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.      

Multi-point observations of earthward fast flow in the plasma sheet by virtual satellites located in the MHD simulation domain

Time profiles of some physical values in earthward fast flows in the plasma sheet are observed at three dimensionally different positions by employing virtual satellites located in the three-dimensional magnetohydrodynamic simulation domain, and these simulations are done on the basis of the spontaneous fast reconnection model. In the spontaneous fast reconnection evolution, the width of the flow channel is narrow in the dawn-dusk direction, and it does not spread until the plasma collides with the magnetic loop. The enhancements in B_z and V_x are larger at the center of the fast flow channel than those at its dawn and dusk edges, reflecting the differences in the reconnection rate in the diffusion region. The enhancement in V_x is shorter near the plasma sheet boundary layer than that near the neutral sheet, reflecting the changes in the thickness of the flow channel.     

近地磁尾方位角流期间的场向电流增强

以往研究表明,地向高速流在近地磁尾可演化为方位角流,电离层内的方位角流和极光膨胀活动发生时,通过磁力线追踪到近地磁尾等离子体片的对应部分通常为地向和方位角高速流.通过对2016-2018年THEMIS卫星计划中THA,THD,THE三颗卫星同时观测到的数据进行分析、甄别后,在筛选出的62个事件中挑选一个典型的方位角流事件,与过去单颗卫星在不同时间段内的观测数据统计平均给出的结果进行对比分析发现,三颗卫星同时观测到的数据与过去单颗卫星在不同时间段内观测数据的统计平均结果存在较大差异.通过比较此事件期间等离子体流在xy平面的流场图发现,场向电流的大小与三颗卫星同时观测到的流场剪切度有较好的相关性.研究结果表明,方位角流期间近地磁尾和电离层通过场向电流耦合时,场向电流并不是在一个较宽的磁地方时内均匀分布,而是在一个局域化区域及较短的时间内产生强场向电流,这可能是由于方位角流在xy平面内的强剪切而造成的.      

Plasma flow in the near and distant geomagnetic tail

Measurements of the bulk flow of plasma in the outer magnetosphere were first made a little over a decade ago with Los Alamos instruments on the Vela satellites. During the intervening years, as flow measurements have been made with improved instruments and by other satellites they have come to play a crucial role in the development of our understanding of the structure and dynamics of the magnetosphere. For example, they were the means of discovery of the magnetosphere's boundary layer and of plasma vortices within the plasma sheet. They were the essential ingredient in the identification of signatures of magnetic reconnection at the magnetopause. And they were indispensible in clarifying the complex phenomena in the magnetotail accompanying substorms and in showing that these phenomena are consistent with a substorm model involving magnetic reconnection at a near-earth neutral line. Most recently, magnetotail plasma flow measurements by the ISEE-3 satellite at distances as great as 230 R_E have been instrumental in fixing the average location of the “distant” neutral line at ～ 60 to 120 R_E and in identifying plasmoids (i.e., severed sections of the plasma sheet), released during substorms and escaping down-tail. This paper reviews the features of magnetotail plasma flow, describes the most recent observations, and discusses their implications for magnetospheric physics.     

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.      

TC-1和Geotail对亚暴过程中近地等离子体片磁扰动的联合观测

以2004年9月28日02:53:20 UT的亚暴为例, 通过TC-1在磁尾约12.5 R_e 和Geotail卫星在近地磁尾等离子体片约8～9 R_e的联合观测, 研究亚暴触发过程中近地磁尾等离子体片中等离子体波动特征. 结果表明, 亚暴触发区是近地磁尾中心等离子体片中较小的一个区域, 在亚暴触发区中低混杂不稳定性在近地磁尾等离子体片中存在, 准垂直传播的低混杂波发生在亚暴触发过程中, 而亚暴触发过程中近地磁尾等离子体片外边界区内的磁场偶极化信号和扰动都非常微弱. 在亚暴触发和亚暴膨胀相过程中出现了多次具有不同特征的磁场偶极化现象.      

Driven reconnection in the near-Earth plasma sheet

In some recent MHD simulations of the near-Earth plasma sheet we studied onset and evolution of reconnection due to non-linear resistive instabilities. In our present contribution we show that these non-linear instabilities can be amplified significantly by inflow through the plasma sheet boundary and we discuss the consequences of that driving mechanism on the global dynamics of the instabilities. For high magnetic Reynolds numbers we find thin current sheets developing.     

