Observations of Magnetic Reconnection in the Magnetotail Associated With Substorms

Magnetic reconnection is one of the most important,dynamic phenomena in the magnetotail in terms of magnetic field line configuration change and energy release.It is believed to occur in the distant magnetotail mainly during southward interplanetary magnetic field periods and in the near-Earth magnetotail in association with substorms.In the present paper,we discuss several important issues concerning magnetic reconnection in the magnetotail associated with substorms,such as reconnection signatures,location,timing,spatial scale,and behavior,from the macroscopic,observational point of view.

Evidence of guide field magnetic reconnection in flapping current sheets

Based on current sheet flapping motion on 27 August 2018 in the dusk flank magnetotail,as recorded by instruments aboard Magnetospheric Multiscale(MMS)spacecraft,we present the first study of guide field reconnection observed in the flux rope embedded in kink-like flapping current sheets near the dusk-side flank of the magnetotail.Unlike more common magnetotail reconnections,which are symmetric,these asymmetric small-scale(λ_(i)~650 km)reconnections were found in the highly twisted current sheet when the direction normal to the sheet changes from the Z direction into the Y direction.The unique feature of this unusual reconnection is that the reconnection jets are along the Z direction-different from outflow in the X direction,which is the more usual situation.This vertical reconnection jet is parallel or antiparallel to the up-and-down motion of the tail’s current sheet.The normalized reconnection rate R is estimated to be~0.1.Our results indicate that such asymmetric reconnections can significantly enlarge current sheet flapping,with large oscillation amplitudes.This letter presents direct evidence of guide field reconnection in a highly twisted current sheet,characterized by enlarged current sheet flapping as a consequence of the reconnection outflow.

Evolution of clustered magnetic nulls in a turbulent-like reconnection region in the magnetotail

t Magnetic null points and flux ropes play important roles in the three-dimensional process of magnetic reconnection. In this study, a cluster of null points are reconstructed in the reconnection region in the magnetotail by applying a fitting-reconstruction method to measurements from the Cluster mission. The number of recon- structed null points varies rapidly, presenting a turbulentlike evolution of the magnetic structure. The electron density and the flux of the accelerated electrons were enhanced in this turbulent-like region. During this unstable reconnection process, a B-As-B null structure was formed, showing flux rope features and resembling a secondary island in the observation.

GENERATION MECHANISM OF MAGNETIC NOISE BURSTS IN THE EARTH’S MAGNETOTAIL

In this paper, we study the electromagnetic instability driven by ion beams and plasmainhomogeneity in the plasma sheet boundary layer and for the first time obtain the growthrate spectrum of electromagnetic waves in a broad frequency range (Ω_i<<0.1ω_(LH)?|ω_r|≤Ω_e,Ω_i and Ω_e are respectively the ion and electron cyclotron frequencies, ω_(LH)=(Ω_iΩ_e)^(1/2) is thelower hybrid frequency). The peak of the growth rate spectrum is near |ω_r|≈ω_(LH) and thesecond peak is near |ω_r|?Ω_e. The fundamental characteristics of the growth rate spectrumare in agreement with the satellite observations of the magnetic noise bursts in the magneto-tail.

Observation of directional change of core field inside flux ropes within one reconnection diffusion region in the Earth's magnetotail

Two consecutive magnetic flux ropes, separated by less than 30 s(Dt \ 30 s), are observed within one magnetic reconnection diffusion region without strong guide field in the Earth's magnetotail by Cluster multispacecraft. The flux ropes are characterized by bipolar signatures of the south–north magnetic field component Bz accompanied with strong core magnetic field By, intense current J and density depletions inside of them. In spite of the small but non-trivial global scale negative guide field(–By), there exists a directional change of the core fields of two flux ropes, i.e.,-Byfor the first one, and Byfor the second one. The directions of the core fields are the same as the ambient cross-tail magnetic field component(By) just outside of flux ropes. Therefore, we suggest that the core field of flux ropes is formed by compression of the local preexisting Byand that the directional change of core field is due to the change of local preexisting By. Such a change in ambient Bymight be caused by some microscale physics.

