磁宁静期磁尾爆发性整体流持续时间多点卫星研究

本文利用星簇CLUSTER的三颗卫星数据分析了磁宁静期磁尾爆发性整体流(BBFs,BurstyBulkFlows)的时间尺度,并与单个卫星的结果做了比较.事例研究表明,利用三颗卫星观测数据判断的BBFs的时间尺度比单个卫星的大一倍左右.对于三颗卫星观测到的同一个BBFs ,BBFs在晨昏方向上的摆动决定了CLUSTER的三个卫星观测到BBFs的先后次序.三颗卫星的观测也显示了BBFs的高度局域化特征.磁宁静期磁尾BBFs寿命的增大,使得BBFs携带的质量和能量的地向输运增加.这种地向输运增加的结果是:磁尾储存的能量得到较为平稳的释放,改变了亚暴起始产生的时间,为解决磁层压力平衡矛盾(PBI,PressureBalanceInconsistency)问题提供了新的思路.

磁尾爆发性整体流与亚暴的关系

利用星簇Cluster的三颗卫星(C1,C3和C4)在2001年和2002年的数据,研究磁尾爆发性整体流(BBF,Bursty Bulk Flow)事件及其与亚暴的关系.三颗卫星可以同时观测到同一次BBF事件,有时只有一颗或两颗观测到BBF,其原因有:(1)等离子体整体流的速度峰值满足BBF选取原则中的峰值要求,但卫星的运行轨道不满足;(2)卫星处于所要求的区域内,等离子体整体流的速度峰值不满足;(3)中性片的复杂结构及其运动使得选取条件不能同时满足;(4)BBF空间分布高度局域化.统计研究结果表明:单颗卫星观测存在局限性,C1、C3和C4卫星独立观测到BBF的持续总时间分别占它们联合观测到的55.07%、77.48%和55.52%;大部分亚暴爆发期间都能观测到BBF,甚至在一次亚暴爆发期间观测到多次BBF;少数亚暴爆发期间没有观测到BBF.

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02:53:20UT的亚暴为例,通过TC-1在磁尾约12 5R_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,业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.在亚暴触发和亚暴嘭胀相过程中出现了多次具有不同持征的磁场偶极化现象.

磁层快速对流事件的多点卫星联合观测

本文利用星簇Cluster的三颗卫星(C1,C3和C4)在2001年和2002年的数据,研究了快速对流事件(RCE,Rapid Convection Event)及其与亚暴的关系.结果显示单点卫星对RCE的观测,不能反映磁尾RCE的真实情况.在2002年7月25日发生的一次RCE事件,C1和C3观测到这次RCE,C4却没有观测到.在三颗卫星联合观测到的306次RCE中,C1观测到215次,C3观测到266次,C4观测到227次.统计研究表明,单点卫星观测到的RCE的平均时间也不能准确反映磁层内的RCE.由此推论在整个中心等离子体片内,快速对流事件所承担的能量和磁通量的输运量,可能远大于单点卫星观测给出的结果.用速度来定义的磁尾爆发性整体流(BBF,Bursty Bulk Flow)与亚暴的关系,比用磁通量定义的RCE与亚暴的关系要更加紧密.

2001年9月15日Cluster卫星对电流片的观测研究

2001年9月15日0430-0515 UT期间, Cluster卫星多次穿越磁尾电流片．由FGM、CIS等仪器获得了电流片磁场,粒子速度等数据变化情况,并得到了磁尾高速流的两次明显反转．本文采用GSM坐标系．利用求磁场空间梯度张量的方法获得了越尾电流,其电流密度的峰值为28 nA/m2．并对电流片在垂直方向上的摆动和在晨昏方向的波动现象进行了分析．数据显示此时电流片为薄电流片并有一个变薄的趋势,其厚度大约为0．2-0．3．Re．磁场重联时地向流与尾向流均超过了1000km/s,并测得了电流片的法线方向和运动速度, 从而得到了磁尾电流片的结构和运动情况．

