux removal from the dayside. Consequently, the dayside magnetopause moves inward and flank magnetopause moves outward.      

磁层氧离子向磁鞘泄漏的机制和效应

１９９７年 １月 １０日磁暴期间， Ｇｅｏｔａｉｌ卫星在向阳侧的磁鞘中观测到了磁层氧离子突增事件．这些氧离子的出现和磁鞘中存在很强的南向行星际磁场有关．事件期间向阳面发生了准静态的磁重联，氧离子流存在由北向南的速度分量．通量突增过程具有逆向和正向能量色散现象，磁层内部只有氧离子有可能被梯度漂移输送到重联区，所以只有氧离子在磁鞘中持续地被观测到．估计氧离子的逃逸速率为 ０．６１× １０２３／ｓ，大约为环电流氧离子输入率的 ３３％．大量的环电流氧离子由磁层跑到了磁鞘，导致环电流指数 ＡＳＹ－Ｈ呈现明显的非对称性．     

On the origin of the high-latitude boundary layer

This paper presents a statistical study of the high-latitude boundary layer (HLBL) performed on 53 Interball-1 magnetopause crossings. In the study we verify if antiparallel merging is the main source of HLBL formation when the IMF is nearly horizontal. To provide such a study we designed a new coordinate system which allowed us to analyze HLBL under varied interplanetary conditions. This coordinate system floats over the dayside magnetopause following the changes in the instant location of the reconnection site. Despite very different interplanetary conditions, the observed HLBL plasma regimes manifest systematic behavior in the “reconnection” frame of reference. We explain the observed pattern in terms of sporadic patchy reconnection in the high magnetic shear region of the magnetopause.     

The magnetopause erosion and the magnetosheath magnetic field penetration into the dayside magnetosphere

The earthward displacement of the magnetopause observed during a southward IMF (or the magnetopause erosion) and its dependence on the solar wind plasma and magnetic field parameters is studied by investigating data of about 30 magnetopause crossings by the ISEE 1 and 2 spacecraft. It is shown that the magnetopause erosion may be explained by a depression of the magnetic field intensity in the dayside magnetosphere caused by the penetration of the magnetosheath magnetic field (component perpendicular to the reconnection line) into the magnetosphere. The penetration coefficient (the ratio of the intensity of the penetrated field to the intensity of the magnetosheath magnetic field) is estimated and found to equal approximately 1.