Double Star TC-1 observation of the earthward flowing plasmoids in the near magnetotail

We analyze Double Star TC-1 magnetic field data from July to September in 2004 and find that plas-moids exist in the very near-Earth magnetotail. It is the first time that TC-1 observes the plasmoids inthe magnetotail at X > ?13 RE. According to the difference of the magnetic field structure in plasmoids,we choose two typical cases for our study: the magnetic flux rope on August 6 with the open magneticfield and the magnetic loop on September 14 with the closed magnetic field. Both of the cases are as-sociated with the high speed earthward flow and the magnetic loop is related to a strong substorm. Theions can escape from the magnetic flux rope along its open field line, but the case of the closed mag-netic loop can trap the ions. The earthward flowing plasmoids observed by TC-1 indicate that the mul-tiple X-line magnetic reconnection occurs beyond the distance of X=?10 RE from the earth.

Application of Galerkin spectral method for tearing mode instability

Magnetic reconnection and tearing mode instability play a critical role in many physical processes.The application of Galerkin spectral method for tearing mode instability in two-dimensional geometry is investigated in this paper.A resistive magnetohydrodynamic code is developed,by the Galerkin spectral method both in the periodic and aperiodic directions.Spectral schemes are provided for global modes and local modes.Mode structures,resistivity scaling,convergence and stability of tearing modes are discussed.The effectiveness of the code is demonstrated,and the computational results are compared with the results using Galerkin spectral method only in the periodic direction.The numerical results show that the code using Galerkin spectral method individually allows larger time step in global and local modes simulations,and has better convergence in global modes simulations.

地球远磁尾中的磁场重联(Ⅰ)流动撕裂模不稳定性

本文数值研究了地球远磁尾中流动撕裂模不稳定性所引起的磁场重联过程.结果表明,在短暂的指数增长之后,当磁岛宽度接近等离子体片厚度时,流动撕裂模不稳定性的增长率大大降低,最终磁岛宽度趋于饱和.磁岛的饱和宽度随着磁Reynolds数S的增大而减小,随着剪切流动层宽度δV的增大而增加.在S→∞的情况下,流动撕裂模将退化为流动颈缩模,磁场重联不再发生.当飞船通过由流动撕裂模不稳定性所形成的磁岛时,即可观测到磁场B_z分量由北转南或由南转北的现象.

2008年1月5日磁层亚暴期间磁尾近地重联事件的THEMIS观测与分析

本文主要应用THEMIS卫星的磁场和等离子体流观测数据,分析了2008年1月5日08:51～08:57 UT亚暴膨胀相期间磁尾的一个近地重联事件.在亚暴膨胀相期间,地面的全天空成像仪清楚地记录到了极光的极向扩展,THEMIS的P5卫星在地球同步轨道附近观测到了磁场的偶极化现象.在亚暴膨胀相末期的08:51～08:57 UT期间,P3(X_(GSM)～-9.12R_E)和P4(X_(GSM)～-9.40R_E)同时观测到了一对方向相反的高速等离子体流.这对方向相反的高速等离子体流是由磁尾的重联现象所引起.重联的位置被估计位于X_(GSM)～-9.12R_E和X_(GSM)～-9.40R_E之间较小的空间范围内.并且,在重联位置的两侧,重联的Hall效应被P3和P4两颗卫星观测到.因此,这一磁尾重联事件发生在距离地球非常近的空间范围内.

近地磁尾重联中哨声波和Hall磁场的Cluster观测

在2001～2003年Cluster飞船通过近地磁尾期间，共探测到14次重联事件，在这些事件中同时还观测到等离子体波活动．本文把14次事件分为三大类，其中：第1类包含了8次事件，它们是在等离子体片内先于重联事件观测到波活动，并且还同时观测到Hall磁场．经过分析判断，这类事件中观测到的波是右旋偏振的哨声波．第Ⅱ类包含了2次事件，这类事件也观测到了Hall磁场和右旋偏振的哨声波．第Ⅲ类也包含了2次事件，这类事件只是普通的重联事件，没有观测到Hall磁场，但是波活动明显先于重联事件．在我们观测的14次事件中，比较强烈的哨声波和Hall磁场是一一对应的，因此哨声波可能主要是在Hall磁场的四极结构区激发的．