太阳风在地球激波前兆区减速的统计研究

本文首次利用完全相同两颗卫星(CLUSTER C1和C3)的数据对地球激波前兆区太阳风的减速和偏转特性进行了统计研究.结果表明,在激波前兆坐标系中,太阳风减小的速度随观测点到激波的距离D_(BS)增大而减小,随行星际磁场与激波法向夹角θ_(BN)增大也减小,在ULF波动区深度D_(WS)小于6R_e(R_e为地球半径)的范围内最为显著;伴随着太阳风减速的另外一个现象——太阳风的偏转,也存在相似的规律.其最大减速和最大偏转角度分别为10 km/s和3°.太阳风减速和偏转,以及随之变化的太阳风动压,可能会引起地球磁层顶位置和形状发生改变,同时也为激波前兆区弥散(diffuse)离子的起源及加热提供了一种可能的机制.

ULF Waves Associated with Solar Wind Deceleration in the Earth＇s Foreshock

在 67 2003 年 4 月上在地球的前震与太阳的风减速联系的 ULF 波浪的特征用波浪望远镜技术被学习。在卫星框架， ULF 波浪是左手极化并且伪反平行的宣传模式，与在大约 18.63 mHz 的一座力量山峰。在 GSE 坐标的波浪向量被估计是 =(? 4.29， 2.28， 1.21 ) 瑮吗？

TC-1对近地磁尾地向等离子体团的观测

分析了2004年07月～09月双星计划TC-1卫星在磁尾的磁场数据,发现近地磁尾存在等离子体团(plasmoid).给出了TC-1对近地(X>-13RE处)等离子体团的观测结果.根据等离子体团内磁场结构的不同,分析两个事件:2004年09月14日磁环(magnetic loop)型的等离子体团具有闭合磁力线结构,2004年08月06日磁通量绳(magnetic flux rope)型的等离子体团具有开放磁力线结构.两个事件与背景流场相比都具有高速地向速度.粒子可以沿着开放的磁力线从磁通量绳逃逸出来,而磁环由于其闭合磁力线结构可以束缚住粒子.TC-1对磁尾地向等离子体团的观测又一次为多X线重联在磁尾的发生提供了证据并表明重联地点应该位于X<-10RE的磁层尾部区域.

THEMIS星座对2008年2月26日磁层亚暴的观测--磁尾重联触发的亚暴分析研究

本文利用THEMIS卫星结合地面极光和地磁的观测,研究了2008年2月26日04:05和04:55UT的两次亚暴事件.Angelopoulos已经对发生在04:55UT的第二个亚暴事件做了分析.本文对两次亚暴的相关活动进行了详细研究,特别对第一次做了深入讨论,并着重分析了磁重联与亚暴活动的关系.在两次亚暴的初始阶段,第一次极光增亮发生在中磁尾磁重联后2～3min,但是持续时间较短,极向膨胀缓慢,与伪暴的特征相似,标志了亚暴的初突发(initial onset).两次亚暴都存在第二次极光增亮和极光的极向膨胀,且时间与近地磁尾观测的地向流和磁场偶极化同时发生,并与亚暴膨胀相的其他活动的发生同步,标志了亚暴的主突发(major onset).在两次亚暴的增长相期间,极盖区开放磁通量持续增加;在亚暴膨胀相和恢复相中,极盖区磁通量迅速减少.表明两次亚暴膨胀相的演化分别与两次尾瓣开放磁力线重联过程相联系的.从亚暴活动的参数分析,这两次亚暴都属于小亚暴范围;从重联率分析,两次磁重联都属于弱重联.本文的观测结果表明,中磁尾磁尾重联首先触发伪暴;高速流将磁通量和能量传输到近地磁尾;高速流减速最终导致亚暴电流楔(substorm current wedge,简称SCW)的形成和电流中断,产生近地偶极化和极光膨胀,引起亚暴膨胀相突发.本文的观测结果是对近地中性线模型(near earth neutral line,简称NENL)和重联-电流中断协同模型(synthesis scenario of MR and CD,简称RCS)模型及亚暴膨胀相两步突发观点的有力支持.

