磁云边界层中的重联慢激波观测分析

在Petschek模型中，排空区边界处的一对慢激波是能量耗散的重要机制.已有大量行星际空间的Petschek型磁场重联排空区观测事件被报道，但是只有少量的排空区边界处观测到了慢激波.针对一例位于磁云边界层中的Petschek型磁场重联排空区观测事件，在排空区靠近磁云一侧边界处证认了一例慢激波.激波跃变层两侧的磁场和等离子体参数满足Rankine-Hugoniot关系，且激波上下游的中间马赫数均小于1，上游的慢马赫数为2.94（>1），下游的慢马赫数为0.65（<1），符合慢激波的观测特征.磁云内部的等离子体β值很低，局地阿尔芬速度高，同时磁云边界层中可能发生丰富的磁场重联活动，这可能是磁云前边界处慢激波形成的原因.      

行星际磁云边界层中的电子分布函数特征

基于WIND 飞船观测的1995---2006 年间的磁云事件, 研究了磁云边界层中电子的流动图样, 以及电子速度分布函数的特点与电子加热和加速的关系, 得出以下结果. ①磁云边界层中存在的电子流动图样, 包括各向同性、双向流动和单向流动等. ② 相比于背景太阳风和磁云本体, 通常情况下磁云边界层中电子分布函数的核心热电子成分 (E<60eV) 增多, 超热电子成分 (E>60eV) 在沿磁场垂直方向上增多, 而在沿磁场平行或反平行方向以单方向增加为主, 此外, 还在近1/10 的磁云边界层中观测到了高能电子的明显增多. ③对比研究了磁云边界层与磁云驱动的激波对电子速度分布函数的调制作用, 经过激波, 电子分布函数的超热电子成分在各方向上都有增加, 不同于磁云边界层中在沿磁场平行或反平行方向上超热电子成分以单方向增加为主, 表明二者有不同的形成机制. ④考察了磁云边界层中的波活动增强和电子分布函数及离子流量增加的对应关系. 上述观测和对比分析进一步表明了磁云边界层是一种重要的动力学结构, 磁重联是一种可能的形成机制.      

行星际磁云与日球电流片相互作用的实例分析

基于1995-2007年Wind飞船观测到的105个磁云事件, 按照磁云在地球附近是否与日球电流片发生碰撞, 将其分为碰撞和未碰撞两类, 得到初步分析结果. ①可能与日球电流片发生碰撞的磁云事件有51例, 约占事件总数的49%; ②发生碰撞时, 磁云与日球电流片之间的相互作用区内呈现动力学相互作用增强的趋势, 例如多呈现局部的磁场方向间断结构(或称局域电流片结构), 伴随磁场强度下降、质子温度升高、部分质子速度分量出现跃变, Langmuir波或100~250,kHz的离子声波出现增强,这些现象可能与碰撞相互作用区域内发生的磁重联有关; 没有碰撞时这些特征一般不明显; ③至少有15个事例中, 磁云前后背景磁场的极性相反, 或磁场极性反转的位置位于磁云内部, 显示磁云可能正在跨越电流片运动; ④在几个磁云事件中发现磁云的亮环和亮核纤维结构. 这些极为初步的分析结果有利于了解磁云与日球电流片之间的相互作用过程.      

磁云边界层与间断的观测研究

利用WIND飞船的磁场和等离子体观测数据,分析了1995年2月至2003年8月之间82个磁云中的边界层事件.我们认为(1)磁云边界层中方向间断DD(Directional Discontinuity)类型中RD(Rotational Discontinuity),TD(Tangential Discontinuity),ED(Either Discontinuity),ND(NeitherDiscontinuity)的分布为37:18:44:1(%),与背景太阳风中的分布51:12:35:2(%)不同;主要区别在于RD与TD的比例变化. (2)磁云边界层的内外边界切向间断的比例很高,占总数的20%,而且两侧的密度和温度平均相对变化分别为|△N/N|=0.24和|△T/T|=0.19,大于边界层内部的平均值,显示了更多的切向间断特征;此外,磁云边界层中方向间断的出现频次约为太阳风中的1.87倍. (3)磁云边界层中方向间断的法向分布在θ-φ平面中不是随机分布,而是以θ=-24.90°,φ=217.49°为几何中心,主要是指向远离太阳的方向,而不是简单的各向同性分布.初步结果表明,间断是磁云边界层中的重要结构,它有着不同于背景太阳风间断类型比,为诊断磁云边界层的形成机理提供了依据.      