我国磁层物理研究的进展和展望

在我国空间科学的开拓者赵九章教授的倡导下，1959年我国开始开展磁层物理研究工作．70年代中期以后，经过20多年的努力，我国磁层物理研究有了很大进展，现已具备重点发展和重点突破的条件本文在文献[1-3]综述的基础上，对近年来我国在磁层物理若干领域内的主要成果进行了补充评述。

模拟IMF北向且B_y分量占主导时磁层顶重联

本文基于自己开发的全球三维磁层模型,模拟研究了IMF(Interplanetary Magnetic Field)北向并且By分量较大(时钟角为60°)时磁层顶三维结构及其重联图像.结果发现,IMF By为正时,在北极隙区附近尾-昏侧存在IMF与地磁场之间稳定持续的重联现象;参与重联的地球磁场既有闭合磁力线也有开放磁力线;IMF在北极隙区与地球闭合磁力重联后一端与南磁极相连的磁力线在尾向运动时还可能与北尾瓣的开放磁力线重联而重新闭合,这种重联与磁力线循环过程不同于同一条IMF磁力线分别在南北半球与地磁场重联的模型.南极隙区的重联发生在尾-晨侧,其动力学过程与北极隙区情形类似.我们的模拟结果表明,IMF By较大时不可能发生IMF同一条磁力线分别在南北极隙区重联的情形,也不会因此而减少尾瓣的开放磁力线.

用于地球磁尾三维磁重联实验的脉冲电源

空间环境地面模拟装置是哈尔滨工业大学承建的国家重大科技基础设施项目,其包含的空间等离子体环境模拟与研究系统是用于提供磁重联过程等基本物理过程的时空演化规律研究的平台。在研究地球磁尾三维磁重联时,使用处于真空环境内的偶极磁场线圈和两个磁镜场线圈来提供研究所需的模拟背景磁场,其中偶极场线圈为一个总电感为17.4 mH、总电阻为30.25 mΩ的单个线圈,而磁镜场线圈为两个线圈镜像对称设置并串联连接,总电感30.16 mH,总电阻58.81 mΩ。为了产生实验所需背景磁场的幅值和持续时间,研制并测试了两套总能量3.36 MJ的脉冲电源,在进行地球磁尾三维磁重联实验时两套电源需要同时工作。用于驱动偶极场线圈的脉冲电源按照实验需求可以在充电压不大于20 kV的情况下,能够提供超过9 kA的峰值电流,95%峰值电流的持续时间超过了5 ms,由峰值时刻降低到10%峰值时刻的时间不超过130 ms;用于驱动磁镜场线圈的脉冲电源按照实验需求可以在充电压不大于20 kV的情况下,能够提供超过8 kA峰的值电流,95%峰值电流的持续时间超过了5 ms,由峰值时刻降低到10%峰值时刻的时间不超过130 ms。

磁层亚暴膨胀相的协同性模型

用一元磁流体力学广义行波展开法进一步研究了近磁尾内边界的位型不稳定性。将笔者以前关于近磁尾赤道面附近的漂移气球模不稳定性工作推广到非赤道区和存在非定常地向流的情形。论证了减切地向流将导致近磁尾内边界绝对不稳定，并使增长率显著增加。研究表明在亚暴电流楔的形成过程中，中磁尾磁重联和近磁尾位型不稳定性可能协同地起着重要作用。在此基础上提出了一个磁层亚暴膨胀相的中性线一电流中断协同模型。该模型能解释卫星和地面的许多亚暴观测结果。不同的亚暴可能有不同的触发过程。

IMF北向时磁层顶重联的模拟研究

本文基于自己开发的全球三维磁层模型,模拟研究了IMF(Interplanetary Magnetic Field)北向时磁层顶重联及磁尾结构.结果发现磁层顶附近存在两种典型的重联过程:一是高纬极尖区IMF与地球磁场的重联,这与空间观测证据和前人的模拟结果是一致的;二是重联后一端在太阳风中另一端与地球相连的磁力线在向磁尾运动中,会发生弯曲、拖曳,在磁尾晨昏侧低纬区域可与尾瓣开放磁力线满足重联条件而再次发生重联.我们认为前一重联会使磁尾等离子片产生与IMF时钟角方向相反的旋转;而后者可重新形成闭合磁力线,可能是LLBL(Low Latitude Boundary Layer)形成的重要原因.