磁场重联扩散区域中磁零点结构的观测研究

磁重联是能量转换的非常重要的基本等离子体物理过程之一.过去磁场重联的理论、数值模拟和观测研究,大多是集中在二维模型下进行,而实际的磁场重联涉及三维非线性过程,对于三维情况下磁场重联及其相关的奇异结构的基本性质现在还未完全解决.通过高斯积分引入Poincaré指数,将其离散化,利用Cluster四颗卫星所测得的磁场,研究了磁场重联扩散区中磁零点结构,通过计算零点位置和轨迹,估算了其运动速度和轨迹,并结合零点附近电流的特征将观测与零点重联模型进行了比较和讨论.

pTC-1 observation of ion high-speed flow reversal in the near-Earth plasma sheet during substorm

Based on measurements of FGM and HIA on board TC-1 at its apogee on Septem-ber 14, 2004, we analyzed the ion high-speed flows in the near-Earth plasma sheet observed during the substorm expansion phase. Strong tailward high-speed flows (Vx ～ -350 km/s) were first seen at about X ～ -13.2 RE in near-Earth magnetotail, one minute later the flows reversed from tailward to earthward. The reversal process occurred quickly after the substorm expansion onset. The near-Earth magnetotail plasma sheet was one of key regions for substorm onset. Our analysis showed that the ion flow reversal from tailward to earthward was likely to be in close relation with the substorm expansion initiation and might play an important role in trigger-ing the substorm expansion onset.

Multi-spacecraft observations of earthward flow bursts

On the basis of the plasma, electric and magnetic fields jointly observed by Cluster and the Double Star TC-1 spacecraft in the Earth's magnetotail, we have investigated the earthward flow bursts by introducing the momentum equation in the X-direction in the ideal conditions of magneto hydrodynamics (MHD). One earthward flow burst with a peak in excess of 500 km/s was selected, when the four spacecraft of Cluster were located around ?16 RE and TC-1 was located around ?10 RE in the X-direction. The inter-spacecraft distances in Y and Z directions were smaller than the statistical spatial scales of the bursty bulk flows. When the Y components of E and ?V×B were compared, there was no clear breakdown of the frozen-in condition during the earthward flow burst. With the measured plasma and magnetic parameters from two spacecraft at different positions in the magnetotail, the X component of the pressure gradient was calculated. Magnetic tension was calculated using the magnetic field measured at four points, which could be compared with the assumed constant in the past research with single satellite. When the pressure gradient and the magnetic tension were put into the MHD momentum equation, some samples of the earthward flow bursts were accelerated and some were decelerated. The braking process of the earthward flow burst was more complicated than what the past results had shown. The accelerated samples accounted for about one third of the whole earthward flow bursts and discontinuously located among the decelerated elements. The original single earthward flow burst event might be split into several short flow bursts when it was moving to the Earth. Our results may partly illustrate that the duration of fast flows during three phases of substorm becomes short near the Earth. The results are consistent with the past results that fast flows intrude to places earthward the typical braking region.

The radial evolution of earthward BBFs during substorm

We here study the occurrence rate,probability function of velocity and duration of earthward bursty bulk flows(BBFs) in the Inner Plasma Sheet(IPS,β>0.5) using the data of Cluster in 2001 and 2002.The occurrence rate of earthward BBFs increases with distance from the Earth up to ?19 RE,which is in agreement with the previous observations of the radial evolution of BBFs.About 54% of earthward BBFs in expansion phase have a velocity larger than 600 km/s,whereas only 38% of earthward BBFs in growth and recovery phases have a velocity larger than 600 km/s.The average velocity of earthward BBFs in expansion phase is 732 km/s,larger than those in growth phase(631 km/s) and recovery phase(617 km/s).The durations of earthward BBFs decrease with the decrease of downtail distance from Earth due to the braking of earthward BBFs.The duration of earthward BBFs in expansion phase is larger than that in growth and recovery phases.The average durations in growth,expansion,and recovery phases are respectively 49.3,71.5,and 47.6 s.Therefore,the ratios of transports of energy of earthward BBFs in growth,expansion,and recovery phases can be estimated to be 0.51:1:0.47.Thus,the earthward BBFs in the expansion phase have the largest capability of the transport of energy and can produce the largest braking effects,such as inertial currents and auroral activities.