来自电离层的尾向流对近地磁场位形的影响

探测一号(TC-1)卫星的观测结果表明,尾向流能够拉伸近地磁尾的磁力线,从而导致磁场位形改变.尾向流具有垂直于磁场的速度分量,这种垂直磁场的速度分量会导致磁力线向尾向拉伸,磁场的结构由偶极型变为非偶极型.而随尾向流的终止,地向流的出现,磁场的结构由非偶极型变为偶极型,磁力线恢复原状.另外在磁场的结构由非偶极型变为偶极型的过程中,伴随磁能的释放热离子温度的迅速升高,温度由各向同性逐渐趋向各向异性.其次,观测结果显示来自电离层的尾向流对磁场By分量有重要的影响,能够引起磁场By分量的显著增强.上述分析结果表明来自电离层的尾向流对近地磁尾动力学过程有着重要的影响.      

A fast tearing mode instability driven by agyrotropic electron pressure

The collisionless plasma environment at the current sheet of the Earth’s magnetotail is subjected to fast dynamic evolutions such as tearing instability. By considering agyrotropic pressure for electron and ion components of a collisionless plasma, we analytically investigate the dynamics of tearing mode instability, in which, breaking the frozen-in condition can either be provided by the electron inertia or by agyrotropic electron pressure. A set of linearized Hall-Magnetohydrodynamic (MHD) equations describes the evolution of tearing mode in a sheared force-free field. The presented scaling analysis shows that if the plasma-β$\beta$ exceeds a specified value, then the main mechanism of magnetic reconnection process is the nongyrotropic electron pressure. In this regime, the role played by agyrotropic ion pressure inside the reconnection layer is out of significance. Therefore, the electron-MHD framework, adequately, describes the dynamics of tearing instability with a growth rate which is much faster compared to the cases with a dominated bulk inertia or a gyrotropic plasma pressure.     

Ion dynamics associated with Alfven wave in the near-Earth magnetotail: Two-dimensional global hybrid simulation

Ion dynamics in the near-Earth magnetotail region is examined during periods of fast Earthward flow with a two-dimensional (2-D) global-scale hybrid simulation. The simulation shows that shear Alfven waves are generated at x ∼ −10R_E, where the strong earthward flow is arrested by the dipole field, and propagate along field lines from the equator to both southern and northern polar ionosphere. Non-gyrotropic ion velocity distributions occur where the large-amplitude Alfven waves are dominant. The simulation indicates that the Alfven waves are generated by interaction of the fast earthward flow with the stationary near-Earth plasma. Beam ions are found to be pitch-angle scattered and trapped in the wave field, leading to the non-gyrotropic ion distributions in the high-latitude plasma sheet boundary. In addition, significant particle heating and acceleration are found to occur behind the dipolarization front due to the effect of wave turbulence.     

Do we understand the inner magnetotail dynamics during substorms?

Substorm evolution of the near-Earth (|X|<15 R_E) plasma sheet has been emphasized recently because the inner tail is thought to link closely to the substorm auroral activity in the ionosphere during the early stage of substorms. In this paper, we discuss how the inner tail substorm phenomena during the late substorm growth phase and early expansion phase are accounted for by the two prevailing substorm models, namely, the near-Earth neutral line model and the current disruption model. We find that the late growth phase features are more satisfactorily accounted for by the current disruption model than by the near-Earth neutral line model. In addition, detailed observations on current disruption show evidence inconsistent with the proposed idea of dipolarization being due to plasma flow braking from reconnection in the mid-tail region, which poses a difficulty to the near-Earth neutral line model as well.     