等离子体彗尾动力学的数值研究

本文基于可压缩磁流体动力学模型，数值研究了尾瓣巾具有超Ａｌｆｖｅｎ速流动的等离子体彗尾的动力学特征．结果表明，等离子体片和尾瓣之间的剪切等离子体流动将会激发流动撕裂模不稳定性，引起彗尾等离子体片中发生磁场重联，形成磁岛和高密度的等离子体团．进而模拟了太阳风引起的局部驱动力对等离子体彗尾中磁场重联的影响，其特征时间远大于流动撕裂模．我们认为一些观测到的等离子体彗尾中的四块和彗尾截断事件可能主要与彗尾中剪切等离子体流动所引起的流动撕裂模不稳定性有关．

磁层亚暴的磁重联-电流中断-电离层、磁层耦合全球模型

磁层亚暴是地球空间动力学的一个重要过程．本文对磁层亚暴进行了理论分析和观测比较研究．研究表明，减速地向流导致近磁尾处磁场位形绝对不稳定，并显著提高其增长率；中碰尾磁场重联、近磁尾电流中断以及电离层－磁层耦合可能协同作用，触发亚暴．该亚暴模型与许多观测现象相符．

地球远磁尾中的磁场重联——(Ⅱ)局部驱动力的影响

基于可压缩磁流体动力学模型,数值研究了由太阳风引起的局部驱动力对地球远磁尾中磁场重联的影响.结果表明,在远磁尾等离子体片中将发生强迫磁场重联,并形成磁岛和等离子体团.形成磁岛的特征时间很大于流动撕裂模不稳定性引起磁岛非线性饱和的特征时间.磁岛宽度随着磁Reynolds数S的增大而减小,随着尾瓣中等离子体压力与磁压之比值β_∞的降低而减小.认为太阳风引起的局部驱动力对地球远磁尾等离子体片中磁场重联的影响,可能不如流动撕裂模不稳定性那样显著.

三维非线性撕裂模耦合和快速磁场重联

基于三维不可压缩电阻性ＭＨＤ方程，在长柱形位型下，数值研究了电阻撕裂模不稳定性所引起的磁场重联过程．研究结果表明，撕裂模的非线性相互作用和耦合，将产生许多模式的强烈解稳和导致快速的磁场重联．这一过程的进一步发展，导致具有螺旋形结构、不同模式的磁岛磁力线重叠，从而引起较大区域内的磁场各态经历和磁力线随机走向，形成撕裂模湍动；该湍动将会产生较大的反常电阻，反过来可以进一步加速撕裂模的非线性发展．这种自激过程，有助于理解日冕中的快速磁重联，并可作为一种可能的太阳耀斑机制．

Excitation of Whistler-Mode （Chorus） Emissions during Terrestrial Substorms

The enhanced growth rate of whistler mode waves has been evaluated during an injection event associated with an isolated terrestrial substorm that occurred at 23：00 UT, on January 21, 1991. The electron phase space density observed by an LEPA instrument on the board of the CRRES spacecraft is modelled by using a bi-loss-cone distribution function （composed of a high anisotropic component and a quasi-isotropic component）. During the injection event, the path integrated gain may increase by a factor of 5 over a frequency range near a few tenths of the electron gyrofrequency, which is consistent with the enhancement observed in the CRRES plasma wave experiment （PWE） emissions. Scattering of electrons by the enhanced whistler mode waves causes the pitch angle distribution of resonant electrons to a quasi isotropic （fiat-top） distribution during the terrestrial substorm injection event.