Statistical research on the motion properties of the magnetotail current sheet:Cluster observations

The origin of the flapping motion of the earth's magnetotail current sheet is one of the most important problems in the magnetotail dynamics.Using Cluster data,we make a statistical research on the motion properties of the magnetotail current sheet of 2001 and 2003.We calculate the velocities of the magnetotail current sheet using new methods and obtain the distribution of the magnetotail current sheet velocities in the X-Y plane in GSE coordinate system.Our results show that although most of the current sheets were propagating toward the tail flanks and those of the exceptions lay in dusk side,which is consistent with previous studies,the proportions of the current sheet which were propagating toward midnight (where |YGSE|=0) were higher than those in previous studies.Motions of the current sheet in the middle area (|YGSE|<8 Re) of the magnetotail are investigated.Relatively high value of the Z component of the velocity further confirms that the middle area of the magnetotail might be a source region for the motion of the current sheets which were propagating towards the tail flanks.According to our case studies,the way the current sheets propagated toward midnight area differs significantly from that toward dusk and dawn side,from which we infer that there might be two different kinds of current sheet motions originated from different sources.The statistical results of this paper may give some clues for further studies on the origin of the flapping motion of the magnetotail current sheet.

Shrinkage of magnetosphere observed by TC-1 satellite during the high-speed solar wind stream

During the interval 06:14―07:30 UT on August 24, 2005, since the Earth’s magneto- pause was suddenly compressed by the persistent high-speed solar wind stream with the southward component of the interplanetary magnetic field (IMF), the magnetopause moved inward for about 3.1 RE. Meanwhile, TC-1 satellite shifted from northern plasma sheet to the northern lobe/mantle region, although it kept inward flying during the interval 06:00―07:30UT. The shift of TC-1 from the plasma sheet to the lobe/mantle is caused by the simultaneous inward displacements of the plasma sheet and near-Earth lobe/mantle region, and their inward movement velocity is larger than the inward motion velocity of TC-1. The joint inward dis-placements of the magnetopause, the lobe/mantle region and the plasma sheet indicate that the whole magnetosphere shrinks inward due to the magnetospheric compression by the high-speed solar wind stream, and the magnetospheric ions are attached to the magnetic field lines (i.e. ‘frozen’ in magnetic field) and move inward in the shrinking process of magnetosphere. The large shrinkage of magne-tosphere indicates that the near-Earth magnetotail compression caused by the strong solar wind dynamic pressure is much larger than its thickening caused by the southward component of the IMF, and the locations of magnetospheric regions with different plasmas vary remarkably with the variation of the solar wind dynamic pressure.

Convective high-speed flow and field-aligned high-speed flows explored by TC-1

From June 1, 2004 to October 31, 2006, a total 465 high-speed flow events are observed by the TC-1 satellite in the near-Earth region (?13 RE < x < ?9 RE, |Y |<10 RE, |Z|<5 RE). Based on the angle between the flow and the magnetic field, the high-speed flow events are further divided into two types, that is, field-aligned high-speed flow (FAHF) in the plasma sheet boundary and convective bursty bulk flow (BBF) in the center plasma sheet. Among the total 465 high-speed flow events, there are 371 FAHFs, and 94 BBFs. The CHF are mainly concentrated in the plasma sheet, the intersection angle between the flow and the magnetic field is larger, the magnetic field intensity is relatively weak. The FHF are mainly distributed near the boundary layer of the plasma sheet, the intersection angle between the flow and magnetic field is smaller, and the magnetic field intensity is relatively strong. The convective BBFs have an important effect on the substorm.

