木星磁层熵参量的计算与分析

空间等离子体熵的不守恒可能来自于磁场位形改变和非绝热过程.熵参量PV~(5/3)被广泛应用于分析地球磁层等离子体片中的输运问题,其中,P为压力,V为单位磁通量管的体积.通过熵参量的分布和变化可以判断磁层的稳定性及揭示磁层中的动力学过程.本文利用地球磁层中熵参量的分析应用,计算了木星稳态磁层模型中磁通量管的熵参量分布.从5R_j(R_j为木星半径)到55_j,熵参量增加了4个量级,55 R)_j之后有所下降,表明所用磁层模型在55R_j之外已经不稳定.同时,假想磁场重联后的单位磁通量管的熵参量分布表明,赤道面中远磁尾的磁场重联是由尾向输运的磁力线管拉伸断裂重联引起的.     

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。      

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.      

利用THEMIS卫星观测结果分析亚暴期间等离子体片尾向膨胀

利用THEMIS卫星观测结果,分析2008年3月13日10:40UT-12:10UT的一次中等亚暴事件在磁尾的全球演化过程.在该过程中,THEMIS的5颗卫星在午夜区附近沿x轴依次排列,离地心距离约8.7~13.2R_e.亚暴触发开始后,磁场偶极化和等离子体片的膨胀依次被在磁尾不同位置的卫星观测到.等离子体尾向膨胀的平均速度约为140km·s-1.在此次亚暴事件中可观测到两种类型的偶极化.一种为偶极化锋面,其与爆发性整体流（BBF）密切相关;另一种为全球偶极化,其与等离子体片的膨胀密切相关.亚暴触发开始约7min后,THEMIS卫星在低中高纬都可以观测到Pi2脉动的发生,且Pi2脉动的振幅随着纬度的升高逐渐变大.此次亚暴事件中的离子整体流速度主要是由离子电漂移速度引起的,测得的电场为局地磁通量变化导致的感应电场.      

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.      

中心等离子体片的地向对流型高速流

在GSM坐标系下, 利用TC-1卫星和Cluster/C1卫星上4s分辨率的磁场和热离子探测数据, 对中心等离子体片内的地向对流型高速流进行了统计研究(-19 Re < x< -9 Re, |y| < 10 Re, |z|< 5 Re). 统计结果表明, 地向对流型高速流会在15 Re以内出现``刹车', 在11.5 Re附近时出现``缺失'. 进一步对其速度特征进行统计分析. 结果显示, 在中心等离子体片内的高速流, 其运动方向主要为地向, 晨昏和南北方向的运动明显较弱; 在对流型高速流的地向输运过程中, 其峰值速度没有明显的下降; 在近地13 Re以内, 等离子体片内的地向对流型高速流具有较强的垂直磁力线运动速度. 这意味着对流型高速流在近地15 Re以内的“刹车”不是由高速流晨昏或南北方向的偏转造成的. 高速流在11.5 Re附近时出现的``缺失'可能与在15 Re以内出现``刹车'密切相关. TC-1卫星和Cluster卫星的观测为了解中磁尾重联高速流地向输运过程及亚暴膨胀相触发提供了重要的观测依据.      

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.      

基于MHD模拟数据的正午午夜子午面磁层顶位形研究

通过分析太阳风-磁层-电离层系统的三维全球磁流体力学(MHD)模型的计算数据, 给出了正午-午夜子午面磁层顶位形的定量模型. 分析表明, 正午-午夜子午面磁层顶位形可以用文献[3]提出的基于卫星观测数据的、描述赤道面磁层顶位形的函数来描述. 与赤道面磁层顶不同, 正午-午夜子午面磁层顶位形更为复杂. 在忽略极尖区(cusp)的简化条件下, 磁层顶位形仍需利用两条曲线来拟合. 太阳风动压Dp与行星际磁场分量Bz是控制磁层顶位形的主要因素. 行星际磁场为北向时, 磁场增强, 日下点距离r0增大; 行星际磁场为南向时, 磁场增强, 磁层顶日下点距离r0减小. 整体而言, 行星际磁场分量Bz由南转北时, r0增大, 且Bz对r0的影响减弱. 太阳风动压Dp是控制磁层顶日下点的主要因素, Dp增大, r0减小. 磁层顶位形的另一个参数磁层顶磁尾张角α, 随着行星际磁场南向分量增强而增大, 即磁层顶张开程度更加显著, 更多的磁通量由向阳侧传输到夜侧; Dp增大, α略增大, 这意味着Dp对磁通量由日侧向夜侧的传输也有一定的贡献.      