强磁云与共转高速流的相互作用两例

本文利用行星际观测数据,分析了1978年8、9月间同一起源的共转高速流两次与磁云发生的相互作用.8月27日,强磁云为共转高速流所追赶,磁云前半部为磁云本体,具有强磁云的基本特征,但后半部则为高速流追赶磁云的相互作用区,流速图象具有明显的双台阶特征.9月25日另一异常强磁云追赶再次重现的共转高速流,磁云前部为相互作用区,后部为磁云本体,但结构特征较简单磁云复杂;磁云前部流速峰值超过900km/s,而在磁云本体.Alfvén波速峰值在600km/s以上,非常接近局部太阳风速.此两事例进一步说明了Alfvén波速增强对磁云加速的重要作用.此外,还就磁云引起的磁暴和宇宙线下降的特性进行了讨论.     

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。      

太阳风中磁场重联离子扩散区的大尺度特征

利用ACE和WIND卫星2007年1月6日的联合探测, 在1AU附近发现了一个等离子体密度极低的Petschek-like重联喷流区. 该喷流区内部出现了非常明显的Hall双极磁场、等离子体密度下降区以及与Hall电流相符的低能段电子投掷角分布. 这些特征与重联离子扩散区的Hall效应非常吻合, 说明很可能在太阳风中观测到了一个离子扩散区. 分析表明, 与之相关的磁场重联为准稳态快速完全反向重联, 其扩散区以一对慢模波为边界, 空间尺度达到80个离子惯性长度, 表现出了大尺度重联的特征.      

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．     

近地磁尾准无碰撞磁重联事件

综合分析了ClusterⅡ－C1飞船在2001年9月15日飞越地球磁尾等离子体片区的热离子和磁场观测资料。结果表明，约在0340－0440UT时间期间，资料多次呈现出较强的尾向离子流（VXGSM＜0），明显的南向磁场分量（BZGSM＜0），以及明显的晨－昏向磁场分量BYGSM等特征。由此可以推断，在磁尾等离子体片中，在径向方向XGSE＞－18．6Re范围内，可能发生了多次磁重联事件，整个事件持续期约1h。磁重联事件的观测特征与准无（或半）碰撞磁重联理论的基本图像符合一致，因此这些事件应当是准无碰撞磁重联事件。     

太阳表面水平运动驱动的磁圈与开放磁场重联的模拟研究

太阳大气的诸多观测事件(如耀斑、喷流等)均被归因于磁重联产生的能量转换. 近年来, 关于太阳风起源, 有研究提出了磁重联使闭合磁圈开放为太阳风供应物质的新模式. 在该模式中, 闭合磁圈被光球超米粒组织对流携带, 向超米粒边界运动, 与位于边界的开放磁场相碰撞进而发生磁重联. 该模式中磁重联的驱动及其效应是本文的研究目标. 磁流体力学(MHD)数值模拟是研究太阳大气磁重联物理过程的重要途径. 本文建立了一个二维MHD数值模型, 结合太阳大气温度和密度的分层分布, 在超米粒组织尺度上模拟了水平流动驱动的闭合磁圈与开放磁场的重联过程. 通过对模拟结果的定量分析, 认为磁重联确实能够将闭合磁圈的物质释放, 进而供应给新的开放磁结构并产生向上流动. 该结果为进一步模拟研究太阳风初始外流奠定了基础.      

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.      