THEMIS observations of two substorms on February 26,2008

Two substorms occurred at ～04:05 and ～04:55 UT on February 26,2008 are studied with the in-situ observations of THEMIS satellites and ground-based aurora and magnetic field measurements.Angelopoulos et al.have made a comprehensive study of the 04:55 UT event.We showed detailed features of the two substorms with much attention to the first event and to the rela-tionship between mid-tail magnetic reconnection(MR) and substorm activities.It was found that in the earlier stage of each substorm,a first auroral intensification occurred 2-3 min soon after the start of mid-tail MR,followed by a slow and very lim-ited expansion.The auroral arcs were weak,short-lived,and localized,characterizing all features of a pseudobreakup.We re-garded the first auroral brightening as the initial onset of the substorms.A few minutes later,a second stronger auroral intensification appeared,followed by quick and extensive expansions.It was interesting to note that the second brightening and related poleward expansion happened almost simultaneously(within a couple of minutes) with the onset of earthward flow and dipolarization in the near-Earth tail and other phenomenon of the substorm expansion phase.We thus regarded the second auroral brightening as the major onset of the substorms.Furthermore,it was seen that during the growth phase of the two substorms,the polar cap open flux Ψ kept increasing,while it quickly reduced during the substorm expansion and recovery phase.These variations of Ψ implied that the evolution of the two substorm expansion phases were closely related to MR of tail lobe open field lines.Analysis of substorm activities revealed that the two events studied were small substorms;while estimate of MR rate indicated that the MR processes in the two substorms were weak.The aforementioned observations suggested that mid-tail MR initiated the pseudobreakup first;the earthward flow generated by MR transported magnetic flux and energy to the near-Earth tail to cause the formation of SCW and CD,which induced near-Earth dipolarization and major auroral brightening,and eventually led to the onset of the substorm expansion phase.These results were clearly consistent with the picture of NENL and RCS models and supported the two step initiation scenario of substorms.

Near-Earth bursty bulk flows and AE index

With the 4-s resolution data of the magnetometer and the ion plasma analyzer on TC-1 from June to November of each year during the period of 2004―2006, we statistically analyzed the occurrence rate of both convective and field-aligned bursty flows (FABFs). A near-Earth bursty bulk flow (NEBBF) occurred during both the quiet time and substorm process. In general, the magnetic field and the plasma density began oscillating with the appearance of the NEBBF associated with a distinct increase of the AE index. The increase of AE index during the NEBBF was more than 100 nT in both quiet time and substorm process. The statistical analysis indicated that the occurrence rates of the FABFs were nearly the same in the dif-ferent stages of the AE index, but the occurrence rate of the NEBBFs was much higher in the growth stage of the AE index, indicating that the NEBBFs were di-rectly related to the growth and expansion phases of the substorm. The observa-tions suggested that the quite large number of BBFs from the mid magnetotail could enter into the near-Earth tail and play important role in triggering the sub-storm onset.

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.

Structures of magnetic null points in reconnection diffusion region:Cluster observations

Magnetic reconnection is a very important and fundamental plasma process in transferring energy from magnetic field into plasma. Previous theory, numerical simulations and observations mostly concen-trate on 2-dimensional (2D) model; however, magnetic reconnection is a 3-dimensional (3D) nonlinear process in nature. The properties of reconnection in 3D and its associated singular structure have not been resolved completely. Here we investigate the structures and characteristics of null points inside the reconnection diffusion region by introducing the discretized Poincaré index through Gauss integral and using magnetic field data with high resolution from the four satellites of Cluster mission. We esti-mate the velocity and trajectory of null points by calculating its position in different times, and compare and discuss the observations with different reconnection models with null points based on character-istics of electric current around null points.