The plasma sheet and boundary layers under northward IMF: A multi-point and multi-instrument perspective

During conditions of northward interplanetary magnetic field (IMF), the near-tail plasma sheet is known to become denser and cooler, and is described as the cold-dense plasma sheet (CDPS). While its source is likely the solar wind, the prominent penetration mechanisms are less clear. The two main candidates are solar wind direct capture via double high-latitude reconnection on the dayside and Kelvin–Helmholtz/diffusive processes at the flank magnetopause. This paper presents a case study on the formation of the CDPS utilizing a wide variety of space- and ground-based observations, but primarily from the Double Star and Polar spacecraft on December 5th, 2004. The pertinent observations can be summarized as follows: TC-1 observes quasi-periodic (∼2 min period) cold-dense boundary layer (compared to a hot-tenuous plasma sheet) signatures interspersed with magnetosheath plasma at the dusk flank magnetopause near the dawn-dusk terminator. Analysis of this region suggests the boundary to be Kelvin–Helmholtz unstable and that plasma transport is ongoing across the boundary. At the same time, IMAGE spacecraft and ground based SuperDARN measurements provide evidence of high-latitude reconnection in both hemispheres. The Polar spacecraft, located in the southern hemisphere afternoon sector, sunward of TC-1, observes a persistent boundary layer with no obvious signature of boundary waves. The plasma is of a similar appearance to that observed by TC-1 inside the boundary layer further down the dusk flank, and by TC-2 in the near-Earth magnetotail. We present comparisons of electron phase space distributions between the spacecraft. Although the dayside boundary layer at Polar is most likely formed via double high-altitude reconnection, and is somewhat comparable to the flank boundary layer at Double Star, some differences argue in favour of additional transport that augment solar wind plasma entry into the tail regions.     

Particle simulation study of substrom triggering with a southward IMF

This paper reports the spatial and temporal development of bursty bulk flows (BBFs) created by reconnection as well as current disruptions (CDs) in the near-Earth tail using our 3-D global electromagnetic (EM) particle simulation with a southward turning interplanetary magnetic field (IMF) in the context of the substorm onset. Recently, observations show that BBFs are often accompanied by current disruptions for triggering substorms. We have examined the dynamics of BBFs and CDs in order to understand the timing and triggering mechanism of substorms. As the solar wind with the southward IMF advances over the Earth, the near-Earth tail thins and the sheet current intensifies. Before the peak of the current density becomes maximum, reconnection takes place, which ejects particles from the reconnection region. Because of earthward flows the peak of the current density moves toward Earth. The characteristics of the earthward flows depend on the ions and electrons. Electrons flow back into the inflow region (the center of reconnection region), which provides current closure. Therefore the structure of electron flows near the reconnection region is rather complicated. In contrast, the ion earthward flows are generated far from the reconnection region. These earthward flows pile up near the Earth. The ions mainly drift toward the duskside. The electrons are diverted toward the dawnside. Due to the pile-up, dawnward current is generated near Earth. This dawnward current dissipates rapidly with the sheet current because of the opposite current direction, which coincides with the dipolarization in the near-Earth tail. At this time the wedge current may be created in our simulation model. This simulation study shows the sequence of the substorm dynamics in the near-Earth tail, which is similar to the features obtained by multisatellite observations. Identification of the timing and mechanism of triggering substorm onset requires further studies in conjunction with observations.     

Ion dynamics of magnetic reconnection in the magnetotail

We discuss the kinetic processes of plasma thermalization, acceleration, and mixing in magnetic reconnection. Non-Maxwellian, gyrotropic ion distribution functions such as anisotropic ion beams in the plasma sheet boundary layer (PSBL) and counter-streaming ions (CSIs) in the plasma sheet are often observed during a plasmoid passage of a satellite in the Earth's magnetotail. Non-gyrotropic ion distribution functions are also sometimes observed just after the passage of the plasmoid. We study the behavior of non-Maxwellian ion distribution functions observed by GEOTAIL. We further study theoretically the ion dynamics by using a particle-in-cell simulation, and discuss the role of non-Maxwellian distribution functions in magnetic reconnection.     

Theory of magnetic reconnection in three dimensions

Magnetic reconnection in traditional, two-dimensional theory can be closely associated with topological properties of the magnetic field, separatrices, neutral lines, or separators. Such well-defined topological properties may no longer exist, or become hopelessly complicated, in more general three-dimensional configurations, that otherwise behave physically quite similarly to the two-dimensional configurations. We discuss generalized definitions of reconnection applied to such situations and illustrate typical properties with simulations of three-dimensional reconnection in the magnetotail.     