2.5维自适应磁场重联MHD模式

磁雷诺数(R_m)是影响磁场重联的重要因素. 真实的物理环境中R_m往往很高, 例如, 在行星际空间和太阳日冕中R_m通常大于104量级. 高R_m条件下的磁重联表现出很多异常特性, 然而高R_m条件下的磁场重联数值模拟需要很高的时空分辨率, 否则很难分辨出重联过程中形成的薄电流片. 本文基于自适应软件包PARAMESH将并行自适应网格技术引入磁场重联数值模拟, 建立了一个2.5维自适应磁场重联MHD模式, 研究高磁雷诺数条件下重联的动态演化过程, 进而将不同磁雷诺数的参数进行对比研究. 结果表明, 该模式可以自动捕捉到磁场重联产生的奇性电流片, 高磁雷诺数条件下产生的慢激波结构可提供一种快速磁能释放机制.      

基于能量色散特征计算磁层顶重联位置的方法研究

磁层顶磁场重联是太阳风向磁层输入能量的主要方式.重联如何触发一直是空间物理研究的难点,其机制仍然有待深入研究.由于卫星穿越磁层顶时,很难恰好穿越重联发生的区域,因此难以观测到重联的触发条件.本文利用THEMIS卫星观测,确立了反演磁层顶重联点的方法.当重联刚开始发生时,卫星能够观测到离子的能量色散特征,可利用其计算卫星到重联发生位置的距离.沿着磁力线模型追踪该距离即可反演出磁层顶发生重联的位置.与其他方法进行了对比分析,结果显示本文方法比其他方法具有更高的精度.      

Where will efficient energy release occur in 3-D magnetic configurations?

The energy needed to power flares is thought to be stored in the coronal magnetic field. However, the energy release is efficient only at very small scales. Magnetic configurations with a complex topology, i.e. with separatrices, are the most obvious configurations where current sheets can form, and then, reconnection can efficiently occur. This has been confirmed for several flares computing the coronal field and comparing the locations of the flare loops and ribbons to the deduced 3-D magnetic topology. However, this view is too restrictive taking into account the variety of observed solar flaring configurations. Indeed, “Quasi-Separatrix Layers” (QSLs), which are regions where there is a drastic change in field-line linkage, generalize the definition of separatrices. They let us understand where reconnection occurs in a broader variety of flares than separatrices do. The strongest electric field and current are generated at, or close to where the QSLs are thinnest. This defines the region where particle acceleration can efficiently occur. A new feature of 3-D reconnection is the natural presence of fast field-line slippage along the QSLs, a process called “slip-running reconnection”. This is a plausible origin for the motions of the X-ray sources along flare ribbons.     

Coronal heating by reconnection

The outermost atmosphere of the Sun, called the corona, is some 200 times hotter than the surface of the Sun. The main source of energy for heating the corona is believed to be the magnetic field which dominates the corona. Magnetic reconnection is probably the most important mechanism for releasing magnetic energy and may, therefore, be important for coronal heating or micro-flaring. The best observational examples of reconnection in the corona are thought to be X-ray bright points, which are small-scale brightenings seen randomly throughout the whole corona. Theoretical models can not only explain the key observations relating to bright points, but they can also explain the complex three-dimensional structures often seen in bright points. In these models magnetic neutral points play a significant role as the centres for reconnection both in two and three dimensions.     

Study of three dimensional structure of the fast convection flow in the plasma sheet by MHD simulations on the basis of spontaneous fast reconnection model

Three dimensional structure of the fast convection flow in the plasma sheet is examined using magnetohydrodynamic (MHD) simulations on the basis of spontaneous fast reconnection model. The fast flow observed in the near-Earth magnetotail is one of the key phenomena in order to understand the causal relationship between magnetic substorm and magnetic reconnection. In this paper, we focus on this earthward fast flow in the near-Earth magnetotail. Our previous studies have shown that the fast reconnection produces the Alfvénic fast reconnection outflow and drastic magnetic field dipolarization in the finite extent. In this paper, the results of our simulations are compared with those of the in-situ observations in the geomagnetotail. They have consistent temporal profiles of the plasma quantities. It is suggested that the fast convection flows are caused by spontaneous fast reconnection.     

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.