MMS星座对磁层顶磁通量绳内离子惯性尺度结构的观测

利用MMS观测数据,对磁层顶通量绳内离子惯性尺度（d_i）的结构进行分析研究.结果发现,许多不同尺度（约1d_i至数十d_i）的通量绳内都存在具有d_i尺度的电流 j _m,其方向在磁层顶局地坐标系的-M方向,即与磁层顶查普曼-费拉罗电流同向,由电子在+M方向的运动（ v _em）携带.这些电流结构具有以下特征:磁鞘与磁层成分混合,磁场为开放形态;离子去磁化,电子与磁场冻结;N方向（即垂直于磁层顶电流片方向）的电场 E _n显著增大,幅度达到约20mV·m-1,并伴有明显的尖峰状起伏,该增强和尖峰状起伏的电场对应于霍尔电场.分析表明,电流、电子与离子运动的偏离以及霍尔电场之间遵从广义欧姆定律,三者密切关联.进一步对磁层顶磁重联的探测数据进行分析发现,在很多重联区内也存在与通量绳内相似的结构,其尺度约为d_i量级,其中霍尔电场 E _N、电流 j _M和电子速度 v _eM均与通量绳内对应物理量的方向相同且幅度相近.基于上述观测事实,采用经典FTE通量绳模型,对通量绳内电流、电子运动和霍尔电场的起源进行了初步探讨,认为其来源于磁层顶无碰撞磁重联区内的相应结构,并且后者在离子尺度通量绳的形成过程中起到重要作用.      

Particle dynamics of the plasma sheet boundary layer

The plasma sheet boundary layer dynamics during substorms are analyzed using ISEE 1 and 2 energetic particle flux data. Considering the plasma sheet boundary layer as a tangential discontinuity and thus its movements as perpendicular to the local magnetic field direction, the differential timing of the two spacecraft data sets is used to calculate the boundary layer velocity and thickness. The velocity values are deduced to be of the order of 40 km/s and boundary thickness varies from a few tens of km to more than 4 R_E with an average value of ∼5000 km. For a majority of the boundaries studied the boundary layer appears simultaneously in all energy channels; within it significant spectral variations are observed and the spectrum is harder at plasma sheet recovery. The experimental data are discussed in the framework of the actual plasma sheet thinning models.     

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.     

Energy requirement of magnetic reconnection during magnetospheric substorms

Methods are discussed to estimate energy transfer from the solar wind to the magnetosphere during substorm growth phases. Observational and modeling constraints are then used to assess quantitatively the total amount of energy stored in the magnetotail. The major avenues of energy dissipation are examined and the energy that is released in the form of plasma sheet heating, ionospheric Joule heating, plasmoids, and energetic particle production during substorms is assessed. Energy sources are evaluated to drive substorm evolution in a particularly well-observed case: stored tail-lobe energy is sufficient to drive observed substorm dissipation processes (by large margins). On the other hand, energy in the closed field lines of the plasma sheet is insufficient to supply the substorm energy. Hence, magnetic reconnection is required on energy grounds during well-observed substorm cases.     

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.     

A statistical analysis of substorm associated tail activity

Substorm onset timing is a critical issue in magnetotail dynamics research. Solar wind energy is accumulated in the magnetosphere and the configuration of the magnetosphere evolves toward an unstable state during the growth phase. At some point, the expansion phase begins and the stored energy is released through a variety of processes that return the magnetosphere to a lower energy state. In recovery the various processes die away. Unfortunately, the ground and magnetospheric signatures of onset, i.e. energy release, can be seen both in the growth phase prior to onset and in the expansion phase after onset. Some investigators refer to each of these events as a substorm. Tail observations suggest that most substorms have one event that differentiates the behavior of the tail field and plasma. We refer to this time as the “main substorm onset”. Each substorm associated phenomenon is timed independently and then compared with main substorm onsets. ISEE-2 tail observations are used to examine the tail lobe magnetic conditions associated with substorms because ISEE-2 orbit has a high inclination and frequently observes lobe field. Approximately 70 ∼ 75% of tail lobe B_t and B_z change are associated with the main substorm onset. If the satellite is more than 3 Re above (below) the neutral sheet, 86% (57%) of plasma pressure dropouts are associated with substorms. We interpret our results as evidence that the effect of the growth phase is to drive the magnetosphere towards instability. As it approaches global instability local regions become temporarily unstable but are rapidly quenched. Eventually one of these events develops into the global instability that releases most of the stored energy and returns the magnetosphere to a more stable configuration.     

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.     

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.     

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。      

Laboratory investigation of physical mechanisms of auroral charged particle acceleration in the field-aligned current layers

One of the major topics of space weather research is to understand auroral structure and the processes that guide, accelerate, and otherwise control particle precipitation and during substorms. The problem is that it is not clear the structure of the magnetic field-aligned electric fields and how they are supported in the magnetospheric plasma. The objective of this research is to study the physical mechanisms of these phenomena in a laboratory experiment. It should be achieved by simulating the charged particle acceleration due to field-aligned electrical field generation in all totality of the interconnected events: generation of a plasma flow, its evolution in the magnetic field, polarization of plasma, generation of the field-aligned currents, development of instabilities in the plasma and current layers, double layers or anomalous resistance regions appearance, electron acceleration. Parameters of the laboratory simulation and preliminary results of the experiment are discussed.