Statistical study of electrostatic solitary waves associated with reconnection: Geotail observations

The role of waves in the dynamics of the magnetotail has long been a topic of interest in magnetospheric physics. The characteristics of Electrostatic Solitary Waves (ESWs) associated with reconnection have been studied statistically in the magnetotail by surveying the large amounts data obtained from Waveform Capture (WFC) which is an important component of Plasma Wave Instrument (PWI) on the Geotail spacecraft. About 150 reconnection events with WFC data available are selected, and approximately 10 thousands of ESW waveforms are picked up by hands for statistical study. The ESWs are observed near diffusion region and near the plasma sheet boundary layer (PSBL). Two kinds of waveforms of ESWs are observed: bi-polar and tri-polar pulses. It is found that the pulse width of the ESWs is in the order of 1–5 ms and the peak-to-peak amplitude is in the order of 0.1–5 mV/m. The amplitudes of ESWs are larger in the near-earth tail region than that in deep tail region. ESWs have been observed with or without guide magnetic field 〈By〉. The characteristics of ESWs in different reconnection region and under different strength of guild magnetic field, their possible generation mechanism will be discussed.     

Global MHD simulations for southward IMF: a pair of wings in the flanks

Global magnetohydrodynamic (MHD) simulations have shown that for southward interplanetary magnetic field nightside reconnection takes place only in a limited spatial scale in the cross-tail direction. Between the end of the nightside reconnection line and the flank magnetopause are regions of closed magnetic field lines which move relatively slowly tailward, compared with the magnetosheath flow on the far side and the jet flow produced by nightside reconnection on the side closer to the midnight. The magetosphere appears to have a pair of extended wings on the nightside. Further refinement of the simulations shows that the wings are not produced by the numerical effects. Similar features have been observed previously and also shown in different simulation models. The existence of the wings in the simulations indicates that the solar wind momentum is transferred to the closed magnetosphere if the simulations are relevant to reality.     

Magnetospheric Physics in China:2014—2015

In the past two years, much progress is made in magnetospheric physics by using the data of Double Star Program, Cluster, THEMIS, RBSP, Swarm missions etc., or by computer simulations. This paper briefly reviews these works based on papers selected from the 191 publications from January 2014 to December 2015. The subjects cover various sub-branches of magnetospheric physics, including geomagnetic storm, magnetospheric substorm, magnetic reconnection, solar windmagnetosphere-ionosphere interaction, radiation belt, outer magnetosphere, magnetotail, plasmasphere, geomagnetic field, auroras and currents.      

地球磁尾的电场模式

地球磁层中的电场是磁层等离子体运动的主要驱动力。目前常用的磁层电场为均匀晨昏电场和投影电场。本文假定磁力线为电场的等位线，地球电离层电场看做磁层电场沿磁力线在电离层的投影。利用Tsyganenko磁场模式（T89），沿磁力线反电离层电场投影到磁尾，得到了一个新的磁层电场模式。文中对偶极磁场和T89磁场模式下的投影场作了比较，说明本模式突破了偶极磁场的局限，在磁层有更大的适用范围。     

近地尾向流和磁场偶极化——TC-1卫星观测结果

2004年10月12日, 在01:30---04:30 UT期间, 位于向阳侧磁层顶附近的Geotail卫星探测到行星际磁场为持续南向. 此太阳风条件驱动了一个小磁暴, Sym-H指数在04:12 UT达到最小值-33nT. 在磁暴主相期间, AE指数维持在较高的水平, 其最大值达400nT. 02:00---03:00 UT期间, TC-1卫星在近地磁尾(-10.6, 3.2, -0.1)Re处观测到明显的亚暴膨胀相特征和磁场偶极化过程. 在偶极化前1min, 有较强的(v_x<-100 km/s)持续时间超过3min的尾向流发生. 分析发现该尾向流具有低温、高密度和沿磁场流动的特点, 这说明尾向流具有来源于电离层风的特征. 尾向流期间, TC-1观测的磁场分量Bx和总的磁场强度增加, 磁倾角减小, 磁场结构变成非偶极型, 说明尾向流对磁场结构有一定的影响, 文中尝试给出了相应的物理解释. 观测表明, 该事例中的近地磁尾尾向流可能对磁场偶极化过程的发生有重要意义.